A Soggy Month (by any measure)

Seattle WA, Oct 31 2016 [Updated: Nov 1 2016]

Seattle has experienced a soggy October this year.  This morning we awoke to another steady and dark drenching rain.  So how wet has it been?

For starters, Seattle typically receives about 3.5 inches of rain in October.  This is based on the 30-year average from readings taken at Seattle-Tacoma International Airport from 1981-2010.

This October, we received 10.05 inches of rain.  November is typically our rainiest month.  In a typical November, we receive about 6.6 inches of rain.  This year, October is more Novemberish than a wet November!
 

Figure 1 illustrates the early and steep start to the autumn rains.  Looking at it a little deeper, two key things stand out:

  • This year’s wet autumn start followed a very soggy beginning to the year; and
  • recent years have had similar early starts to the autumn rains.

Figure 1. Cumulative precipitation.
Click to enlarge.

Figure 2, a scatterplot of average monthly temperature vs. total rain for the years 1981-2016, shows a couple of things.  

  • It shows how much of an outlier 2016 is in terms of precipitation to all but one of the other 36 years plotted.  That year was 2003, when 8.93 inches of rain fell.  However, one day in in that month, October 20, had nearly 5 inches of rain fall.  Take that outlier day out of that month, and October of 2003 looks very average.  Seattle had about 13 days with precipitation in the previous record-breaking month of October 2003.  This year, October had 25 days with some level of precipitation.  The rains of October 2016 were scattered much more broadly across the month.  Figure 3 and Figure 4 show this.
     
  • Figure 2 also illustrates that the Octobers of 2014-2015 also were wet and warm as well, outside of the climate norms established from 1981-2010.

Figure 2. October temperatures and precipitation, 37 years, Seattle-Tacoma International Airport.
Click to enlarge.

Figure 3. October 2003 precipitation distribution by day.
Click to enlarge.

Figure 4. October 2016 precipitation distribution by day.
Click to enlarge.