What a wet weekend it has been! Many places across the I-5 corridor have picked up 0.25-0.75 inches of rain over the past 48 hours, but the Olympics and Coast Range have seen much more, with 1-2 inches common. Vancouver Island has been the wettest of all, with two stations on the Southern Vancouver Island picking up nearly 4.5 inches.
April 2020 was the 4th driest on record for Portland, which with only 0.79″ saw its lowest April total since 1956. So this weekend’s rain was not only refreshing, it was critical for alleviating our rapidly-worsening drought situation.
We even saw some snow in the mountains! Alpental, located at a mere 3,140′ in the Central Cascades, saw some sticking snow overnight. Too bad it isn’t open for business; talk about some epic late season powder!
But by Thursday, any fresh snow will be a distant memory as a blistering early-season heat wave takes hold over the area. Friday-Sunday will easily feature our warmest temperatures thus far of 2020, and a few daily records could even fall for spots in Western Washington and Oregon.
Despite the Pacific Northwest’s reputation for long, gloomy spring weather that persists through the summer solstice (ever heard of “Juneuary”?), spring can also feature some remarkable temperature extremes and heat waves. Our sun angles are as high now as they are in early August, so if we have a strong ridge of high pressure over our region to give us clear skies and warm temperatures aloft, we can see temperatures rising well above their summertime norms.
On Thursday, a strong ridge will begin to build over the West Coast and warm air will begin to migrate northward.
Credit: University of Washington
But while Thursday will be plenty warm (highs near 80 for Portland), the real hot stuff comes Friday and Saturday. Taking a look at the surface pressure and 925mb (approx. 2,500 ft.) temps, we can see a “thermal trough” spreading north from the Central Valley all the way through the northern tip of Vancouver Island. Thermal troughs are troughs of surface low pressure that are induced by warm air, as warm air is less dense than cold air. Technically, they should be called “thermally-induced troughs,” but the acronym might be a little too racy for peer-reviewed journals!
The below images show the evolution of the thermal trough from 5pm Thursday through 5pm Sunday. With the 5pm Sunday chart being a 180-hour forecast, the details are certain to change; the thermal trough may weaker-than-modeled or further to the east (I don’t imagine it would be much stronger than the model below already shows!). Still, models have been pretty darn consistent in bringing a thermal trough up the I-5 corridor and giving Portland temperatures reaching at least the low 80s on Friday and Saturday, and potentially even into the upper 80s or 90 if some of the more extreme solutions come to fruition.
Credit: University of Washington
Credit: University of Washington
Credit: University of Washington
Credit: University of Washington
These thermal troughs also induce offshore flow due to the offshore pressure gradients they create. This can result in additional warming, particularly in locations that see strong subsidence from this offshore flow. The foothills of the Cascades/Olympics/Coast Range are most susceptible to additional warming, and while Portland tends not to see much additional warming due to the fact that the Columbia River Gorge is a roughly sea-level passage through the Cascades, we do see an influx of much drier air from the Columbia Basin. The combination of dry air, hot temperatures, and pre-existing drought will result in high fire danger, and I wouldn’t be surprised if a Fire Weather Watch or even an early-season Red Flag Warning was issued for the Willamette Valley.
Lastly, be aware that even though temperatures will be hot this weekend, rivers will be running high and temperatures will be cold (generally 45-55 degrees) from all the snowmelt. These early-season heat waves are dangerous in that they can seduce people into doing water activities that may be much safer during August or September when rivers are much warmer and running lower, so use good judgment and always wear a lifejacket. And of course, remember to adhere to the CDC’s social distancing guidelines… we are all in this together and the more we keep things under control, the sooner we can phase back into normalcy.
I’ll have more on this heat wave as it approaches. In the meantime, have a great week and thanks, as always, for taking the time to read my blog!
Charlie
