Seattle's (relatively) cool winter, Part 2

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This is Part 2 of a two-part blog post highlighting the cool winter season we’ve generally experience in the Seattle area this year.

  • For both posts, I am using a ‘seasonal year’ which runs from November 1 through the follow October 31.

  • Technically, November is late autumn, but snow is not uncommon in the Seattle area in November, and it is generally the beginning of our long winter-spring season, where on any given day it may seem more wintry or springlike during this period. Low temperatures can typically drop into the 30s ˚F and rarely rise above 60 ˚F, and often barely reaching the 50s ˚F.

  • Days are often cloudy in this period as well in addition to being cool.

  • Part 1 explore the Accumulated Average Daily Temperature.

  • Part 2 explores the Average Daily Temperature by day of the seasonal year for the entire period.

 

Figure 1 is an animated GIF which pages through the years 1948 through 2023 (to-date) and shows the Average Daily Temperature (ADT) recorded at Seattle Tacoma International Airport. In the background, three basic time-series traces exist: (a) light gray traces of 15-day running averages of the ADT for each year; (b) a red trace showing the  15-day running average of the ADT for the highlighted year; and (c) a dark hairline curve of the 15-day running average for the mean of all years. Individually recorded daily ADTs are show as red points.

 

The animation highlights the wide year-to-year variability in the ADT as the red highlighted year trace fluctuates across the broader gray traces for each year. Some temporary trends exist though you need to look at them quickly. In the early-mid 1950s, the ADT traces are often in the low half of the temperature band. In the 2010s, the opposite is true. Still, even with this, year-to-year variance fluctuates commonly across the gray temperature traces.

 

Each highlighted year’s chart is shown in Figure 2. Scroll down to compare. It is easier to compare still images more accurately and studiously. The animation simply shows the wide year-to-year variance.

 

Figure 1. An animated GIF showing the daily Average Temperature at Seattle-Tacoma International Airport for the years 1948 thru April 2023. Also shown is the 15-day running average and the mean daily temperature for all year, also plotted as a 15-day running average.

Figure 2. Time-Series Plots by Year

Click any chart to enlarge. Hint: Once an annual year shows up, you my be able to navigate using your keyboard’s left and right arrow keys

Seattle's (relatively) cool winter, Part 1...

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Updated on April 6, 2023



I first published this blog post on March 18. The day before,  it warmed to the 60’s for the first time since October in and around Seattle. This felt very nice to my cold bones. It has been a relatively cool winter this year if you look at the December – February traditional winter period. January was relatively mild, warm, and dry for a winter period in Seattle. But the traditional meteorological winter was bookended by a coolish December and February. Extend the bookends of the period to November and March (to-date), four of the past five months have been cooler than normal.

My charts only contained data through March 15 when I first published the post on March 18. Now, a week into April, I have data through the end of March. The updated figures below reflect this.

I’ve posted several charts below. Figure 1 is an animation. The chart is a series of traces which track the cumulative average daily temperature recorded at Seattle Tacoma International Airport (KSEA) for the seasonal period of November 1 through March 31 for the years 1949 through 2023.

NOTE: The animation is an animated GIF file programmed to loop through twice. Each loop takes roughly 25 seconds. Refresh the browser window (command+R) to restart the animation.

Figure 1. Accumulated daily average temperature traces by seasonal year, 1949-2023, at Seattle Tacoma International Airport.

A few notes about Figure 1 follow:

  1. I’ve chosen a “made-up” metric. I don’t know if this is used by the National Weather Service, meteorologists, or climatologists with any regularity. I call my metric or index the Accumulated Degree-Day (ADD). It simply takes the average daily temperature recorded at KSEA and adds it to the previous days. It is similar to Heating Degree-Days and Cooling Degree-Days which are commonly summed over a period of time but my ADD does not use a threshold temperature of 65˚F to compare against. But it does give one an idea of how cool or warm the seasonal period has been in comparison to similar periods in other years at a common location.  I suppose it’s units would be ˚F-Days.

  2. The gray traces in the background display each year from 1949-2023 for an aggregate comparison.

  3. A blue trace is highlighted for each individual year as the animation is played.

  4. Since each seasonal period plotted crosses two calendar years, I’ve chosen the latter of the two calendar years to identify each individual trace. So, the trace for the November 2022 – March 2023 period is identified as ‘2023.’

  5. The highlighted blue traces extend to March 31 for each year.

  6. An average ADD trace for all years has been added to the chart for comparison. The ADD trace is shown as a black  hairline. For the total November–March season period, the total average ADD value is 6482 ˚F-Days.

  7. Some data went uncollected by the National Weather Service on March 16-17, 2023 at Seattle Tacoma International Airport which made it difficult to calculate a daily Average Daily Temperature for these dates. The missing data is explained in the chart and the days impacted are shown in a red block on the chart.

  8. Finally, I name the animation a dog tail chart because when played back, especially at higher frame rates, these types of charts look a little like a wagging dog tail. 

 

So why an animation? Animations display motion, and motion display movement over time and, possibly, patterns. You may want to ask yourself; year-to-year is there any rhythm, pattern or trending that becomes obvious that might be difficult to discern when comparing a series of still images or charts. This is especially true when comparing 74 still images if each chart was published separately (and they are further below).

 And there does appear to be some patterns at the distal ends of this period we’re investigating. In the late 40’s and through the mid-1950’s there are a series of coldish winter seasons(7 of 9 between 1949-1957). This is well documented in old photographs of heavy snows in Seattle during this period. And one could argue that the 2010’s have a pattern of warmer than normal winters (6 of 8 between 2014 through 2021). But mostly, this animation shows the high level of variance fluctuating between cooler-normal-warmer winter cycles. The late 2010’s may have shown warmer periods, but this season is quite cooler. And there were warmer winter seasons in the 1950s.

*****

The individual annual temperature traces which make up the animation are shown below. The are shown in order of the earliest year (1949) to the most current (2023). Click on any individual image below to see more detail and to compare.

San Francisco's rainy season...

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California is in the midst of a wet and snowy winter. It sounds as if more is due this coming week. Although the copious amounts of rain and snow have wreaked havoc in some places, after several years of drought, this influx of moisture was very much needed.

 Not much in the way of charts to show here on this initial post on California. But I’ll post two charts. The data comes from the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Information (NCEI) site where you can download historical climate data records for most major weather stations in the United States.

 The data shown below is for San Francisco International Airport and ranges from July 1, 1945, through the present. I plotted these charts using a water year calendar. The water year calendar is a convention often used when plotting precipitation or snow data because on the West Coast these are typically cyclic or seasonal events. Most precipitation events fall during 4-6 months of the year beginning in October. This is true up and down the coast, though the precipitation season typically longer in higher latitudes. Therefore, the water year calendar typically runs from October 1 through the following September 30 to encapsulate the data for an entire season, which traditionally spans across two standard calendar year-dates. So, for the current water year, data is included from Oct 1, 2022, through September 30, 2023. This span is referred to the 2023 water-year (WY), as nine months of it exist in 2023.

 I disregarded the early data period from July 1, 1945, to September 30, 1945, since it only represents a small part of the 1945 WY.

***

 Figure 1 is called a dot plot and it shows the total accumulation of precipitation for each WY from 1946 to 2023. The current water-year is only partially complete. The data has been sorted showing the rainiest water seasons on top. I’ve highlighted the current 2023 WY in red. I’ve also highlighted the previous nine water seasons in blue. You can see much of the period between 2014 and 2021 were dry years though 2015-16 were close to the mean for total rain. The 2022 and 2019 seasons were also relatively close to the mean for annual precipitation. The 2017 WY, now six years past, was quite wet (for San Francisco). This year is so far close to the mean, but we are only five months into the water-year. But those are the wettest five months.

 
 

Figure 2 is a short animation (~30 sec.) which shows a cumulative precipitation trace across the water year of each year from 1946 to 2023 (through March 2, 2023). It can be seen from this chart that year-to-year, (a) the basic pattern is the same (lots of rain early followed by a long dry spell until October, and (b) the total amount of annual rainfall varies widely. And playing the small animation through there doesn’t appear to be a year-to-year trending pattern other than what was mentioned in the previous paragraph.

Helpful hint: Clicking on the gear wheel in the lower playback element allows you to slow or spead the animation playback rate.

Figure 2. Precipitation traces for San Francisco International Airport, 1946-2023 (current). Traces are displayed one at a time and follow the water year calendar starting on October 1 and ending the following September 30. The water year number takes on the year value for the January to September dates.

NYC's Central Park in Snow...

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I’ve never been to New York City though I’ve always wanted to visit and even live there. I remember when I told a colleague I worked with in the Seattle area in the mid 90s and who grew up there, he said 

“Dave, it’d eat you alive.”

 I replied, what do you mean? I’ve been around.

 “You haven’t been to New York. It eats up people like you.”

 I protested, People say this all the time. The whole city’s made up of people from other places.

 “Trust me Dave. It’d eat you alive.” End of conversation. He moved on to another task. I laughed.

