Showers were already being observed in the Columbia Basin and Yakima Valley before 04:00 Monday thanks to an upper level low pressure system centered near Central Oregon. This pattern is pulling moisture up from the southeast in a setup similar to what occurs during monsoon season late in the summer. I’m labeling it a fake monsoon, however, because the North American Monsoon may just now be starting.
It typically aims moisture from the Pacific at and around the Mexican states of Durango and Sinaloa before turning northward toward the American Southwest closer to July. On its own, moisture from the real monsoon is not able to make it to the Pacific Northwest. A low pressure system like the one in place today is usually needed to pull the humid air up from the Desert Southwest.
Storms wind down after sunset with some clearing toward Tuesday morning. Thunderstorms are again possible in the afternoon but they aren’t likely to be as widespread. The upper level low gets pulled into the broader flow pattern midweek and temperatures are likely to turn hot again toward the weekend under sunny skies.
Modeling is strongly indicating a pattern change for near average temperatures next week, but historically model guidance has been quicker to remove strong upper level ridging than actually occurs. Confidence should increase as we approach the weekend.
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