UPDATE EARLY THIS AFTERNOON: The situation is getting even more serious. Nearly certain the many west-side stations from Seattle south will exceed their all-time temperature records, some by a lot.
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The weather model solutions are now converging and we are close enough to the event to have some confidence in the forecast.
The bottom line: we are about to experience a historic regional heatwave, which will break both daily and all-time temperature records.
A heatwave that is made even more unusual by its timing, occurring far earlier than most of the major extreme-warmth periods of the past.
But the heat will not be uniform, and some locations, such as Northwest Washington and the coast, will escape the extremes.
Previous Record Highs
Before I provide the torrid forecast--and tell you why I am confident in it-- let's review how warm things have gotten in our region in the past.
The all-time record high for Washington State, based on observations going back approximately 125 years, is 118F at Ice Harbor Dam near Pasco on August 5, 1961. In Oregon, the record high is 119F observed in Prineville and Pendleton, on July 29th and August 10th--both in 1898.
Here in Seattle, the record high is 103F on July 29, 2009, and in Portland, a sizzling 107F on July 2, 1942,
You will note that many of these record temperatures are well in the past. A plot of record temperatures I found on plantsmap.com, shows that eastern Washington has lots of record highs from 110-115F, while southwest WA has surged into 100-108F territory. Northwest WA is substantially cooler.
As I will describe below, I expect temperatures will approach and/or exceed these temperatures over the weekend and Monday, and there is an excellent chance that some all-time records will fall.
The Forecast
Prepare yourself for what I am about to show you. Quite frankly, it is stunning. The model solutions are quite similar for Saturday and Sunday, so let's start with those.
The generally highly skillful European Center model shows maximum temperatures Saturday reaching 100F from roughly Olympia southward, with temperatures climbing to about 105 in the northern Willamette Valley. 103-110F in the Columbia Basin. Much cooler near the water along the coast and Northwest Washington.
The much higher resolution UW WRF model (4-km grid spacing compared to about 9 km for the European Model) driven by the U.S. GFS global model is quite similar, but slightly cooler, with the off-white and white colors show where temperatures are forecast above 100F at 5 PM Saturday.
But Saturday is just a "warm up" for the extraordinarily threatening temperatures predicted for Sunday and Monday.
The Sunday maximum temperatures from the European model are shown below.
Just amazing. From Olympia south, temperatures rise to 105F and more, with Portland, reaching 110F-- just smashing the previous all-time record of 107F. Sea Tac gets to 101F. Above 110F in portions of the Columbia Basin. But it is the west-side temperature extremes that really are really insane, leading to many stations breaking their all-time records.
And then there is Monday. And quite honestly, it is hard to believe my eyes. Around 105F at SeaTac--breaking the all-time record of 103F. Around 114F in Portland---7F greater than the previous record. 110F in Olympia and peaking around 115 near Richland. Folks....this is unparalleled, dangerous meteorological territory.
But these are just single forecasts. What about ensembles of many forecasts? A key tool for determining probabilities and confidence estimates in our forecasts.
For Saturday and Sunday, most of the European Center and U.S. ensemble members are on the same page, suggesting we should have confidence in the forecasts.
The ensemble average tends to be highly skillful and the European Ensemble mean forecasts for Seattle and Portland are shown below: up to 103F in Seattle and 110 in Portland.
Monday is far more uncertain than the weekend....it could be nutty warm and perhaps a minor cool down in the West. Stay tuned for that as we get closer.
In any case, it is essentially certain that the Willamette Valley and western Washington from Seattle southward will have a historical heatwave, one beyond the experience of many residents.
An important issue is that the minimum temperatures will not fall below 70F during the coolest periods, making it more difficult to cool homes and apartments. For portions of the heatwave, the dew points will rise into the mid-60s, making it seem more humid than normal.
In a region where air conditioning is not widespread, this event will be potentially dangerous for the elderly and I am glad the City of Seattle Emergency Management folks are working on cooling centers and other approaches to mitigate the heat.
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