 …

Anyway, it is probably too late for me to ever live there, but maybe one day I’ll visit New York. And as a daily reader of the New York Times, I sometimes come across a local story which gets me wistful about visiting or living there sometime. And whenever I do that, I think back to that brief 30 second conversation I had underneath a giant 747 being tested prior to rolling out to the flightline for further finishing work.

 And that happened tonight when I came across a story that NYC might get its first meaningful snowfall of 2023, on nearly the last day of February. I’ve always loved photographs of New York in winter. The old black and white ones, but also the color ones – especially the color ones from the late Saul Leiter. It’s hard to imagine New York getting through a winter with little to no snow. But what do I know?

 I decided to look at how much NYC sees in a typical winter. So, I went to the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information (NCEI) data portal to see where data of records were collected for New York City. Sure enough one of those locations was in Central Park with records going back to January 1, 1869. I downloaded the temperature and precipitation data for this location just for grins and looked.

  And it appears, New York City gets quite a bit of snow in Central Park certainly when compared to the city I live in, Seattle.  It looks like on average about 26 inches over the course of a typical winter falls in Central Park, though that varies widely.

 I put together several charts to see if any patterns or trends over time could be detected. Frankly, I don’t see many from this broad (and quick) overview. In fact, I was surprised how uniform the data looked whether years were grouped across decades or individual snow events were plotted across time.

 You can see what I found with this very cursory look tonight in the following five (5) figures. Click on any chart to expand.

Figure 1. Accumulated snowfall in Central Park. Group in 20 year bins or bars. Note that records only began in 1869. The first bar only includes data for 12 years. The most recent 20-year grouping began only three years ago. Therefore the last bar is quite small on this account.

Click on to expand.

Figure 2. Individual snow events grouped by years (e.g. not individual event dates).

Click on to expand.

Figure 3. Individual snow events grouped plotted on event dates.

Click on to expand.

Figure 4. Individual snow events from 2000-2023 and grouped by years (e.g. not individual event dates).

Click on to expand.

Figure 5. Individual snow events from 2001-2023 and plotted on event dates.

Click on to expand.

DATA SOURCE: NOAA NWS NCEI DAT PORTAL, https://www.ncdc.noaa.gov/cdo-web/, accessed 27 Feb 2023. Station Name: Central Park NYC.

Temperature changes at SeaTac...

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Weather data collected at Seattle Tacoma International Airport (SeaTac) is generally considered the main data of record for the Seattle metropolitan area, though there are several other sites in the region where meteorologist and climatologists gather their information. Data has been collected at SeaTac continuously since 1948, though some data is missing from the records for the period of October 1, 1996 through October 31, 2005. This missing data is generally the snow and sky cover observations. Temperature, the focus of this page, and precipitation data was collected during this period and is available from the National Oceanic and Atmospheric Administration (NOAA) and their publicly accessible National Centers for Environmental Information (NCEI) Climate Data Online (CDO) tool.

So, having temperature data from 1948 through yesterday (Feb 22, 2023), how much has the average daily temperature changed at SeaTac over the past 75 years (inclusive) if at all? And are there any trends? If so, one would assume that the temperatures have slowly warmed over the years based on all the science and reporting published over the past decade or longer.

 

An undulating plot

The National Weather Service generally reports the daily minimum temperature (Tmin) and daily maximum temperature (Tmax) at the weather stations. The average of these two values is calculated and recorded as the average daily temperature (Tavg). Often when looking at or downloading past data from a weather station, the user receives only the Tmin and Tmax values for a given date. This is common on older records. Sometimes the data includes the calculated Tavg. Regardless, knowing that Tavg is the average between the Tmin and Tmax values, the Tavg shown in the following charts has either been calculated by the NWS and reported, or this author has performed the calculation in the spreadsheet I’ve imported data from the NWS into.

If you plot the TAVG value for every date from January 1, 1948 through today as shown in Figure 1 you get an undulating or wavy curve form. Obviously, the average daily temperature of a summer day is much higher than the same on a winter day in Seattle. It is difficult to determine if there has been a fall or rise in the average daily temperature and, if so, by how much by just looking at this wave form.

Figure 1. Average daily temperatures trace from 1948 to 2022, Seattle Tacoma International Airport.

I’ve added a linear trend line in red in Figure 2. This trend line shows a modest rise in daily temperatures over the 75-year period. The average daily temperature recorded at Seattle Tacoma Airport has risen from roughly 50˚F to about 54˚F during this period, a rise of roughly 4-4.5 ˚F, or 2.2-2.5 ˚C.

Figure 2. Average daily temperatures trace from 1948 to 2022 with red trendline, Seattle Tacoma International Airport.

Annual Cumulative Degree-Day Index (ACDD)

Another method of looking at temperature changes over time and comparing these to other years over a long period is to plot the cumulative total of daily average temperatures and plot that accumulation over the course of a year. Then plot a similar curve for each year in the period of study. This is shown in Figure 3.

A simple way to check this value is described here. Say the average annual daily temperature at SeaTac is 50˚F. This is the average daily temperature over the course of a year. Some days will be cooler and some warmer. If you multiply (50F DEGREES) X (365 DAYS) you’ll get a value of  18,250 DEGREE-DAYS, or 18,250 DD. I’ll use the term Annual Cumulative Degree Day (ACDD) to indicate the degree-days accumulated over an annual period at a specific weather station.

Looking at the y-scales on Figure 3 shows that summing the average daily temperatures for each day of the year will get you to approximately 18,250 ACDD.

…

Note: My use of an index I call degree-day is not the equivalent of Heating Degree Days (HDD) or Cooling Degree Day (CDD), both indexes used in the building systems world to calculate required heating or cooling loads for designing heating and air-conditioning equipment and systems and both reported on in daily summary statistics from NOAA/NWS data repositories. But it is similar in the sense of understanding the total amount of heat received over the course of a year (as defined by the sum of the daily average temperatures) in any given year at SeaTac airport. Each gray line or trace shown in Figure 3 represents one year in the 1948-2023 period.

Figure 3. Annual Cumulative Degree Days (ACDD) plots for each year, 1948-2022, Seattle Tacoma International Airport (KSEA).

 

Comparing individual years

Figure 4 shows ACDD traces of four individual years (highlighted in red) plotted against similar ACDD traces all years. Of the four selected, 1948 and 1985 started off relatively cool through the end of June and then each had a cool autumn and early winter. The plot for 1966 shows it too started cool through spring and into early summer before a late summer surge gave it a relatively average ACDD Index value for the entire year. The plot for 2015 shows the year had an average winter and spring temperature-wise for this region, followed by a warm period extending through the remainder of the year. For the 74 full years between 1948-2022, this year (2015) was the warmest on record in Seattle.

Figure 4. Individual ACDD trace samples for four select years, Seattle Tacoma International Airport.

 

Small Multiples

Figure 5 is a series of plots grouped as a whole. Each individual plot can be clicked on to expand it. This form of data graphic is collectively called small multiple. It allows one to compare one year to other years easily. It also allows for a deeper dive into any individual year.

Detecting trends requires a little more study, but you can see that, in generally, the ten years from 1948-57 were cooler than average at SeaTac. One can also see that the 10-year period from 2013 through recently completed 2022 have been warmer than average. There was another five-year period running from 2003-07 which were warmer than average. Most of the other years hovered near average, though it is common over any ten year stretch to see back-to-back years running much cooler or warmer than average.

Figure 5. A small-multiples data graphic comparing the Annual Cumulative Degree Days (ACDD) tracing for each of 74 consecutive years at Seattle Tacoma International Airport.

Another interesting thing that shows up when plotting the Degree-Day number for each year is best seen going back to Figure 3. Notice how in the winter and early spring months, Seattle tends toward either warmer or cooler springs. Very few annual periods are “average”. Notice the “gap” that exists between early February through May. This period in the calendar tends towards two groups or strands of tracings which braid together in midsummer. By the end of each calendar period, the tracings have advanced towards split ends.

I don’t know why. I wonder if La Nina years tend towards one strand and El Nino years towards the other. Mathematicians, meteorologists, or atmospheric scientists might understand this “strand” phenomenon or “strand theory” better than I.

An Animated Chart

Finally, Figure 6 is an animated chart. It is an alternative to the small multiples chart shown in Figure 5. It displays each year’s trace above the gray traces of all years sequentially. You can imagine it looking like a dog’s tail wagging. This means of presenting data highlights the year-to-year variability. It also can show trends if over a course of periods of years, the “tail” wags high or lower. You can witness by watching the video the early years of recorded temperatures tend towards cooler days and as time marches forward, the wagging tail inches up the chart. Year-to-year the tail wags randomly (annual variability), but over time a pattern emerges of the tail drifting upwards into warmer cumulative temperatures (periodic trending).

Figure 6. ACDD traces for Seattle Tacoma International Airport, 1948-2023. The animated GIF should loop through once, probably upon initial loading of the web page. Notice that the wagging red line slowly drifts higher as the years progress. Drifting high indicates the total daily temperature load is increasing (e.g. days are getting warmer on average).

