Weekend Heat Wave

The warmest days of the year so far  over the Northwest are coming up this weekend and some folks in western Washington will be sweating with temperatures exceeding 90F.  Today got to 81F at SeaTac...as perfect a day as one can imagine.

The Weather Channel revs up Seattle's temperature to 86F on Saturday and 89F on Sunday, before cooling down on Monday.  I suspect temperatures will be considerably warmer east and south of Seattle this weekend.

To get the big picture, let's start by looking at a sequence of forecasts of sea level pressure (solid lines), surface winds, and lower-atmosphere temperatures (cold shading).

At 5 PM today, one notes low pressure over the CA interior and western Oregon that is coincident with warm temperatures (hence the name, thermal trough).  This thermal trough is associated with offshore (easterly) flow forced by building pressure east of the Cascades.  Today, with the thermal trough south of western Washington,  there are cool northerly winds over Puget Sound.

Tomorrow at 5 PM is much warmer over western WA and Oregon, as the easterly flow moves northward.

 By 5 PM on Sunday, major changes are occurring.  Temperatures peak over western WA and the thermal trough has just "jumped" over the Cascades into eastern WA.  The warmest day for Seattle, but cooling starting on the coast.

 The effects of the thermal trough moving east of the Cascades is really obvious at 5 PM Monday.  Marine air and a large pressure difference has moved into western Oregon and Washington.

Taking a look at the surface (2-meter) temperature forecasts from the super high resolution UW WRF forecast model, today at 5 PM shows low 80s into SW Washington and southern Puget Sound.

 24 hour later, a MUCH warmer story, with upper 80s and lower 90s over SW Washington and 90s in the Willamette Valley.

 Sunday at 5 PM is even warmer, particularly over the western interior and the Columbia Basin.  Portland will be white hot!  And you will notice some coastal cooling.

It is, of course, dangerous to look at one forecast solution....so let's check out the latest National Weather Service higher-resolution ensemble (called SREF) for Seattle.  Most of the forecasts are for low 90s in Seattle.  Quite a bit of confidence in this forecast.

So find a fan and a cold drink...you will need them during the afternoon and early evening on Saturday and Sunday.