Our current cool weather has been associated with an upper-level trough of low pressure over the region (see 500 hPa–about 18,000 ft–map below for 8 AM this morning).
But things change over the weekend as a significant ridge of high pressure and heights builds along the West Coast…check out the same map for Sunday at 2 PM… 180° change!
Ridging brings sinking and warming air over the region…and more important than that, inland pressure rises resulting in offshore (easterly) flow that cuts off western Washington and Oregon from the cool, coastal air.
Let me show you the predicted warm up!
The forecast surface air temperatures and winds for 5 PM Saturday, shows 80sF in the Willamette Valley and the Columbia Basin, but cooling northerly flow over Puget Sound where temperatures only climb into the lower 70s. Very nice day. Good one for outdoor activity.
But the situation is considerably different, and more toasty, on Sunday at 5 PM. Notice the development of easterly flow over southern WA and northern OR–easterly flow rapidly warms as it sinks and compresses on the western slopes of the Cascades. Our big heat waves all have strong easterly flow crossing the mountains. Portland jumps into the upper 90s, as does the Tri-Cities area.
But the real show-stopper is on Monday (5 PM shown)–the warmest day of the sequence, except for the immediate coast. The Columbia Basin and the Willamette Valley are torrid….with significant areas above 100F. Seattle gets into the upper 80s to near 90F. But look closely and you can see cool air moving inland from the coast.
There will be a weak marine push late Monday and Tuesday, with modest cooling west of the Cascade crest. And there will be major cooling…and the chance of rain later in the week.