HINT: Refresh (Command+R) the web page to cycle the GIF in sequence. And remember to scroll back down to the bottom of the page after refreshing.

That’s all for this post, just some interesting new ways at looking at annual temperature profiles for data from selected NOAA / NWS collection sites. I used SeaTac Airport as an example as I live in Seattle.

Seattle Snow

They say we may receive some lowland snow next midweek but it’s highly uncertain right now. And we’re running out of winter this late in February. I always like a good snow, but by this point in the winter I’m ready to move onto spring and lighter days with a higher chance of sun.

 

So how much snow do we receive in Seattle typically. Looking at the historical records, by year, it’s not uncommon. But it’s also not a lot of snow either. In the last half of last century, we received approximately about 11.6 inches of snow per calendar year. Since 2006, it has been about 7.24 inches per calendar year.

 

A side note: I’m specifically stating calendar years as opposed to “winters” or “seasonal years”. Snow is a winter phenomenon in Seattle for the most part and winters typically cover parts of two calendar years, running from November through the following February. So, in a lot of ways, it would make sense to use winters or seasonal years to describe accumulated snow. But there is a bit of data gymnastics involved introducing new calendar constructs and developing the charts as such requires extra work often with labelling winters as, say 2022-23. So, I’m sticking with using calendar years in this discussion since the data is delivered grouped in years. Therefore, when speaking of, say, the amount of snow received in 1993, I’m speaking of the accumulated snow which most likely occurred in January, February, November, and December of 1993.

 

Seattle-Tacoma International Airport Records

 

Figure 1 shows the cumulative snow by calendar years at Seattle Tacoma International Airport (KSEA). Full year records at SeaTac begin in 1948 and continue through today. SeaTac averaged about 11.6 inches of snow from 1948 through 1995, a period of 48 years Several years saw no or very little snow, about nine in total, a little less than 19% of the years. On the other hand, 11 years saw more than 20 inches of snow, close to 23%. There were sever whopper years: 1950 saw about 60 inches of snow. This was a cold period, and the massive snow year was bookcased by snowy years, roughly 25 inches in 1949 and 27 inches in 1951. They must’ve sold a lot of snow shovels in those years.

 

Figure 1 also shows something else: missing data for an extensive 109-month period from the National Oceanic and Atmospheric Administration (NOAA) and National Weather Service (NWS) data records for the SeaTac site. The year 1996 stands out. During January and February 1996, Seattle received about X.X inches of snow. That is what is reported on their csv data extracts for 1996. However, anyone around in Seattle during December 1996 knows we received quite a bit of snow in the end of that month. However, the NOAA/NWS written records on standard forms and published in monthly PDF reports do account for that December snow (sort of – too much to go into here). So, I’ve added it in a gray bar on Figure one for 1996. Similar monthly published PDFs show approximately 5 inches of snow for 2001 unaccounted for in the standard database extracts, so I’ve added a gray bar for 2001 as well. Suffice it to say that the data between October 1996 and October 2005 should be taken with a grain of salt.

 

Figure 1. Seattle accumulated snow by year, 1948 – Present.
Click to zoom.



Other Local Weather Stations of Record

 

Seattle has several other local weather stations of record. Several of these are even located in the city proper. SeaTac is about 15 miles SSW of downtown Seattle. And some of these stations have or had records extending well before SeaTac’s, which only go back to 1948.

 

These stations and their period of records for snowfall are listed below:

 

(a)    Seattle Naval Air Station, STA USW00024244, NE Seattle, 1929-1970;

(b)    Sand Point Weather Forecast Office, STA USW00094290, NE Seattle, 1986-Present;

(c)    The University of Washington Weather Station STA USC00457478, N Seattle, 1909-1983;

(d)    Portage Bay Weather Station, STA USW00024281, N Seattle, 1894-1998; and

(e)    Boeing Field Seattle, STA USW00024234, S. Seattle, 1948-Present (but not all for snow).

 

Figure 2 shows all these stations and their respective recordings of measurable snowfall from 1894 through the present, depending on each station. So, some of that missing data in Figure 1 could be somewhat covered for by two of these stations, Portage Bay (partially) and Sand Point (for the entire period) though, in general, they are 20-25 miles from Seattle-Tacoma Airport.

Figure 2. Accumulated snow in Seattle from various recording stations, 1894 – present.
Click to zoom.

 

 Figure 3 thru Figure 8 highlight snow records for each of these stations individually and the period of records they kept respectively.

Figure 3. Seattle Tacoma International Airport, KSEA (current).
Click to zoom.

Figure 4. Seattle Naval Air Station (historical).
Click to zoom.

Figure 5. Sand Point NOAA/NWS Weather Forecast Office (current).
Click to zoom.

Figure 6. University of Washington (historical).
Click to zoom.

Figure 7. Seattle Portage Bay (historical).
Click to zoom.

Figure 8. Seattle Boeing Field (historical for snow).
Click to zoom.

on the current Pacific Northwest heat wave...

Seattle, WA

We’ve been experiencing the first heat wave of summer 2022. It has been hot all over the Pacific Northwest, including the metropolitan areas of Seattle, Portland, and also, or especially so, on the east side of the Cascades Range of Washington and Oregon.

High temperatures in the Puget Sound basin have reached into the mid-90s for the past four days. In Portland, they’ve exceeded 100˚F. In the Yakima Washington / Pendelton Oregon area east of the Cascades, the maximum temperatures have well exceeded 100˚F.

It is not uncommon for the Northwest to experience summer heat waves. How do these current heat waves compare to previous ones? The followingf time-series charts show the historical maximum daily temperatures for Seattle-Tacoma International Airport, Portland International Airport and the National Weather Service Station in Pendelton OR. I’ve also included Austin, TX as a reference point (mostly because they and much of the Southwest are also experiencing heat waves and were recently featured in a recent New York Times story along with the Pacific Northwest cities).

*****

>90˚F

The first series of charts highlight in red the typical number of days reaching or exceeding the arbitrarily chose 90˚F threshold defining a “hot” day.

Please keep in mind, for all charts shown below, we are only half way through summer 2022.

Days exceeding 90F at SeaTac Airport thru 7/29/2022.
Click to enlarge.

Days exceeding 90F at Portland International thru 7/29/2022.
Click to enlarge.

Days exceeding 90F at Pendelton, OR thru 7/29/2022.
Click to enlarge.

Days exceeding 90F at Austin, TX thru 7/29/2022.
Click to enlarge.

>100˚F

The next series of time-series charts show the same four locations, but here, the arbitrary temperature of 100˚F was chosen as the threshold for misery.

Days exceeding 100F at SeaTac Airport thru 7/29/2022.
Click to enlarge.

Days exceeding 100F at Portland Internationalt thru 7/29/2022.
Click to enlarge.

Days exceeding 100F at Pendelton, OR thru 7/29/2022.
Click to enlarge.

Days exceeding 100F at Austin, TX thru 7/29/2022.
Click to enlarge.

1993 vs. 2022

in ,

Precipitation data updated on June 30. This is the final update to this post.

The spring and early summer of 1993 was memorably wet for some of us who have called Seattle home for many years. That was the one year where I belonged in a van pool to go to and from work. each day. One dreary morning following a series of similar days in June or July, several people in the van were complaining about the weather. I can’t recall if I was one of them or just in my mind agreeing with that sentiment. I do remember a fellow vanpooler and native of Seattle chastising the complainers and suggesting this weather was normal for Seattle and telling us if we didn’t care for it, we should move. I had mixed feelings but did not completely disagree with her.  Nevertheless, that was my 10th summer in the city, and I hadn’t recalled any of the others being so miserable weatherwise.

We appear to be experiencing a similar wet and cool spring and early summer in 2022. It has brought up memories of that gray early summer of 1993. I was just speaking to a neighbor yesterday about the very thing though she hadn’t lived in Seattle in 1993. What triggered my discussion was looking at the daily rain total from the day before and seeing we had set a record for that June day. The previous record was in place since 1993.

So how does this wet spring and early summer compare to a similar period from 1993? In all honesty, I didn’t know if 1993 was our wettest spring since 1984, my first year in Seattle, let alone for the recorded history of spring weather at Seattle-Tacoma International Airport (KSEA) going back to the 1940s. But it was, for me, a memorable wet period, as is this spring, so let’s compare.

*****

 

Precipitation

Figure 1 shows the cumulative precipitation traces for calendar periods starting on October 1 of one year through September 30 of the following year recorded at SeaTac airport. This period is often considered the water year on the U.S. west coast due to our seasonal weather patterns of a strong wet season usually followed by a strong dry season. The midpoint of this multi-calendar year span is April 1 which roughly represents the transition period from wet-to-dry seasons.

 

Click to enlarge.

Figure 1. Cumulative precipitation traces for water years 1948-49 through 2021-22 at Seatlle-Tacoma International Airport (KSEA).

