Portland hit 99 degrees on Saturday, a pretty hot end to August! It looks like we might top that number this Thursday and possibly Friday as well.
A strong pressure of the sea air kept all the places on the west side trapped in grey today, with maximum temperatures of only about 21 degrees, that’s almost 15 degrees less in just two days. This happens at the end of the warm season in our climate; we tend to see more fluctuations in temperatures. Not only from day to day, but also from day to night due to the longer nights.
We’re calling Thursday and Friday FIRST ALERT WEATHER DAYS because both days are expected to be particularly hot. Additionally, temperatures above 100 degrees Fahrenheit are rare in Portland during the first week of September.
Note that we experienced the latest 100-degree temperature in Portland on Thursday (5th). On that day in 1944, it was 101 degrees. This Thursday, we expect 100 degrees.
Over the next two days, a strong ridge of high pressure will quickly form over the west coast. On Wednesday, its center will be above us.
Highs are a little higher and the atmosphere is a little warmer/hotter than it was late last week. For the geeks, 850mb temperatures are forecast for Thursday and Friday afternoons between +25 and +26 (C). The absolute September record for 850mb temperatures is 28.2 degrees, set the day PDX reached 105. On September 2, 1988. That’s one of only two days we hit 100 degrees in September. I remember that day because I was shoveling hot carrot/corn mush that night on the night shift at a cannery. It was a well-paying college job…but tough.
I digress. The point is that the atmosphere doesn’t look THAT hot, but almost. It’s possible that Thursday will be the hottest September day since I graduated high school…a long time ago!
The ridge is a little weaker, but still above us on Friday
Under that ridge, all models are forecasting a thermal low pressure system to form over the Willamette Valley on Wednesday, then shift toward the coast for at least the first half of Thursday. That means another wave of east winds coming in from the Gorge and Cascades. Thankfully, it won’t be a strong east wind, but it will be very similar in strength to the one we experienced last Thursday. Gusts were generally in the 20-30 mph range in the urban area and 35-50 mph in the western Gorge. Fire danger will be higher this time as fuels continue to dry after the late August rains. That’s our forecast for Portland for the next 7 days. If that’s true, Thursday’s 100 will be the last 100 ever. If we somehow get consecutive 100 degree days? That would be a first for September in Portland
During this event, humidity will again be low due to the dry easterly wind, especially on Thursday and Friday.
We regularly use the GRAF model for our forecast graphics. This model is created and operated by TWC (The Weather Company). It has generally predicted 2-4 degrees too cool during heat waves this season. During the event late last week, it was almost right. Tonight’s heat is surprisingly hot, with temperatures above 100° predicted for Thursday along the I-5 corridor. That would be the highest temperature Portland has seen this late in the season. It also projects that easterly winds could stick around the northern Oregon coast long enough to push temperatures above 90 degrees in some spots. Here’s the forecast for Thursday at 3 p.m. You can’t see it, but it expects a pressure gradient of about 4 millibars through the canyon this Thursday morning; pretty similar to what we experienced last Thursday.
The model’s text output shows the very dry air and easterly winds that moved into the city on Thursday. I keep these text products here: https://www.marknelsenweather.com/public_html/GRAF_ECMWF_Images.html
That’s it for tonight. Enjoy the sunny, warm weather on Tuesday!
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