The ridge is forecast to bring a late-season heat wave. The heat wave is forecast to peak west of the Cascades on Thursday and east of the Cascades on Friday or Saturday.####
Heat wave forecastECMWF ensemble probability of reaching 100°F (38°C) on Thursday (left) and Friday (right). (WeatherBell)Some local TV meteorologists in Portland have already put 100°F (38°C) on their forecast graphics for Thursday. I think it is possible for Portland to reach that mark, especially near downtown where the urban heat island is strongest.
The ECMWF ensemble indicates that the Northern Willamette Valley has a higher chance of tapping 100° on Thursday. It generally looks like the warmest area will be from Salem to Wilsonville.
Parts of the Columbia Basin may tap 100°F on Thursday (especially around Boardman), but Friday and Saturday will be the warmest days. Pasco, Hermiston, and Boardman are likely to hit 100°.
Yakima and Moses Lake will be close on Friday. An isolated 100° reading is also possible around Salem and Hillsboro.
Onshore flow west of the Cascades on Saturday brings an end to the 100° weather but the Columbia Basin and Yakima Valley may even be a degree or two warmer. Given the current forecast, Walla Walla could join the 100 club.####
ClimatologyThis is unusually late in the season for 100° temperatures, but they aren’t unheard of. The record latest 100° day in Portland was September 7, 1981. Salem is likely to tie their record of September 5, set in 1944. They have about a 30% chance of hitting 100° on Friday, which is September 6.
Despite being further inland, the record latest 100° day in the Tri-Cities isn’t that much later than Portland. It currently stands at September 9, 1940. I don’t think there is a very good chance of this even being tied.
The record could be broken in Yakima, where their latest 100° day so far was September 3, 1988. At the Hermiston Airport, the record is September 1 but records there only go back to 1998.####
Extended forecastTemperatures begin moderating west of the Cascades on Saturday and early next week for the Columbia Basin. Models currently favor upper level troughing becoming dominant to bring cooler temperatures and, for some, a chance of rain.
https://ingallswx.com/2024/09/03/columbia-basin-and-willamette-valley-may-hit-100f-later-this-week/
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