 

I’ve highlighted two specific annual traces: the 1992-93 and the 2021-22 water years. The first obvious thing one notices is that since October 1 of last year, this has been one of Seattle’s wettest periods. It is in the top five wettest water years so far. Ironically, two of the other 74 traces in the top five category are also recent water years, 2015-16 (#3) and 2016-17 (#1).

A second obvious feature is that in the first six months of water years 1992-93 and 2021-22 leading up to April, it has been far wetter this water year (2021-22) than 1992-93. On April 1, SeaTac Airport had already seen 35.8 inches of rain during the previous six months. On April 1, 1993, SeaTac Airport had seen 21.35 inches of rain in the previous six months. This is a difference of 14.45 inches, or more than two average November’s worth of rain. Memory plays tricks on us, but I’d argue that up to June 10, this water year has been much drearier than 1993.

There are two boxes on Figure 1. These boxes encapsulate the period between April1 – June 10 for each trace being compared, 1993 (red) and 2022 (blue). The total amount of precipitation for each period is similar, 8.94 inches and 8.83 inches respectively. The typical Seattle summer dry spell in 1993 began on or around July 30. It remains to be seen when this will occur in 2022.

Figure 2 shows the same data as Figure 1, cumulative precipitation at SeaTac for the years 1948-2022 except the it shows precipitation for the months of April through August. The values have been zeroed to March 31 for comparison purposes. Plotting this shows just how wet this spring has been. Aside from 1948, 1993 and 2022 have been two of the wettest springs recorded at SeaTac Airport. The spring of 1993 ended up being the second wettest spring recorded to-date at SeaTac. The spring of 2022 ended up being the fifth wettest.

Click to enlarge.

Figure 2. Cumulative precipitation since April 1 at Seattle-Tacoma International Airport (KSEA) for the years 1948 through 2022.

Figure 3 is a dot plot which ranks the precipitation periods from April 1 to Jun 17 and is analogous to Figure 2 showing the values for all traces between 1948 to 2022. In mid-June, the springs of 1993 and 2022 were virtually tied for 2nd place. But the latter half of June 2022 was relatively dry and the final tallly of April-June precipitation for 2022 dropped the year to fifth place on Figure 3. June 1993 ended up having .68” inches more of precipitation than June 2022.

Five of the top ten wettest years for this April-Jun period have occurred since 2010. At mid-month it was six. The 2017 period dropped to #12 on the list by the end of the month.

Figure 3. Ranking of spring-summer precipitation at Seattle-Tacoma International Airport (KSEA).

Figure 4 shows the same data displayed in Figure 3 but lays the data out in chronological order to see if any trends over time are evident. Any trending appears to be mild, but there appears to be a slight increase in spring and early summer precipitation in recent years.

Click to enlarge.

Figure 4. Cumulative spring-summer precipitation trends, April 1 through June 30, KSEA.

Temperatures

This spring has been much cooler than normal since April. How do the temperatures of Spring 2022 compare to Spring 1993?

Taking a look back at April through June daily average temperature departures from normal (30Y average for 1961-90) shows that spring and early summer was normal to slightly warmer than normal. The average April daily temperature was about 1˚F warmer than 30Y normals. The average May daily temperature was about 4˚F warmer than long-term normals. And the average June daily temperature was just about normal.

Contrast this with this spring and early summer (to-date). This year at SeaTac airport, the average daily temperature in April was -4˚F, the average May daily temperature was -4.6˚F, and the average June daily temperature (to June 17) is -2.4˚F. These are averages across an entire month. Of course there were nice days during these periods, even some warm one, but in general it has been a very cool spring and early summer. These differences are shown in Table 1.

Table 1. Monthly Average Temperature Deviation from 30-Year Normals

 

Figure 5 shows the daily temperature departures for the period from April 1 to June 30 for 1993. A 7-day average line plot is included.

Figure 6 shows the same data for this year through the most recent data point.

Click to enlarge.

Figure 5. Daily temperature departures for April through June 30, 1993.

 

Click to enlarge.

Figure 6. Daily temperature departures for April through June 18, 2022.

Looking back, spring and early summer of both 1993 and 2022 were wet and gloomy. Some people like the rain and clouds. Come spring and summer I’m ready for light, increased sun, sunrises and sunsets, and warmer temperatures to separate from our normal 6 month wet and cool winterspring climate.

But this spring and early summer has been much cooler than those of 1993. However, the current spring / early summer did get a boost of warmth in the last two weeks of June.

It’s also been notably darker in terms of solar radiation compared to recent years (I don’t have similar data for 1993).

Based on this, barring any sudden change in weather for the last two weeks of June, I’d have to say this year takes the cake for weather gloominess.

 

An update on this winter's rain and (lack of) sunshine...

in

Originally posted on or around January 22, 2022.

Charts 1 - 3a have been updated on June 19, 11 AM PDT.

Solar Radiation: How much sun are we getting…

A little background first: the Union Bay WSU Solar Radiation Station has been recording solar radiation data since late 2011. Here I’m comparing water-year (WY) cumulative solar radiation totals in Figure 1 thru Figure 3. Water-years begin on October 1 and run through September 30 of the following year. Water-year 2011-12 is incomplete for the Union Bay station – WSU only began publishing data sometime after October 1 that year. Therefore, I’ll compare cumulative traces for the 2012-13 through 2021-22 water-years, a total of ten water-year cycles

***

Weatherwise, it was a dim and grim start to winter, and this extraordinary low level of light has continued through most of winter and through spring 2022 as well. Figure 1 shows since October, we’ve received the least amount of sun over this 10-year period. The total solar radiation received so far this water year equates to roughly 93% of the mean for the previous nine years. Previously, the winter half of 2012-13 had the lowest cumulative amount of solar radiation in this limited dataset.

Figure 1. Cumulative solar radiation since October 1 at the Union Bay Solar Radiation Station. Last updated on June 19 with data thru June 18.

Click to enlarge.

Figure 2 is a Cleveland-style dot plot sorted by cumulative daily solar radiation to-date at the WSU Seattle Solar Radiation Station. This shows the 2021-22 water year cumulative solar radiation total is quite a bit less than the other years in this data set. This past February was abnormally dry (see Figure 4). On February 24, 2022, the solar radiation totals at the WSU station in Seattle for 2021-22 rose above totals for the same period in 2012-13. This elevation in ranking persisted for about three weeks. However, this cloudy spring has dropped 2021-22 back to the lowest total solar radiation levels for the past ten years.

Note too that the X-axis values start above 2000 MJ/sq. meter. Starting the x-axis at zero would show that the totals between years are not as disparate as it might seem in Figure 2. 

Figure 2. Dot plot of solar radiation accumulation since October 1 for the past 10 years.

 

Figure 3 shows the same data as Figure 2, except here in a traditional column chart ordered by year. The recent winter-spring seasons of 2017-18 through 2019-20 were quite a bit lighter (sunnier) than this past year. Perhaps this is the reason the darkness has been so noticeable the past 7-8 months.

Figure 3. Cumulative solar radiation beginning each water year on October 1.

 

Figure 3a is new to this post as of May 30. While Figures 1-3 show the total solar radiation measured on site in Seattle’s Union Bay station, Figure 3a shows the cumulative daily cloud cover or sky ratings beginning on October 1 of each trace and extending through May 29 of the following year. This data was gathered at SeaTac airport by the National Weather Service.

A sky cover rating is assigned a value of 0-10 for each day. A sky cover rating of zero indicates no clouds. A sky rating of 10 indicates 100% total cloudiness the entire day. Partially cloudy days are given a rating in whole number values in between this two extremes.

Each trace was plotted by adding up the daily rating values from October 1 of the previous year through the end of May. Steeper line traces indicate cloudier years.

Figure 3A shows that the current October 1 through mid-June period has been the cloudiest over the past 15 years.

Figure 3a. Cumulative daily sky ratings beginning each water year on October 1.

Precipitation: How wet have we been…

Figure 4 displays the cumulative precipitation for all water years at Seattle-Tacoma International Airport (KSEA) from the water years 1948-49 through 2021-22. This represents 74 years of data. SeaTac Airport is approximately 16 miles south of the Union Bay solar radiation site and is the official site of record for Seattle area weather. 

Figure 4 shows a rainy start to our winter season. Seattle had a total of 26.7 inches of rain in the period between October 1, 2021, and January 14, 2022. This placed this year’s early winter rainy season as third rainiest since the late 1940s, following the 1955-56 and 2006-07 early winters. Most of this rain fell after October 19, about 25.5 inches. The October 19 through January 14 period represented 86 days. Therefore, the precipitation rate for this period was 0.30 inches per day. The often sunny, pleasant days we usually see in October seemed elusive this past year and this chart shows why. 

It was relatively dry most days from January 13 through February 26, a period of 43 days. A total of 1.24 inches of precipitation fell during this period, a rate of approximately 0.029 inches per day, or 1/10 of the rate for the previous three months.

