A strong upper level ridge (high pressure) has been in over our region and will continue to amplify for the next day. Here is the 500 hPa (around 18,000 ft) heights (like pressure) for 5 PM Saturday to illustrate. Huge, broad ridge over the West Coast. In contrast, a cool trough is over the eastern U.S.
This ridge brings warmer temperature aloft and the development of a thermal trough that will progressively extend up the West Coast.
Here is the 24 h forecast of sea level pressure, surface wind, and low-atmosphere temperatures (925 hPa) for Saturday at 5 PM. There is a tongue of low pressure in western Oregon that extends into western WA...that is the thermal trough. VERY warm temperatures in Oregon. The northern portion of the trough is associated with strong offshore and downslope flow.
A close up view of the temps at the same time shows that Portland should be over 100F on Saturday. and southwest WA will reach the 90s. Puget Sound is cooler. Why? Because with the thermal trough centered south of us, there will be a north-south pressure gradient that will drive northerly winds and cooler air from north. Low to mid 80s around Seattle. Cooler over NW Washington.
At Sunday at 5 PM, the thermal trough will extend north and start to transition over the Cascades. This will be the warmest day around Seattle.
The temperature at 5 PM Monday illustrates this...cooler over western WA--perhaps reaching 80F in Seattle. But 100F in portions of the Columbia Basin. That should ripen some crops.
Temperatures will further moderate during next week, dropping into the 60s by Wednesday.