The first half of August has featured comfortable summer temperatures, but a change is in store for the weekend as a brief heat wave is expected to take hold over the Pacific Northwest.
One way to illustrate heat waves is by looking at model projections of 500 hPa geopotential height anomalies and wind speeds. My favorite source for these maps is a page designed and maintained by Dr. Alicia M Bentley. Positives anomalies are generally associated with ridges and seasonally warm temperatures.
On Wednesday morning the GFS model shows the opposite of a heat wave–a trough with seasonally cool temperatures over all of the PNW:
By Friday morning the trough will have moved on and zonal (west to east) flow will prevail:
Over the weekend a rapid and dramatic change will occur. The upper-level pattern is expected to amplify, with a strong ridge over the western US and an associated trough over the NE Pacific.
The GFS model is also predicting that lower-level winds (at 850 hPa and below) will be from the east by Sunday morning.
Easterly winds across the Cascades are known to supercharge heat waves in western Washington and Oregon. As air descends from the mountains to the lowlands it compresses and warms. In summer, this local effect can often be the difference between a very warm day (upper-80s) and a hot one (90s).
Will that be the case this Sunday? Possibly. NWS Seattle is calling for upper-80s in Seattle on Sunday, but the potential exists for widespread 90s depending on how the forecast evolves.
Eastern Washington is a near-lock for 90s to low 100s by Sunday. Tri-Cities airport has already reached 100 degrees 11 times this summer. The NWS is currently forecasting highs of 101 on Sunday and 102 on Monday.
How long will the heat wave last?
Not long, but there is some uncertainty. The deep trough over the eastern Pacific (seen in the above GFS maps) is expected to approach the coast by mid-next week.
For longer range forecasts, it is preferable to look at the distribution of model ensembles rather than a single, deterministic model run. The ECMWF ensemble 850 hPa temperature spread suggests high confidence that the heatwave will peak on Sunday in western WA (and Monday in eastern WA), but there is high uncertainty regarding how rapid the cool down will be.
At this time I would plan for another hot day on Monday except along the coast. Hopefully cooler air will reach most of the state by Wednesday at the latest.
Heat waves are especially dangerous for vulnerable members of the population, including the elderly and people experiencing homelessness. COVID-19 has further exacerbated these risks due to the closing many air-conditioned public buildings and water fountains. It is fortunate that Seattle has largely avoided extreme heat so far this summer, and hopefully the models will be correct about the upcoming heat being short lived.