Record rains hit starting on February 27 and continued through the next day. A total of 7.9 inches of rain fell from February 27 through March 31. This represented a daily precipitation rate of 0.23 inches per day – a high rate, but not quite the rate of the first three months of the wet season. 

It has been a relatively wet spring as well including through May, especially in late May and early June. This water year’s ranking has moved from the 12th most rainy at SeaTac Airport to the 6th most in just the past few weeks.

Figure 4. Cumulative rain totals at Seattle-Tacoma International Airport for yearly periods beginning on October 1.
Click to enlarge.

This wet spring has moved the SeaTac total precipitation level to the #9 position in the rankings since the late 1940s as shown in supplemental Figure 4a. However, two recent winters have been wetter than this year, 2015-16 (#3) and 2016-17 (#1).

Precipitation traces for past water-years by decade at SeaTac can be found here.

Figure 4a. Rankings of winter precipitation for water years beginning on October 1 at Seattle-Tacoma International Airport (KSEA).

 

Winter / Spring Temperatures: How cold have we been…

It was a chilly winter in 2022, chillier than normal. March ended with a spate of warmer days which took some of the chill off the winter temperature pattern. But April and May have seen a return to the chilly weather. On average it’s been roughly 5-10 F cooler than normal most days since the end of March.

Figure 5 shows the daily average temperature departures from 30Y climate norms at SeaTac Airport. I’ve included data for December 2021 since December is the meteorological start to winter.

Figure 5. Daily average temperature departures from normal, SeaTac Airport. Click to enlarge.

 

Figure 6 takes the same data shown in the previous figure and extends it back to October 1, the start of the 2021-22 water year. It has been cooler than normal for most of the start of the rainy season this year. However, the whole of November may have been slightly milder than most Novembers. You can see the warm period quite clearly in late November when a series of atmospheric rivers blew in across the Pacific and dumper huge amounts of rainfall in Washington and southern British Columbia flooding large parts of the areas around the Fraser and Nooksack rivers.

Figure 6. Daily average temperature departures from normal at SeaTac Airport starting October 1, 2021.
Click to expand.

 

Stepping back further in time, Figure 7 shows the average daily temperature departures from climatic norms from October 1, 2000, to the present. This chart also highlights days where the average daily temperature exceeded or fell short of the daily normal for a given date by 20˚F or more. A particular warmish period when warmer-than-normal days outnumbered cooler-than-normal days began sometime around 2012-13 and continued for about five years before moderating somewhat. With the most recent coolish last few months, that trend appears to have moderated even more. It remains to be seen if this will continue.

Figure 7. Average daily temperature departures from 30Y climatic normals, SeaTac Airport, 2000-2022.
Click to expand.

Sources:

[1] Precipitation and temperature data: NOAA / NWS Climate, Observed Weather: https://www.weather.gov/wrh/climate?wfo=sew
[2] Solar radiation data: Washington State University AgWeatherNet Current Conditions Map: http://weather.wsu.edu

Data Graphics:

All dataworks created using DataGraph 5.0, Visual Data Tools, Inc. for macOS.

on balance...

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The Pacific Northwest experienced an extraordinary heat wave last summer. The heart of the heat wave lasted three days; however the days leading up to and trailing those three core days were also very warm. The average daily temperatures at Seattle-Tacoma International Airport (KSEA) for June 26-28, 2021 exceeded normal daily temperatures by 22˚F, 25˚F and 23˚F respectively for those three hottest days. The actual daily maximum temperatures for this period were 102˚F, 104˚F and 108˚F. This event was covered prominently in local, national and international news and continued to be for months afterwards.

However, in the Seattle area (just a relatively small area impacted by the heat wave), aside from another relatively hot period towards the end of July, the rest of summer was very pleasant. In fact, by mid-August, mean daily temperatures began to fall short of normal August temperatures. September and October, often pleasant and warm months in the Seattle area, were both cloudier and cooler than normal. And aside from a warmish period in late November 2021 when atmospheric rivers from the tropics swept in warm winds and huge rainfalls causing massive flooding in parts of the Pacific Northwest and British Columbia, the past winter and spring has continued to be rather cool and showery.

It has been 317 days since the heat wave broke. I plotted the average daily temperature departures from 30Y-normal mean daily temperatures at SeaTac (KSEA) for these 317 days. I also included the 317 days prior to the heat wave as well in the plot. Including the 3-day heatwave, a total of 637 days are plotted in Figure 1.

Click to enlarge.

Figure 1. Daily departure from normal temperature trends, August 13, 2020 through May 11, 2022.

 

The temperature trends in Figure 1 appear to show a certain seesaw symmetry around the brief heat wave. The linear fit curve (for just the shown data range) crosses the X-axis about a month after the heat wave broke. The pre-heatwave period had 183 warmer-than-normal days at an average of about 4.8˚F warmer per day. The post-heatwave period had 183 cooler-than-normal days. at an average of roughly 4.3˚F cooler per day. One key difference is that the pre-heatwave period had almost twice the number of neutral days than the post-heatwave period.

I suspect this symmetry is likely more coincidental than anything else and likely had little to do with the large heat dome that set in place last summer and caused temperatures to peak. It likely has much more to do with the strong La Nina pattern we’ve been in this winter and spring. Atmospheric systems are huge and are likely slow to change. Though 637 days sounds like a lot of data points, it’s less than two years and it’s perfectly normal for warm and cool trends to settle in for long periods. But just looking at the chart and thinking in medical terms, it appears the “fever broke” after the extraordinary heat wave.

The truth is, if you look at the longer record and extend this chart backwards for many years as shown in Figure 2 the cooling trend appears much more moderate. This chart goes back to 2000. And a 365-day smoothing curve of average daily temperature departures shows a much more gradual cooling trend (after an extended warming trend reaching back to at least early 2017).

Click to enlarge.

Figure 2. Daily temperature trends, Seattle-Tacoma International Airport, 2000-2022.

Washington National, January 1982

Supplemental information to a story in The Washington Post on the 40th anniversary of the 1982 air disaster of an Air Florida aircraft crashing into the Potomac River in Washington D.C. during an afternoon snow storm.

Figure 1

Figure 2.

Data Source

NOAA National Centers for Environmental Information Database

https://www.ncdc.noaa.gov/cdo-web/search;jsessionid=1F85704FD4D14094B2BAA7E69B4CF8F9

Weather Station: USW00013743

A dark start to winter...

in , ,

Updated: December 29, 2021, 6:30 AM PDT

Figure 1 updated with most recent data.
Figure 2 has been updated. Note that the minimal value on the x-axis begins at 320 MJ/sq.meter. Figure 3 has been updated. Note that the minimal value on the y-axis begins at 300 MJ/sq.meter. Figure 4 has been added to show the daily temperature departure for 2021 to date.
Figure 5 has been added to show the daily temperature departures since 2000 at SeaTac airport.

If it seems like it has been pretty dark and rainy this autumn and early winter in Seattle you would be right. I’ve already posted about the high level of precipitation we’ve seen in Seattle and the extended Western Washington area since the start of the 2021-22 water year on October 1. The areas surrounding Bellingham and Vancouver B.C. have been especially hit hard with rain.

But it has seemed very dark in recent months as well, and by dark, I mean low levels of light. Of course all this rain comes in leaden skies. And looking at solar radiation data collected near Husky Stadium and Union Bay (47.66, 122.29) in Seattle confirms this has been the darkest start to winter in the past ten. Washington State University operates a solar collection station in this area and exposes the processed data to the public on their web site. A link to this data is listed at the bottom of this page.

Figure 1 shows the cumulative daily totals of solar radiation (MJ/sq.meter) from October 1 through December 31 for 2013 through the present year. At this writing we are only partly through December this year. But it is clear that this year has been quite dark compared to the other years. Lower traces indicate lower levels of solar energy measured. This year’s trend line is the heavy line. The Seattle area began with a normal October, but around mid-month the rate of cumulative solar energy slowed considerably.

I choose a start date of October 1 since this is the traditional start date of the Northwest water year, the time of year when storms begin to blow off the Pacific with increasing frequency. This date provides a good start or “zero point” for the start of the wet season.

Figure 1. Solar radiation measured at Seattle’s Union Bay data collection site.

 

Figure 2 was added on December 15. This figure is a dot plot showing the total cumulative solar radiation received at WSU’s Solar Station in Seattle. The dot plot is sorted by total solar daily accumulation for the stated period.

This year’s accumulation of sunlight since the start of October is marked by the black dot in the lower left corner. It is substantially lower than the sunnier years at winter’s start. It is substantially lower than the median value for all years shown. The median value is shown by the dash vertical line.

Looking at the years’ positions on the y-axis, there does not appear to be any pattern to the order for the very limited number of years for which data exists.

Figure 2. Dot plot showing cumulative solar radiation sorted by year.

 

Figure 3 essentially shows the same data as Figure 2. In this bar chart, the years are in order along the X-axis (horizontal). It’s a little easier for the mind to see the lack of any pattern by time in this small sample size – we are more often used to seeing date-format data along the horizontal axis, increasing in time from left-to-right. It’s also clear to see how low sun levels have been this Oct-Dec period compared to recent years. All years show data through the date in the title.

