On Thursday, associated with the movement of an upper level trough across the region, temperatures dropped substantially west of the Cascade crest as marine air and low clouds pushed in. The 10 AM visible satellite image shows low clouds on the west side and unfortunately lots of smoke over NE Washington.
As shown by the sea level pressure forecast for later on Thursday, there was a large pressure difference across the Cascades (the lines are isobars, lines o constant pressure; a large change in pressure results in strong winds). Cooler, denser air west of the crest increased pressure over western WA while warmer air remained to the east.
Here is a table of the pressure differences across the Cascades (Seattle minus Yakima). The pressure difference increased to over 9 mb (hPa) across the Cascades. That is quite large.
Using aircraft data at Seattle-Tacoma Airport we can document the surge of marine air into the interior (red is temperature, the blue barbs are wind, height is pressure (700 is 10,000 ft) and time is in GMT (UTC)). The plot runs from 11AM on Wed to 11 AM on Thursday (later is to the left). Big wind switch to southerlies at low levels and a cool off.
As shown by the winds at Ellensburg, the eastern slopes winds responded on Thursday, gusting to 40 knots at that location.
Here are the max gusts for the eastern slopes: many locations reached 30-40 mph.
I should note that these eastern slope winds were not exceptional for this summer. To illustrate, here are the winds are Wenatchee over the last 12 weeks. The eastern Cascade slopes often have strong winds late in the afternoon....enhanced when western WA cools.
Temperatures will warm this weekend as a minor ridge pushes over the Northwest, but major changes are coming. By mid-week a deep trough will develop over the eastern Pacific and temperatures on BOTH sides of the Cascades will slip below normal.
The NOAA Climate Prediction Center 6-10 day forecasts are for above normal precipitation and below normal temperatures over our region (see below) and UW WRF model predictions suggests little lightning. A chance to make progress on the fires.
Finally, let me show you one of the most amazing smoke photos I have every seen, from the NASA MODIS satellite on Thursday. DENSE smoke extending from the North Cascades into Montana. Extraordinary.