Sopped in Seattle

Table 1 shows the precipitation totals for five consecutive annual periods beginning one October 1 for each period. The most recent listing is incomplete, as we are only in April at the time of this writing, more than five months from a complete year.

Three of the five years show a large deviation for annual precipitation totals from the 30-year Seattle average of around 37.5 inches per year.

The 30-year climatic average of precipitation totals is derived from National Weather Service values established from 1981-2010.

Initially, Table 1 included totals for the period beginning on 10/1/2011 through 9/30/2012 to include all recent years outside of the climatic average data set. It seems we have been experiencing wet winters of late, so I wanted to explore the data to see if this was true. The precipitation totals for the annual period stretching from 10/1/2011-9/30/2012 appeared to match the averages of a typical year. Plotting this year would not help in determining whether recent years were trending exceedingly wet. The first annual period outside of the 1981-2010 climatic averaging period which exhibited a significant deviation from average began in October 2012. This is the first data line in Table 1.

Both annual periods ending in 2014 and 2015 were close to the average of 37.5 inches.

The annual period ending in 2016 was similar to that ending in 2012, a very wet winter. We received 11.8 inches of rain more than average last year.

This winter has also been exceptionally wet as well. Though the actual total on data line 5 is almost five inches less than the previous year, keep in mind that the previous year includes values up until September 30th. We still have more than five months remaining this year to reach the same date in September.


A Five Year Look

Figure 1 shows the accumulation of precipitation beginning on 10/1/2012 (starting with the first one of data in Table 1)..  At the end of the first annual period, on September 30 2013, we had about 10 inches more precipitation than normal (Point A). However, one year does not make a trend.

Point B shows where we stand today across the five year time scale for values listed in Table 1. Seattle has had nearly an additional year of rain than normal over this stretch. However, it is worse than this chart shows.

Take a look a Point C. This represents February 11, 2014 on the timeline (X-axis). Though Seattle was 10 inches beyond normal on October 1, 2013, the next 15 months were relatively quiet in terms of precipitation. This can be seen by comparing the slopes of the blue and gray lines betweens Points A an C. By February 11, 2014, the total amount of precipitation fallen since October 1 2012 was back to around what one should normally expect for that period.

Figure 1. Precipitation at SeaTac Airport since 10/1/2012 (click to enlarge).


A Shortened Look

Figure 2 shows the precipitation deviation beginning on February 11, 2014 (Point C). The extra year of precipitation we've experienced has fallen since early 2014. 

In slightly more than three (3) years, Seattle has had four (4) typical years worth of precipitation.

No wonder the moss is so thick.

Figure 2. Precipitation at SeaTac Airport since 2/11/2014 (click to enlarge).