Figure 3. Cumulative Solar Radiation, Oct 1 through December, sorted by years, 2012-21.

 

In addition to lower solar radiation levels in Seattle since roughly mid-October, it has been a relatively cool autumn and start to winter. September had mare days that were cooler than normal than days warmer than normal. Same for October. November was pretty mild with a series of atmospheric rivers coming in from the tropics. Those brought warmish, moist air with them. And plenty of flooding in parts of the Pacific Northwest and southern British Columbia. December has been relatively colder than most recent Decembers. This can be seen in the large number of days where average daily temperatures have been below 30Y climatic normals.

Figure 4 shows the daily temperature departures for 2021 up through the most recent date.

Figure 4. Daily Average Temperature: Departure from 30Y normals. Click to enlarge.

Figure 5 shows similar data as Figure 4, daily average temperature departures from 30-year normals going back to January 1, 2000. It appears in recent years, the trend is back towards the climatic normals of the past 30 years after a warming period for several years in the mid-2020s.

The 30Y normal daily average temperature reference values for all years except 2021 are based on the years 1981-2010. In 2021, the comparison is against the normal daily temperature range from 1991-2020. It’s possible that the 1991-2020 30Y normals cycles have increased from the previous 1981-2010 30Y cycle and this year’s lower trend is simply reflecting that change - departures may be comparing against a high reference line..

This data is for the National Weather Services’s Seattle-Tacoma International Airport site.

Figure 5. Daily normal temperature departures for SeaTac airport from 2000 to present. Click to enlarge.

SOURCE DATA
Washington State University AgWeatherNet: http://weather.wsu.edu
NOAA/NWS Climate Data Seattle/Tacoma https://www.weather.gov/wrh/climate?wfo=sew
FULL DISCLOSURE
I am not a meteorologist, climate scientist, data scientist, geologist nor hydrologist. I am simply a (retired) engineer who has some familiarity with numbers, basic statistics and probability statistics who enjoys looking at readily available public data and trying to make sense of things. I enjoy building data visualizations from data I find much like others enjoy working daily crosswords or sudoku puzzles. Local weather, climate and hydrology science are complex subjects. Take what you read and find here with this in context.
 

Rearview Mirror: November 2021

in ,

The November 2021 Temperature Departure chart is shown in Figure 1.

In general we’ve had a somewhat cooler than normal autumn, but November was slightly warmer than normal, with a monthly temperature average of 47.9˚F, about 1.4F above normal. Several strong atmospheric storms hitting the Puget Sound region from the tropics to the southwest passed through the city mid-month and towards the end. These storms brought warm, tropic air with them with plenty of moisture. These multi-day temperature spikes can be seen in Figure 1.

Figure 1. Daily temperature departs from 30Y climatic normals through November 30.

Click image to enlarge.

Normally, I’d add a chart showing new monthly precipitation totals, but I’ve reported these in several previous blog posts over the past few days. There’s no sense repeating that information here. Click to Prev button at the bottom of this post to navigate to those posts if you’re interested.

DATA SOURCE
NOAA / National Weather Service Local Climate: https://www.weather.gov/wrh/climate?wfo=sew

On Seattle-Tacoma precipitation traces, Part 1...

in ,

This post will display a history of water-year cumulative precipitation traces from Seattle-Tacoma International Airport. A water-year defined here crosses parts of two calendar years and runs from October 1 of year 1 to September 30 of year 2. For instance, the water year 1972-73 runs from October 1, 1972 to September 30, 1973.

The reasons for using water years is simple. The Puget Sound area can be thought of at a higher level of having two basic seasons: a wet season running from October through March when most of our annual precipitation falls, and a dry season which runs from April through September.

Figure 1 is the baseline chart and shows cumulative precipitation traces at Seattle-Tacoma International Airport from 1948-49 through the start of the 2021-22 water years. The data comes from the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS). These data extracts normally list daily precipitation totals. I’ve summed these values up for each water year.

Figure 1 also shows some other graphic elements. In the lower left corner, a blue box appears highlighting the first two months of the water year. The reasons for this box is simple. We just completed this two month period for the 2021-22 water year. It was a very wet period in Seattle and the Pacific Northwest in general. I placed this box on the chart to allow for easy comparisons of this year’s heavy rains with other years’. As you will see later, this year’s start to the water year was one of the wettest since the late 1940s, but not the wettest.

Two other key elements added to the baseline chart include a 30Y climatic normal reference line for two climate periods. These periods are the 1981-2010 climate normals for precipitation at SeaTac and the 1991-2020 climate normals. These are standard 30 year daily averages published by NOAA/NWS. As you can see, the more recent 30 year normals show a higher level of daily precipitation than the 1981-2010 period, adding up to a little more than 2 inches additional rain per year at SeaTac.

One final observation is that precipitation rates vary widely within each annual trace and between the final annual totals of all traces. The wettest years had roughly 51-52 inches of rain over the course of 12 months, the driest years had less than 25 inches over a 12 month period.

Figure 1. Precipitation traces for each water year from 1948-49 through 2021-22 (to the current date).

I only have two full water-year traces from the 1940s. These are shown in Figure 2. Interestingly, both years began the first two months with similar trace patterns. After about mid-December, each trace line diverges for the remainder of the year, one towards the high side of annual precipitation totals, the other towards the low end. At a glance, and without performing the statistical calculation, it appears as if the final year-end totals span the first or second standard deviations of all year-end precipitation totals.

Figure 2. Cumulative precipitation traces for available SeaTac data in the 1940s.

The 1950 traces in Figure 3 show that the start of the wet seasons varied widely during that decade and that, in general, it was a decade when annual precipitation totals hewed towards current climate normals or were generally higher in precipitation.

Figure 3. Cumulative precipitation traces for available SeaTac data in the 1950s. Tracks in the 1950s trend towards years with greater precipitation.

Figure 4 shows the 1960s traces were distributed about evenly above and below current climate normals and tend to not have extraordinarily wet or dry annual totals.

Figure 4. Cumulative precipitation traces for available SeaTac data in the 1960s. Precipitation tracks in the 1960s hover more closely to 30Y climate tracks about evenly above and below these normals.

Figure 5 shows the 1970s traces. It appears most years in the 1970s began relatively dry. From December on, the annual totals varied widely. This might be the decade with the widest and most uniform spread between very wet and very dry years.

Figure 7. Cumulative precipitation traces for available SeaTac data in the 1970s. Half the 1970’s traces hover close to the climate normals. The remaining traces are biased towards both extremes: wet years or dry years.

Most annual traces in the 1980s hewed close to current climatic normals, though biased on the dry side of those normal. Figure 6 shows a few years were substantially drier by year’s end.

Figure 8. Cumulative precipitation traces for available SeaTac data in the 1980s. The 1980’s traces tend to be biased towards drier than climatic normal years. The also tended to start the years tightly around climatic normals.

The precipitation traces for the 1990s, illustrated in Figure 7, appear to be evenly distributed around the current climatic norms much like the 1960s traces. However, the 1990s traces are dispersed more widely, reaching towards the wettest and driest annual limits for all of the traces. Collectively, the 1960s and 1990s traces in the early part of each year exhibit pretty similar patterns, hewing close to current climatic normals except for one particularly dry autumn in each subset.

Figure 7. Most years began with fairly normal precipitation levels except for perhaps two. By years’ end, the traces were evenly, if broadly, distributed around current climatic normals. This decade saw the most varied ditribution of precipitation totals at the end of the year.

Figure 8 shows the traces for the 2000s. The year end precipitation totals tend not to trend towards extremes and are maybe slightly biased towards drier years. But the start of the wet season (October-November) varied widely for this period. Some of the wettest starts to winter began here and even a drier one as well. Many other decade charts showed must tighter distributions around the current climatic normal lines.

Figure 8. Cumulative precipitation traces for available SeaTac data in the 2000s. Traces appear to show wide autumn variations then trending towards an even distribution around climatic norms at years end.

The 2010s year-end precipitation totals trended towards wetter years. These traces are illustrated in Figure 9. This decade, along with four very wet years in the 1990s might explain why climatic normal curves jumped for the 1991-2020 period. The starts of the water-years in the 2010s also trended towards wetter Octobers and Novembers in most years. The 2010s also hosted some very long, dry summer periods.

Figure 9. Cumulative precipitation traces for available SeaTac data in the 2010s. Trends show wide variation in the autumn months and a bias towards wetter year-end totals.

The 2020s are too new to host many precipitation traces. In fact, we’ve only completed one year of this decade and only started a second water-year trace. In Figure 10, the single, complete year hewed very closely towards climatic normals after a wet late winter and early spring. This was followed by a very long and dry late spring and summer period.

The start of its second year has been very wet.

Figure 10. Cumulative precipitation traces for available SeaTac data in the 2020s. The 2020-21 year followed current climatic normals closely. The current year has begun very wet.

