Seattle’s Rainiest Day

Filed in Weather Records Posted on October 21, 2012 4:10 pm by 5 Comments

Sure, it rained yesterday, and sure, it rained pretty hard in some spots. Boeing Field, after all, saw .18 inches from a midday thunderstorm, and heavy showers pounded areas north of Everett as a strong line of storms—including a waterspout by Whidbey Island—moved through.

Compared to what fell from the sky nine years earlier, however, yesterday’s downpours were mere sprinkles.

Torrential rains swept through Seattle on Oct. 20, 2003, making for the wettest day in the city’s history. 5.02 inches of rain was measured in the gauge at Sea-Tac Airport, shattering the previous daily rainfall record of 3.41 inches from Nov. 20, 1959. The mark for the most rainfall in a 24-hour period was also easily broken, with the 3.74 inches that fell from Oct. 5-6, 1981 well over an inch shy of the October ‘03 deluge.

Seattle's rainiest day

Seattle was submerged in record rainfall nine years ago, with 5.02 inches falling at Sea-Tac Airport on Oct. 20, 2003—the wettest day in Seattle history.

The heaviest rains from the Oct. 20 storm actually fell over the southern half of the metro area, with Renton  picking up 4.48 inches and Boeing Field 4.04. Further north, at the National Weather Service office in Sandpoint, 3.59 inches landed in the gauge. North and west of Seattle, rainshadowing from the Olympics cut further into precipitation totals, with 2.36 inches recorded in Port Angeles and “only” 1.79 inches in Everett.

To put that day’s amazing rains into perspective, the steady downpour that brought traffic on area highways to a crawl last Thursday night amounted to .82 inches of rain—only one-sixth of what fell on Oct. 20, 2003. Even more, our October rainfall to-date this year stands at 2.26 inches—not even half of the infamous soaking Seattle received nine years ago.

With no big storms on the horizon this week, any “soaking” over the next several days will be limited to half an inch of rain at best, likely in the form of strong—but short-lived—afternoon showers. The reason? An upper level low remains off the Washington coast, forcing the more widespread rains to our south.

Today’s models keep the cold upper low in our vicinity through Thursday—meaning we’ll continue to see highs in only the lower 50s—before shifting it eastward at the end of the week. That should eventually open the door to warmer, wetter systems capable of dumping an inch of rain on Seattle—true October gullywashers by almost any definition.

Just not by Oct. 20, 2003.

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  1. In Seattle, ‘Rainiest Day Of The Year’ Defends Its Title | my blog | November 20, 2012 5:21 am
  1. kari simonson says:

    Hello Justin,
    I am wondering what the average rainfall amount per event is in Seattle… any ideas on where to find this information?

    • Justin Shaw says:

      Hi Kari,

      That’s a good question. From mid- to late-November—the wettest time of the year in Seattle—the average amount of rain per day is around .23 or inches or so. This number levels off to about .18 inches per day during December and most of January, and drops below .10 inches from May through September. Here’s a link to a graph from the Western Regional Climate Center that shows this (scroll down to the “Precipitation” area in the left-hand column, then click on “Daily Average”): http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?waseat

      Hope this works!

      • kari simonson says:

        No stats for the average rain event size? just monthly averages?

        • Justin Shaw says:

          Seattle averages 155 days a year with .01 inches of rain or more—so the average rainfall per rain day is .24 inches. That stat is somewhat misleading, though, since our precipitation isn’t spread out uniformly throughout the year (the rain we get in the summertime tends to be very light)—but it’s fairly accurate for the months of November through February.

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