Record Heat This Weekend

Hi all, I’m back after a hiatus! We really haven’t had much to talk about and I’ve been busy spending lots of time with my family, so blogging and weather updates have fallen by the wayside. However, we’re expected to see what could be the strongest heat wave of 2020 this weekend, and such blistering heat definitely deserves a few words.

Blog Highlights:

• Saturday should be the hottest day of the year for Portland. We’ve already hit 100 twice this year, but I think Saturday could be 101-102. With low humidities, breezy easterly winds, and these extremely hot temperatures, fire danger is extremely high and Red Flag Warnings are in effect.
• The hottest weather will spread north into Western Washington on Sunday, with Seattle likely reaching the mid-upper 90s. Portland is expected to be slightly cooler than Saturday, with highs around 100. However, the Oregon coast will be about 10-15 degrees cooler than Saturday due to a switch to onshore flow.
• Afternoon/evening thunderstorms will form over Southern Oregon on Saturday and spread northward on Sunday along the Cascade Crest and points east. There is the potential for some of these thunderstorms to drift into the far eastern Portland metro Sunday night, but the vast majority of storms will stay at or east of the Cascades. Given the extremely dry fuels and hot temperatures, we’ll likely see some new fires Sunday/Monday from lightning strikes. Any storms that do impact the Portland metro should be elevated and have little/no precipitation.
• Temperatures will slowly decline next week but are expected to remain above-average. Portland is expected to stay dry for the next 10 days.

Take a look at the current watches/advisories/warnings issued by the National Weather Service across the Western US. I don’t know if I’ve ever seen so many excessive heat warnings across the entire Western US. Excessive heat warnings span all the way from Mexico northward to Canada.

This extreme heat is courtesy of an extremely strong ridge of high pressure building over the Western US. These ridges are associated with warm temperatures for two reasons; first, they displace tropical or subtropical air from warmer, lower latitudes towards higher latitudes, and second, the ridges themselves are favored areas for air aloft to sink towards the surface. As the air sinks, it warms further via a process known as adiabatic compression.

Strong, summertime ridges are a pretty common occurrence over the Western US and frequently bring triple-digit heat to the Intermountain West and 110+ degree temps to the Desert Southwest while the West Coast sees onshore flow, sunny skies, and only slightly-above-average temperatures. For a true West Coast heatwave to occur with 90s in the Puget Sound lowlands, the lower 100s for the Willamette Valley, and the upper 100s for the Central Valley, you need offshore flow to prevent any moderating influence from the Pacific. To get offshore flow, the ridge has to be far enough west that the clockwise circulation around the ridge will direct a tongue of hot air the I-5 corridor and cut off any moderating, onshore influence.

The model chart below shows the predicted 500mb (approx. 18,000 ft.) height anomalies at 5pm PDT Saturday. Note the huge ridge over the entire West and the smaller upper-level trough offshore. This ridge is close enough to the coast that it is able to direct a thermal trough of air into the Pacific Northwest, giving many of us what will likely be our hottest days of the year on Saturday and Sunday.

This ‘tongue’ is known as a “thermal trough,” as the extreme heat at the surface causes the atmosphere to expand and creates relatively low pressure at the surface. As the 5pm Saturday model chart below shows, the thermal trough will extend from the Central Valley NNW to the Oregon and Washington coasts, creating relatively lower pressure along the Coast/Coast Range and higher pressure inland. This will create offshore pressure gradients and winds, with hot, dry, continental air from Eastern Washington/Oregon making its way into Western WA/OR and further warming as it descends the west slopes of the Cascades.

Offshore gradients should be moderate on Saturday with perhaps a peak Portland-Hermiston (KPDX-KHRI) gradient of -3 to -4mb Saturday afternoon/evening. This could drive 10-20 mph winds through the Columbia River Gorge, with gusts of 25-30 mph possible near exposed locations at the western end of the Gorge such as Crown Point, Rooster Rock, and Corbett. However, unlike our wintertime easterly wind events, these winds will not be limited directly to the Gorge. Some of the RAWS stations over the Southern Washington/Northern Oregon Cascades and the station at Timberline Lodge could also see 25-30 mph gusts.

Sunday will be similarly hot to Saturday for Portland but much cooler for the coast as the thermal trough moves inland and flow turns onshore there. Note the coastal southerly winds in the model chart below at 5pm Sunday and compare them to the coastal northerlies in the 5pm Saturday chart above.

Some thunderstorms are expected to develop over the Cascades and points east Saturday night-Monday, with most storms firing up in the afternoon as atmospheric instability increases and weakening at night. The most recent 00Z WRF model shows some of these storms making their way into Portland Sunday night/Monday morning. There’s still pretty low confidence in this solution and the National Weather Service forecast discussions mention thunderstorms over the Cascades but don’t discuss the potential for them to drift west of the Cascades.

Temperatures will slowly moderate next week as the thermal trough slowly moves east of the Cascades and onshore flow returns to Western Washington and Oregon. However, this ridge of high pressure will remain over the Western US, allowing warmer/sunnier/drier-than-average weather to persist in spite of this cooling trend. Temperatures will remain extraordinarily hot over California with temperatures hitting the triple digits for all of the Central Valley and much of Southern California, and the Desert Southwest will see widespread 110+ degree highs next week. August is usually one of the wettest months of the year for the Desert Southwest (especially eastern Arizona and New Mexico) due to monsoonal thunderstorms, but they’ve been tinder dry and even hotter than usual over the past month and that trend doesn’t show any signs of letting up soon.

Finally, here are some heat tips for you and your pooch courtesy of your local Portland National Weather Service office. Enjoy your weekend and please be safe!