Conclusions and Notes

Looking at the traces, I’m not really sure I’m seeing any convincing trending decade-to-decade. The only long-term trending I’m seeing is between the climatic normal lines where wetter late winters and early springs seem to be trending. But these two lines are drawn from only two sets of averaged daily data. I would be more convinced if other 30-year climatic normal periods were plotted here, say for the 1951-80, 1961-90 and 1971-2000 periods. I may do so for Part 2 of this blog entry.

Sure, the 2010s appear to be quite wet. But that decade was preceded by several decades of precipitation traces distributed pretty evenly around the climatic normal curves. And the 1970s and 80s trended towards the drier side of the distributions. They, in turn, were preceded by a pretty normal series of plots in the 1960s and, again, wetter trending in the 1950s. Who knows what’s in store for this decade. It’s way too soon to tell.

And this second point brings up an important third one: the arbitrary definition of a decadal dataset. I used the common calendar dates to group my datasets and plots, begin each set of plots with a year ending in a zero and enclosing the dataset with a trace from a year ending in a nine. But I could have just as easily chosen decadal sets such as 1974-83; 1984-93; 1994-03; etc. Who knows what trends we might detect then?

***

I think I’ll plot a Part 2 post to this topic. It will take some time due to the need to post-process the data. I have data preceding the 1950s going all the way back to the 1890s for the Seattle area. However, this data is typically from other locations, including the Portage Bay area in North Seattle and Boeing Field in South Seattle. These areas are 10-20 miles from Seattle-Tacoma Airport.

I think too, I will plot climatic normal curves for other 30-year periods to see if any trending is detected within those longer-term datasets.

That’s all for now.

Pacific Northwest autumn 2021 rains...

in ,

Updated: December 1, 8:00 AM PDT

This post has been substantially updated on Wednesday morning, December 1.

  • One chart has been replaced by five (5) charts with further explanations and comparisons.

  • Data has been updated to include precipitation totals through November.

  • Vancouver’s annual precipitation totals have been corrected to state the 1981-2010 30Y average for Vancouver International Airport as 46.8 inches. In the original blog post, it listed the 30Y annual average for Vancouver’s Harbour climate station site. This site has a considerably higher average than the airport.

  • On Figures 2 thru 5 a new reference line has been added for the recently released 1991-2020 climate precipitation normals for Seattle-Tacoma International Airport. As you can see, the most recent 30Y normal reference line show it has been wetter in Seattle in recent decades.

All figures subject to correction and, if required, will be noted.

Click on any chart to expand.

The Pacific Northwest almost always experiences rainy Novembers. This year precipitation rates have been high, if not extraordinary. Heavy flooding occurred in northwest Washington and southern British Columbia in mid-month. Since then, rains have levelled off a little but are accumulating nonetheless. This weekend we are expecting more rain in Seattle. Bellingham and Vancouver will likely see even more.

So how does this start to the rainy season compare to others? The charts below shows several comparisons:

Figure 1 sets the table for this data and subsequent charts and shows the cumulative precipitation traces at Seattle-Tacoma International Airport (SeaTac) for the water years 2002-03 through 2021-22 (to-date). Water years are calendar constructs which help visualize the wet season precipitation totals vs. dry season in the Pacific Northwest. Typically a water year begins on October 1 of a given year and runs through September 30 of the following year. Looking at cumulative precipitation traces in this time frame usually show distinct wet / dry season patterns.

Figure 1 also highlights the 2006-07 water year. Seattle experienced an extraordinary level of precipitation in November 2006, the most of any traces for SeaTac in this set of data. The blue box in this and subsequent charts highlights for comparison the level of precipitation versus the first 61 days of the 2006-07 water year and corresponds to the months of October and November.

Figure 1. Record-setting Oct-Nov at Seattle-Tacoma International Airport, 2006.

Figure 2 highlights the amount of precipitation SeaTac has received so far this water year. As mentioned in the updates, the 1991-2020 climate normal reference trace has been added to this and subsequent charts.

Figure 2. Seattle-Tacoma International Airport cumulative precipitation for Oct-Nov, 2021.

Figure 3 shows the accumulated precipitation trace (red) for Bellingham International Airport. Bellingham is approximately 90 miles north of Seattle and was hard hit by the mid-November flood.

Figure 3. Oct-Nov accumulated precipitation for Bellingham International Airport compared to SeaTac totals, 2021.

Figure 4 shows the accumulated precipitation (blue) trace for the Vancouver International Airport weather station. Vancouver is approximately 50 miles north of Bellingham. In the last few days of November, Bellingham has seen more rain than at Vancouver’s airport climate station.

Figure 4. Vancouver International Airport and Bellingham International Airport cumulative precipitation totals for Oct-Nov, 2021.

Figure 5 compares the accumulated precipitation at Vancouver’s airport in it southwest corner to its Harbour climate station located near Stanley Park. This station is near the core of downtown Vancouver. The station at Vancouver’s Harbour has seen far more rain this water year than at the airport.

Figure 5. Vancouver International Airport and Vancouver Harbour precipitation totals for Oct-Nov, 2021.

Preliminary observations:

  • Seattle and Bellingham have had similar precipitation totals through this period though they arrived at this point in slightly different manners. Over the past 48-96 hours Bellingham has received substantially more rain than Seattle.

  • The precipitation totals at Vancouver International Airport for October-November are similar to Bellingham’s.

  • The precipitation totals witnessed at Vancouver’s Harbour climate station far exceed totals in the Washington cities and at Vancouver’s airport. I don’t know if this level of disparity is typical. However, with regards to the comparison with Washington cities, Vancouver typically sees approximately 46.8 inches of rain per year; Seattle sees – with the new 30 year climate normals – about 39.3 inches of precipitation per year.

Sources:

[1] NOAA / NWS Climate\Observed Weather: https://www.weather.gov/wrh/climate?wfo=sew for SeaTac and Bellingham data.

[2] Canadian Government Historical Climate Data: https://climate.weather.gc.ca/historical_data/search_historic_data_e.html

 

ABOUT THIS BLOG / FULL DISCLOSURE

I am not a meteorologist, climate scientist, data scientist, geologist nor hydrologist. I am simply a (retired) engineer who has some familiarity with numbers, basic statistics and probability statistics who enjoys looking at readily available public data and trying to make sense of things. I enjoy building data visualizations from data I find much like others enjoy working daily crosswords or sudoku puzzles. Local weather, climate and hydrology science are complex subjects. Take what you read and find here with this in context.

Nooksack flooding, 2021

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It has been a rainy start to autumn in Seattle in 2021. Figure 1 shows a plot of the cumulative precipitation at Seattle-Tacoma (SeaTac) International Airport since the beginning of the 2021-22 water year. Water years in Seattle are date-based construct starting on October 1st and running through September 30 of the following year. These are often used when discussing precipitation events since the west coast of North America and specifically the Pacific Northwest of the U.S. can often be divided into a wet season (late fall through early spring) and a dry season (late summer through early autumn).

The plot below highlights the current water year precipitation accumulation but also includes similar traces for all water years at SeaTac since 2002-03. Compared to recent years, this year – just starting – is quite wet.

Click on any figure below to expand it.

Figure 1. Cumulative rainfall, Seattle-Tacoma International Airport, current water-year. Also show are traces from previous water-years, 2002-03 through 2020-21.

Figure 2 highlights on of the past two decades wettest start to winter for comparison purposes. This was the 2006-07 water year. November of that month registered close to 15.6 inches of rain.

It has not been nearly as wet this November with roughly 8 inches of precipitation so far, about two-thirds of the way through the month. But if you compare the two lines on the chart, Seattle has had a much wetter October in 2021, so the total precipitation to date is similar between the 2006-07 and 2021-22 water years.

Figure 2. Comparison of current water-year cumulative precipitation with the 2006-07 water-year trace.

Figure 3 shows an additional year with a very wet start, the water year of 2016-17. This wet year reached approximately the same total precipitation as 2006-07 for the two-month, OCT-NOV period, a value of roughly 17 inches total. Normal precipitation totals for Seattle-Tacoma for that two-month period is typically about 10 inches. In the 2021-22 water year, with November still one week away from completing, we have seen 13.66 inches of rain since October 1.

Figure 3. Current water-year cumulative precipitation at SeaTac Airport with the 2006-07 and 2016-17 precipitation traces highlighted.

Figure 4 is a little bit different than the first three charts. I often track weather out of SeaTac. But I rarely step out to track weather in other areas, even areas not so far from home. Last week, in the northwestern part of Washington State and in southern British Columbia, large volumes of rain, arriving in the form of an atmospheric river, flooded the region. The flooding was quite severe, impacting many homes and farms, and creating great havoc and damage to many roads and highways. Many people were stranded by the flood waters and needed to be rescued. Several people have died in this storm.

I’ve plotted the cumulative rain totals recorded at the Bellingham International Airport since October 1. Bellingham is a mid-sized city approximately 90 miles north of Seattle, and very close to the flood zone. In fact, parts of Bellingham were under water for several days after these recent storms.

