A dangerous heat wave will impact the Pacific Northwest this week. It won’t be anywhere near as strong as the “Heat Dome” of June 26-28, but it will still bring record highs to large portions of the Pacific NW Wednesday – Friday, with record highs likely holding on for the inland Pacific NW on Saturday. Smoke from California fires will move into Western Oregon Thursday afternoon – Saturday and will likely decrease high temperatures from current forecasts (especially on Friday and Saturday), but both days will still feel extremely hot.
Excessive Heat Warnings are now in effect for the Willamette Valley and SW Oregon from noon Wednesday – 10pm Saturday, with Thursday being the hottest day. Excessive Heat Watches are in effect for the Oregon Coast Range & Cascades, portions of North/Central Oregon, and most of Washington.
This extreme heat is due to an extremely strong ridge of high pressure moving over the Pacific NW & Western Canada. Still, this ridge will be weaker than the ridge that brought the unprecedented “Heat Dome” of June 26-28 and will be a bit more progressive, meaning that temperatures at upper levels will be cooler than they were during June 2021. Additionally, we won’t see the same sort of brisk offshore flow with this heat wave that we saw with the one in late June, meaning we won’t have the additional warming effect due to downslope flow off the Cascades into the Willamette Valley and Puget Sound lowlands.
Credit: WeatherTogether Models
The weaker offshore flow also means that this heat wave will have lower fire danger than the late June heat dome, which caused several fires across the Pacific Northwest, including a fire that destroyed the majority of Lytton, BC just a day after it recorded Canada’s all-time high of 121F. However, fuels are drier this time around and the potential for new fire starts west of the Cascades will be very high due to the hot and dry environment. Some models even show thunderstorms over portions of the Pacific Northwest Friday evening on the tail end of this heat wave, and any storms would bring a very high threat of new lightning-caused fires given the critically dry environment.
Temp Forecast:
For Portland, I’m forecasting highs of the low/mid 100s on Wednesday, the mid/upper 100s on Thursday, the upper 90s/low 100s on Friday, and the 90s on Saturday. Things will be significantly cooler up in Seattle – highs should rise from the mid 80s on Wednesday to the mid 90s Thursday/Friday and fall back to the 80s on Saturday. Below are the high temperature maps from the 12Z 8/9/2021 European operational model for Wednesday, Thursday, Friday, and Saturday.
Most models, including the one above, are likely a bit too hot for Friday and Saturday. This is because smoke from California fires is expected to be overhead at the time and block out solar radiation, and models do a very poor job with estimating the attenuation of solar radiation due to smoke and subsequent impact on temperatures. Models could be up to 5 degrees too hot Friday and Saturday for Portland, but we’ll have a better idea when we get within 48 hours and can see the smoke plumes from the HRRR-Smoke model.
Friday Night T-Storms?
On Friday night, an upper-level low will approach the Pacific NW from the southwest. As it does so, it will spread moisture inland at lower levels while cooling temperatures aloft, resulting in increasing atmospheric instability and CAPE (convective available potential energy). Additionally, upper-level lows are associated with rising air, which tends to lead to clouds/precip and, if there is sufficient instability, thunderstorms. The below image shows the precipitable water (amount of liquid water if all water vapor in a column of the atmosphere were condensed) and 500mb heights at 5pm Friday – note the upper-level low to the southwest and the area of enhanced moisture out ahead of this low.
Here is the simulated satellite image at the same time. Those white blobs are thunderheads. The best chance of thunderstorms with this feature is over the Cascades. I can’t rule out a thunderhead moving into the Willamette Valley, but the chance appears to be slim at this time.
Temperatures will cool through the weekend, and by Monday or Tuesday, temperatures may even be slightly below average. If you need any help getting through the heatwave, just put Monday/Tuesday’s forecast on your calendar, and hopefully that’ll give you the strength to get through this week!
One final note before I sign off for the evening – there is an outside chance that temperatures could hit or exceed 108 degrees on Thursday for Portland. If this were to happen, we will have beaten Portland’s previous all-time record high temperature on two separate occasions in the same summer. It’s a testament to the extreme drought over the Western US/low soil moistures, the strength of the heat wave patterns this summer, and the long-term warming of our planet as a whole.
Charlie
