Ridging is building over the American West today into the weekend, leading to a regional heat wave in the Pacific Northwest. Above average temperatures are forecast for the Columbia Basin, though things aren’t looking quite as warm as they were originally expected. The Tri-Cities can expect a high in the mid 80s on Friday and near 90 over the weekend.
Heat is also forecast west of the Cascades where highs in the low 90s for Portland and mid to upper 80s in Seattle have prompted heat advisories. The treshold for heat advisories west of the Cascades is generally lower than the threshold the National Weather Service uses for the Columbia Basin because of the milder climate as well as there being a much higher portion of the population not having air conditioning.
Warm temperatures west of the Cascades will create a thermal trough, which is an elongated area of lower pressure, along the coast Saturday and Sunday. With strong high pressure in Montana and Alberta, air will be moving from there toward the coast for breezy easterly and northeasterly winds in the Columbia Basin. Relative humidity values possibly in the 10-15% range both afternoons will create an elevated fire risk.
An upper level low develops over Oregon Sunday, strengthening into Monday. This will pull moisture up from the south to generate thunderstorms in Central and Southeast Oregon on Sunday with a few possible in the Blue Mountains. Models support scattered thunderstorms in the Columbia Basin with widespread mountain thunderstorms on Monday in a pattern that will be very similar to the sort that brings monsoonal moisture into the Pacific Northwest during the summer months.
Dewpoints in lowland areas are only marginally supportive of heavy rain. It is possible that a majority of the cells that track over lowland areas will be dry thunderstorms, bringing another day of elevated fire risk.
This upper low meanders for another day or two before being absorbed into the broader upper level flow pattern for a low chance of lowland thunderstorms on Tuesday. Ridging rebuilds as the low no longer dominates the weather pattern, keeping warm temperatures in place for most of next week. Another heat wave is possible next weekend but there’s quite a bit of uncertainty associated with forecasting that far in advance.
