Today, clouds thickened a bit more rapidly than forecast, keeping temperatures down below the forecasts; but one area of western Washington did get into the low 70s: the western foothills of the Cascades. Here is the proof, today's maximum temperature. Lower 70s along the Cascade foothills, but only mid to lower 60s near the Sound.
Why warm temperatures in places like North Bend and Enumclaw? Easterly offshore flow, with air descending the western slopes.
Earlier today, high pressure developed over eastern Washington, with lower pressure west of the Cascades. The result (see map at 11 AM Friday, solid lines isobars, lines of constant pressure)) was easterly flow across the Cascades.
As easterly flow descended the western slopes of the Cascades, it was compressed and warmed, just as a bicycle pump warms when used to compress air. We started with relatively warm air over the region and the downslope flow enhanced the temperatures further along the slopes. The loss of solar heating by the relatively dense mid-level clouds had a substantial effect: without the clouds the foothills would have hit 75-80F and the Puget Sound region would have hit 70F in many locations.
But now the other shoe is about to drop: a very potent atmospheric river has now reached the NW, dramatically illustrated by the forecast atmospheric vertical total moisture at 2 AM Saturday morning (see graphic).
The latest image from the Langley Hill coastal radar shows something we are not used to: a field of moderate precipitation approaching western Washington.
Want to be impressed? Here is the forecast 48-h precipitation ending 5 AM on Monday. 2-5 inches for the higher terrain in the Cascades. There will be plenty of water to help fill local reservoirs, with rivers low enough not to worry about flooding. I am going to have to look for my umbrella, which is collecting dust in some closet.