In Figure 4 you can see the total amount of rain falling in Bellingham has been very similar to that of SeaTac. Until about November 8, Belingham was tracking about 2 inches less in rain compared to Seattle from the start of the new water year. But from November 9-15, Bellingham received approximately 6.6 inches of rain, about 2.5 inches more than Seattle in that week.

I regularly keep track of these precipitation vs. time charts for Seattle-Tacoma International Airport. Adding Bellingham totals to the chart was only for comparison reasons only. Inthe most recent storm, it’s likely the Seattle area was protected to some extent by the Olympic Range to our west. Bellingham is not as well protected.

Figure 4. For comparison purposes, this figure shows the current year precipitation trace at Bellingham International Airport with the past 20 year traces at Seattle-Tacoma International Airport. Note: Bellingham is approximately 90 miles north of Seattle.

The United States Geological Service (USGS) keeps track of river flows across the country. Some of this data is exposed to the public. Two gaging stations the USGS keeps near the Whatcom County flood zone are the stations on the Nooksack River at Everson and at Ferndale. Everson is about 12 miles ENE of Ferndale as the crow flies. The monitoring station at Ferndale appears to have records published over a much longer time period, so this data will be used for comparison purposes. The Nooksack river flows from the Mount Shuksan and Mount Baker watersheds. On the other hand, the station at Everson, also on the Nooksack, is much closer to the Sumas River which presumedly caused much of the flooding in the Fraser River Valley in British Columbia.

The Nooksack river major flood stage at Ferndale is 23 feet. The Nooksack river crested above this stage on November 16 (Figure 5, lower chart) at 23.7 feet.

Figure 5. Nooksack River stage charts from Ferndale WA from November 15-22, 2021. Chart is courtesy of the USGS.

Figure 6. Nooksack River stage charts from Everson, WA from November 15-22, 2021. Chart is courtesy of USGS.

Table 1 shows the values of historic stage crests on the Nooksack River at Ferndale for crests above 12.25 feet. The values were copied into this table from the source listed below the table. I entered the most recent value (blue entry) for 11/16/2021 since the web site values had not been updated yet.

This November’s flood stage ranked with the top twenty events at this gage site.

Table 1. Record gage levels for the Nooksack River at Ferndale WA. Source: NOAA / NWS.

Source link

Figure 7 is a dot plot of the historic crests on the Nooksack River at Ferndale ordered from great to least. Color coding indicate the decade of the event.

Figure 7. A dot plot showing the historic flood stages on the Nooksack River at Ferndale WA.

Figure 8 is a series of dot plots of the same data as shown in Figure 7. The data has been grouped in decades to see if any trends can be found due to time. Decades are an arbitrary grouping. But often we group data in years, decades and centuries to see patterns time-related data, so I used decade groupings here.

I don’t see any obvious patterns in the decade groupings other than some early decades had fewer but more severe events and more recent decades had more events but events which appeared less severe in terms of gage height. Of course, the 2020s are just beginning. It is possible, I suppose, that less severe events in the early 1910-30 period were not as diligently recorded. And it might also be true that less severe events in more recent decades may do more damage due to greater populations and development in the area. These are just guesses.

Click on any chart to enlarge. You can use left and right arrow keys to navigate between decadal sets of data.

Figure 8. A series of dot plots of ordered historic crests on the Nooksack River at Ferndale WA grouped by decade. Click to enlarge.

 

Figure 9 shows the data in Table 1 in a time-series chart format (which is basically a horizontal dot plot sorted by time rather than rank).

Again, strong patterns are hard to find, other than the first half of the timeline tends to show greater historic crests. I don’t know the reasons for this. Maybe storms were stronger in the past. Perhaps physical river control methods have been installed over the years to help regulate flow. Since 1965, it is difficult to discern any distinct pattern to the historic crests other than every 15 years or so, a moderate-high or major flood event occurs.

Figure 9. Time-series chart of historic crests on the Nooksack River at Ferndale, WA.

 

Finally, it should be noted that parts of northern Whatcom County and much of the Fraser River Valley were flooded and not on the Nooksack River, but rather on the Sumas River. The Nooksack flows west to Puget Sound from the North Cascades. The Sumas River flows north into Canada. The U.S. border town of Sumas was severely flooded the past week as was much of the nearby Fraser River Valley.

There is a point near Everson, WA where the Nooksack river runs very close to the Sumas River. The surrounding land is generally flat. Literature suggests past overflows of the Nooksack have flowed into the Sumas near Everson. The distance appears to be less than two miles at the closest points

I suspect this is what happened this past week. I’m not a hydrologist, so I don’t really know. But with all of the water falling last week and the weeks leading up to this event, it seems likely. River gage data on the Sumas for the public was difficult to find.

Figure 10 shows the proximity of the two rivers. Directionally, north extends into the top background in the image.

Figure 10. Proximity of the Nooksak River to the Sumas River. Image courtesy of Google Earth. Click to enlarge.

The Bellingham Herald is the paper of record in Whatcom County and Northwest Washington. The paper has excellent coverage of the recent floods and photo essays of past historic floods. Flooding in this region is not uncommon. It is a flat landscape at the base of very steep mountains subject to heavy winter rains and runoffs.

 

FULL DISCLOSURE

I am not a meteorologist, climate scientist, data scientist, geologist nor hydrologist. I am simply a (retired) engineer who has some familiarity with numbers, basic statistics and probability statistics who enjoys looking at readily available public data and trying to make sense of things. I enjoy building data visualizations from data I find much like others enjoy working daily crosswords or sudoku puzzles. Local weather, climate and hydrology science are complex subjects. Take what you read and find here with this in context.

"Summer" Precip at Seattle-Tacoma International

Summer can mean a lot of things. The dates to anyone can be somewhat arbitrary. Some people consider summer runs from the Saturday of the U.S. Memorial Day weekend through the Monday of Labor Day weekend. That’s been my traditional choice.

Others consider the calendar months of June-July-August. Still others may select June 21 through September 21. And in the Puget Sound region, many believe summer does not begin until roughly July 5 and runs through the end of September.

This summer, 2021, has been a dry one in Seattle. How dry has it been? Choosing the “summer months” of June-July-August, the chart below shows this is not the driest summer we’ve had at SeaTac since 1949, but it has certainly been dryer than the median summer is for precipitation. The median is about 3.11 inches of rain for that period from 1949-2021. Seattle had 2.02 inches of rain during that period in 2021. SeaTac had as much as 8.43 inches of rain in the summer of 1968. And it had as low as 0.84 inches in 1987.

What stands out about 2021 is that of the two inches of rain it received in the summer of 2021, almost all of it fell in the 4-day period from June 10-13. Since June 14, we were pretty dry for the remainder of the summer.

Figure 1. Summer precipitation time-series chart for Seattle-Tacoma International Airport, 1949-2021.
Click on chart to zoom in.

 

A dot plot showing the summer precipitation at SeaTac since 1949 and ranked in order from wettest to driest is shown in Figure 2. The value for the current year, 2021 is highlighted in red text.

Click on the image to expand it.

Summer Precipitation ranked, wettest to driest.

Click on image to enlarge.

Seattle-Tacoma's hottest days...

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Charts updated: August 14, 2021, 8:00 AM PDT

We’re in the midst of a second heat wave this summer. Our first, in late June, was a scorcher, setting an all time high record for Seattle at Seattle-Tacoma International Airport (SeaTac) on June 28 of 108˚F. We had three days of temperatures greater than 100˚F in that period.

This week’s heat wave (and now smoke wave) isn’t quite as hot. But temperatures are back above 90˚F. This is a arbitrary threshold, but it is as good as a threshold for a heat wave as any, so we’ll use it.

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So, how often does it get to 90˚F or greater in Seattle, The Land of No Air-Conditioning? It is pretty much a summer only phenomena and is not as uncommon as one might think. Figure 1 shows maximum daily temperatures at SeaTac going back to January 1, 1949. Most summers record at least one or more days of 90˚F, but not all. This year is about average. We’re not through with this current heat wave, so one can expect to see another red dot for today and probably tomorrow. I can’t predict beyond tomorrow whether we will see anymore really hot days. But it is becoming late summer. Days are gradually getting shorter and nights longer and we only have about 2-½ more weeks until September begins.

Ironically, in the six or so weeks between this summer’s heat waves, we’ve had one of the most pleasant, if a bit dry, summers in my 38 years here.

Click on Figure 1 to expand it. I’ll update it over the next few days as this heat wave plays itself out.

Figure 1

Figure 2 was added on August 14. It shows a zoom-in of Figure 1 for the last 10 years. Most summers in Seattle experience at least a few days with temperatures reaching 90˚F.

Figure 2 also shows the extraordinary heat wave Seattle experienced in June 2021 where temperatures exceeded 100˚F for three straight days.

Figure 2

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Tags: Tmax, maximum temperatures, weather, Seattle-Tacoma International Airport, KSEA, summer, heat