First rain of August in Western Washington signals the end of peak summer.

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A weak frontal system brought a rare first-week-of-August rainfall to the Seattle area, ending a very nice stretch of summer weather. Fans of rain happily welcomed the return of an old friend, while fans of summer were given a brief but ominous signal of the approaching transition to autumn.

The morning began with benign conditions, as seen from the Space Needle cam at 6:30 AM:

Space needle cam, 6:30 AM August 6 2020

At 10 AM there wasn’t much to see except the “winter” shade of grey.

Space needle cam, 10:00 AM August 6 2020

The culprit was a trough of low pressure with a weak surface cold front.

August 6, 2020 12 UTC (4 AM PDT) 500 hPa heights (green), wind (yellow), and IR cloud cover (white)

Rainfall totals were generally around a tenth or two tenths of an inch, with higher amounts on the Olympic Peninsula, and to the north of Seattle. A weak convergence zone was responsible for the higher totals in Snohomish County.

24 hr precipitation observations

The weather system was notable because it brought the first rainfall following the climatologically driest time of the year. Sea-Tac recorded 0.09″, the first measurable rainfall since July 22. At my home in West Seattle it was the first measurable rain since July 17, ending a 3 week long dry spell.

Rain on August 6 has been almost unheard of in the past two decades.

As the chart below shows, the last week of July is the driest week of the year in Seattle. The odds of measurable rain (0.01″ or greater, black line) on a given day are about 1 in 9. The first week of August is the beginning of the transition to fall, with precipitation odds increasing at a steep but steady rate through early November. By September 1 the odds of measurable rain increase to 1 in 4 and by late October the odds are 1 in 2. Heavier rainfall amounts follow a similar trajectory, although the odds increase more rapidly beginning in October.

Probability of daily rainfall exceeding various thresholds by time of year at Sea-Tac airport.

Model forecasts confirm that the “peak summer” is likely over, meaning western Washington is unlikely to see another long run of sunny, warm days in the near future. The average upper-level pattern for the next 7 days, as pictured by the 500 hPa geopotential height anomaly (below), shows anomalous ridging (yellows) over the Pacific to the west of Washington with anomalous troughing to the north and northwest over western Canada and Alaska. This setup opens up a pathway for weather systems to reach the PNW from the west-northwest, roughly following the red arrow.

ECMWF 500 hPa height anomaly, next 7 days (image via WeatherBell)

How much additional rain can be expected? Models show a secondary weak front will bring some sprinkles on Friday night. After that, there is little agreement on the following week, although with the above upper-level patten I suspect that additional weak weather systems will make their way into the PNW over the next week. Better enjoy the warmth while it lasts!

5 comments

  1. Do we know what the reason is for the upper-level pattern? Does it have anything to do with the Great Pacific Warm Patch?

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