A building ridge of high pressure this week is forecast to bring a late-season heatwave to the Pacific Northwest. This has prompted the National Weather Service in Pendleton to issue a heat advisory from 11:00 to 20:00 Wednesday for the Lower Columbia Basin and Yakima Valley. This includes the Tri-Cities, Yakima, and Hermiston, but excludes Walla Walla and Pendleton which will be a few degrees cooler.
The National Weather Service has highs currently forecast to land between 97° and 103° in the advisory area. Model guidance suggests that the warmest areas around the Tri-Cities and Hermiston may see highs more around 101° to 105°.
A weak shortwave trough clipping the Pacific Northwest Wednesday should bring a surge of westerly wind through the Columbia Gorge that afternoon into the morning Thursday. Breezy conditions are possible as far as the Tri-Cities and Hermiston, but won’t reach those places early enough to limit the highs. This pattern should, however, moderate temperatures by a few degrees on Thursday with highs only expected to be around 100°.
Right now it looks like Friday may be the warmest day of the week with highs possibly in the 103° to 108° range. That afternoon a stronger, but still weak, shortwave may sweep across the Pacific Northwest bringing another westerly surge across the Cascades for breezy conditions in the basin late Friday into Saturday and more moderated temperatures into the holiday weekend.
A bit more uncertainty exists with the high temperature on Friday with the timing of the incoming system. Wednesday’s disturbance, which is only about 48 hours out, has pretty decent odds of coming in within an hour or two of when models show. Friday’s system is four days out and it coming in a little early could make a big difference in afternoon temperatures. Still, I lean toward things running hot that afternoon.
Temperatures above 100° aren’t rare in the Tri-Cities for late August and early September, but these two days bring a chance of breaking the daily record for Wednesday (currently 101° set in 1967) and the monthly record for September (104° set in 1930). Right now, the Tri-Cities stands at 26 days at or above 100° for the year – tying the record which was set in 2021 and 1898. These two upcoming warm days may add to that total to set a new record.
In 2013, the Pendleton National Weather Service office was issuing heat advisories clear to mid-September. Looking forward beyond the weekend, above average temperatures are favored with ridging likely to continue. That said, it does look somewhat subdued which would lead to somewhat less heat.
