data — Seattle's Past and Present Weather and Climate in Charts and Graphics

The past year (and then some)

This is my final post on this cold snap / winter topic. First, I think the weather is changing and winter really is exiting. Second, as a subject, it has become a little tired. So, here goes.

I suspect the extended coolish pattern we’ve experienced over the past three months has finally had its run. I’m not a meteorologist nor a climate scientist. Just someone working on my writing and data presentation skills. It just seems that we’re approaching the end of a pattern. The longer term forecasts seem to point towards more normal temperatures.

No doubt, we will still get cooler-than-normal days. Just as we’ll get warmer-than-normal days. But I think the persistent patterns are breaking down. Looking at those cooler-than-normal patterns on the right of Figure 1 shows that up until about March 9, we were still seeing strings of continuous cool days. However, the last few days have been mild, and the temperature deficits have been shrinking since about early January.

A couple of things seem clear from Figure 1:

This coldish spell was persistent and long — almost 3-½ months long (with a few brief interruptions);
it was preceded by at least 11 months of predominantly warmer-than-normal weather; and
the change from one state to the other was quite abrupt in early December. It was as though a switch was flipped.

Figure 1. Temperature departures for the past 15 months. Click to enlarge. — **Figure 1.** Temperature departures for the past 15 months. *Click to enlarge.*

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Temperature Deviations - A longer comparison (2000-2017)

Figure 2 shows similar temperature deviation data to Figure 1, except it covers a much longer stretch of time backwards. The red zones depict the approximate stretch of winter (Dec-Feb) for each annual period.

I mentioned in the section above that prior to the recent three month cold snap, Seattle experienced 11 months of warmer-than-normal temperatures. Well, looking at a longer timescale backwards, the stretch of warmer weather goes back at least about three years, aside from occasional brief spikes of temperatures in the opposite direction.

For most of this period, specifically the dozen or so years extending from 2000 through 2012, the daily differences from normal temp appear to be more evenly distributed between short warmer and cooler periods. The year 2011 appears to have a long stretch of cooler temperatures — more in the early spring-to-fall time frame rather than winter. And the two year period of 2003-04 appeared warmer than normal in winter as well. I recall during this time thinking to myself that after 20 years, I was finally becoming accustomed to cool, rainy Northwest winters and they weren’t as bad as they previously seemed to me. It turns out, I may had just been experiencing a milder series of back-to-back winters.

Figure 2. Comparison of temperature deviations over time (2000-2017). Click to enlarge. — **Figure 2.** Comparison of temperature deviations over time (2000-2017). *Click to enlarge.*

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What about the rain

The temperature discussion above focuses on the traditional winter months of December through February. In Seattle, our average winter day temperatures don’t differ much than late fall or early spring temperatures. We have mild winters, mostly and coolish springs. In fact, I’ve always considered the period between mid-October to mid-April as one long winterspring season. This time period nearly coincides with the first half of the west coast meteorological water year which typically runs from October 1 to September 30 each year. Seattle, and much of the northern West Coast has a wet season and a dry season. So I’ll use the Oct-Sep water year as a time period to discuss this winter in terms of precipitation.

Figure 3 shows the accumulated precipitation since the beginning of the water year. This year’s line is the blue dashed line. The orange line represents the climatic averages from the years 1981-2010 from SeaTac International Airport. Clearly, we are experiencing a very wet rainy season. So far — and the rainy season for this year is not over — we have had about 11 more inches of rain than normal since October 1 of last year.

Figure 3. Cumulative precipitation since October 1, present year and historical. Click to enlarge. — **Figure 3.** Cumulative precipitation since October 1, present year and historical. *Click to enlarge.*

So we're having an extraordinarily wet year. Big deal — some years are wet; some are dry. However, are these wet years becoming more frequent? I don't know if we have enough data to establish this claim for the long run, but recent years seem to be.

Figure 4 shows more recent years in the same chart as Figure 3. Last year, the 2015-16 water year, is the heavy black line. Last year was also an exceptionally moist year, the wettest of the previous 14 years shown. The medium gray lines reflect the water years: 2014-15; 2013-14; 2012-13 and 2011-12, the years precluded from the 1981-2010 average. The light grey lines represent the nine preceding years before 2011-12 since I have the data handy and they give some balance to the discussion.

The chart in Figure 4 illustrates:

Though it has been very wet since October and we are approach last year’s surge of rain, it is unlikely we will catch it. We are currently about 5 inches behind and are slowly approaching our drier months. Back-to-back years of this flow of water lends understanding as to why many are griping about this winter;
This year’s totals are in a tight race for second place of the wettest years of the past 15 years. Today, we are about even with 2006-07, which produced a notably wet start to winter [1];
Of the six (6) years since the 1981-2010 climate reference period, three are near the reference average and three are well above it (assuming the 2016/17 water year keeps accumulating precipitation at even the climatic average rate). Simply put, in an annual accounting of rain, we have been in a pretty wet period since 2010.

Figure 4. Cumulative precipitation based on the water year calendar, multi-years. Click to enlarge — **Figure 4.** Cumulative precipitation based on the water year calendar, multi-years. *Click to enlarge*

So, what happens if you take the averages of the most recent past six water years, not included in the climatic data range, and compare them to the historical averages. Keep in mind that the 2016/17 water year is not even half way completed. This is shown in Figure 5. There has been a significant bump in the amount of precipitation we've seen recently. From the chart, it appears most of this bump has occurred in the months of October, February and March. September also shows an uptick in precipitation as well, but not as pronounced as those three. One could also make the case that summers are slightly drier (seen by the flattening of the blue curve in comparison to the orange one for these months).

