One last day of near 100° highs are expected in the Tri-Cities on Wednesday before an upper level trough passes through the region. This is forecast to place the jet stream right over the Washington/Oregon border, allowing cooler air from the north to dip into the Pacific Northwest.
This trough in the upper level flow is expected to be dry. Not even areas west of the Cascades are likely to see any precipitation out of it. The changing wind pattern in the mid- and upper-levels of the atmosphere is also likely to significantly decrease the amount of monsoon moisture making it into Oregon and Northern California.
Temperatures Thursday through Saturday are likely to be quite a bit cooler than the last week and a half. Friday is forecast to be the coolest, with highs possibly not reaching into the 90s throughout the entire Columbia Basin. The trough kicking out the monsoon moisture for a time means lower dew points allowing temperatures to get lower after the sun sets and overnight lows should get down to the upper 50s.
The trough will continue its trek eastward so that by Sunday the Tri-Cities is forecast to be getting back near 100° as the Pacific Northwest heads into another heatwave. This one is not expected to last as long or be as strong as the one that is drawing to a close. Monday will probably be the hottest afternoon with highs reaching near to just above 100° in the Tri-Cities and Hermiston with upper 90s elsewhere in lowland areas.
Moderation comes as early as Tuesday with only about a 50/50 chance of highs getting to 100° in the Tri-Cities. It is still a ways out, but the general trend beyond Tuesday indicates another short lived relatively cooler period followed by no real strong signal for anything in particular. I expect typical summertime conditions to continue, with my best guess for the second half of next week coming out to a little warmer than average for this time of year.
As always, the forecast is likely to change as we get closer to the next heatwave. There is reasonable model consensus as of Wednesday morning for temperatures up to Monday, but beyond that confidence begins to wane.
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