Figure 5. Comparing the past six years to the climatic average. Click to enlarge. — **Figure 5.** Comparing the past six years to the climatic average. *Click to enlarge.*

Seattle has received over 246 inches of rain since October 1, 2011. Climatically, Seattle would be projected to receives about 214 inches of rain during this period. We've received nearly 34 inches of additional rain in the past six years. Table 1 illustrates this clearly.

This is almost an entire year's worth of additional rain squeezed in those six years. Much of that additional precipitation has occurred in the past two years.

Actually, it is worse than that. I included 2011-12 in Table 1 since it is not included in the 1981-2010 historical averages. I wanted to include all recent years not included in the historical average in an exploration to see where the data took me.

Since Table 1 shows the 2011-12 water year to match essentially the historical average for the period, you can leave that year out of the discussion. Our near additional year of rain has been squeezed into the past five years. And — again — this water year is not half over yet.

In this context, one may like the rain but should also understand the weariness people are experiencing and the accompanying complaints.

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Shades of gray

Aside from rain, the most common characteristic of Seattle winters (and wintersprings) is the near constant cloud cover. So, how did we stack up with regards to cloud cover? I’ll use the first half period of the water year for the time scale, since this is traditionally when the clouds roll in.

We began the water year with one of our rainiest Octobers. This was followed by a normal November, typically our rainiest month. So that in itself a sign that it might be shaping up for a pretty dismal winter. However, if you look at Figure 3, the rains softened in December. This can be seen by looking at the slope of the blue curve in December. Further, the last half of January saw little rain. In fact, December and January were cold, but not too rainy. If the skies were not completely clear, we experienced some sun during those months. More than usual, I’m guessing, without looking deep into the data.

The rains returned in February at an accelerated rate. February 2017 was one of our wettest Februarys. The month of March, to date, has been dismal as well.

The National Weather Service gives a rating for sky cover for each day based on an 11-point scale, going from 0-10. In maybe an oversimplification, a day rated 0 is clear with no clouds. A day rating of 10 has 100% cloud cover. We can sum the scores for each day since October to see how this year compares to other years.

Figure 6 shows this. So far, this water year has been cloudier than all of the previous 10 annual periods except for one. But, with this chart it doesn't appear to be much.

Figure 6. Seattle Cumulative Sky Cover ratings, 2016-17 water year. Click to enlarge. — **Figure 6.** Seattle Cumulative Sky Cover ratings, 2016-17 water year. *Click to enlarge.*

Figure 7 illustrates this method of comparison with a close-up of the data shown in Figure 4. So far, through March 10, SeaTac Airport has accumulated about 50 more sky cover points than the average for previous 10 years. These additional sky points don't buy you an upgrade; only a slight downgrade, unless you love clouds.

However, spread evenly, 50 sky points divide by 161 days (Oct 1 to Mar 10) account for only about .3 sky points per day. Or, another way of thinking things, the maximum sky cover points any day can have is 10. Divide 50 sky points by 10 points per really cloudy day and we have had roughly 5 extra really cloudy days more than average during this time period.

Figure 7. Cumulative Sky Cover rating points for annual periods running from October 1 through September 30. Click to enlarge. — **Figure 7.** Cumulative Sky Cover rating points for annual periods running from October 1 through September 30. *Click to enlarge.*

A simpler way of observing the cloudiness of the past 5-½ months is to look at the total number of cloudy days in a table and compare this year’s numbers to past years. Table 2 shows this comparison.

To date, Seattle had about a 68% chance of seeing an all cloudy day this past wet season. Normally, in recent years, this would be a 64% chance. This winter, Seattle also had about an 8% chance of seeing a pretty clear day. In the recent past, we would have a 12.5% chance of seeing a reasonably clear day. The chances of a partly cloudy day are essentially the same for this year and the recent past.

So, yeah. It’s been a bit cloudier than normal; though not that much more. Still, every little bit adds up.

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Final thoughts...

Yeah, this winter in the Pacific Northwest has been pretty dismal. More dismal than normal. More dismal than most. The coldish streak was a bit long — I grew very weary of it. But the low temperatures were not that cold. Consider the Midwest or Northeast in any normal winter. Our coolish winter temperatures this year were still pretty moderate in comparison.

We had snow a couple of times. Deep enough for some fun and some nice photos. Not too much to cause trouble for commuting in the lowlands, though the mountain passes got hit hard at times.

This area doesn’t seem to suffer too often from daily extreme temperatures, humidities, or even heavy rains. We get strong winds blown in off the North Pacific occasionally, but we don’t see hurricanes or tornados often.

Where we see extremes, it seems to me, is that we settle into long, long patterns, whether long heat streaks; long coldish streaks, long rainy streaks, and — for me the most dreaded — endless cloudy stretches.

This winter combined most of those long streaks. I’m happy to see it ending and am looking forward to longer days, brighter skies and warmer temperatures.

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References:
[1] City That Takes Rain in Stride Puts on Hip Boots; Yardley, Wm.; New York Times; November 27, 2006.
[2] Data Sources NOAA NWS Seattle Local Climate site (click on text for link).

Charts and Graphics Notes:
Data from NOAA/NWS Climate site was post-processed in Microsoft Excel for Mac 2016.

Figures 1-7 were produced using DataGraph 4.2.1 software from Visual Data Tools, Inc. and available on the Apple App Store. A terrific product that keeps improving — support good engineering and design.

Base charts were copied to Adobe Illustrator CS6 for post process refinement and scaling.

Tables 1 and 2 were developed in Adobe InDesign CS6.

A warm and dry summer

Temperatures

Precipitation

A winter to forget...

The past year (and then some)

Temperature Deviations - A longer comparison (2000-2017)

What about the rain

Shades of gray

Final thoughts...

The cold return...