Today has been a real nice late summer day. We made it up to 84 in Portland (as of 6pm), above the average high of 80 for this date
Yet it feels a bit different outside the past couple of days doesn’t it? I think that’s due to several changes we see every late August. First, the sun angle is getting lower, the midday sun just doesn’t feel as intense as it did in June and July. Second, days are getting shorter. In late August the loss of daylight begins to accelerate. In fact we’ve lost two hours of daylight since the summer solstice in late June.
Of course the longer nights mean we’re starting cooler in the mornings. We’ve dropped into the mid 50s the past two nights in Portland, and have seen 40s for a few nights in outlying areas as well. Fall is getting closer!
Saturday will be a cooler day with more cloud cover, definitely a September feel for one more day. That’s due to a very weak weather system dying over the Coast Range as it moves onshore right around sunrise. Sunday turns sunnier and warmer as we onshore flow weakens. It will be the sunniest day of the weekend.
But the main story the next week will be a hot spell (heat wave might be too strong wording, depending on how long/intense the hot weather will be). Models have been doing a good job showing upper-level ridging (high pressure) developing right over the Pacific Northwest beginning Monday. Right now we’re in weak westerly flow, very warm to the south and cooler than average in southern Canada.
By Monday the ridge has popped up right over us
It’s still there, although a bit weaker, next Thursday
Models are warming 850mb temps to the mid teens Monday, then lower 20s both Tuesday and Wednesday afternoons. That’s the warmest forecast we’ve seen since the early June heat. And it appears for the first time since June we’ll get a breezy offshore flow to go with it. Check out the surface map for Monday afternoon
Surface high pressure is centered to our north, a gusty north wind is blowing down the coastline and Willamette Valley. And it appears a very light east wind is trying to develop in the Cascades and Gorge. A “thermal trough” is beginning to develop west of the Cascades. That’s an area of low pressure that develops due to easterly wind going over the Cascades; lower pressure develops on the “backside” of the mountains (west side) in this setup. By Tuesday afternoon, surface map shows we are in a classic east wind hot weather pattern, which we haven’t seen since that early June heat. Look at the thermal trough stretching north from Redding all the way to Puget Sound. In fact it appears to briefly get out to the coastline at some point earlier Tuesday. This means that may be the day the northern Oregon coast reaches into the lower 90s as well. Everything has to work out perfect for that to happen.
Regardless, this would be a very gusty east wind day in the western Gorge, Cascades, and metro area since the easterly flow extends up to around 4,000′ or a bit higher. Temperatures in the lowlands should soar that day. My chart says August patterns with this setup have pushed us as high as 98 or 99. More likely 94-98 degrees. By Wednesday the trough is weakening and onshore flow is about to arrive in the western valleys of Oregon and Washington. The 95+ temps will probably be for only Tuesday/Wednesday, then back to 90 or a few degrees by Thursday
Sure, it looks hot next week, but two factors make this one a bit different:
- Relative humidity will be very low with the easterly wind. This won’t be a humid setup, at least through Wednesday
- Long nights at the end of August mean we still drop down to 60 in the city, and well down into 50s outlying areas even after a 95 degree day. Much better than June through early August.
By the way, there’s no rain in sight through at least the early part of Labor Day Weekend. The ECMWF model ensembles only show a few members trying to bring in showers at some point during that weekend.
I have tomorrow off and then will be back on all next week Sunday through Saturday. I’ll keep you updated as we go through the week.
Chief Meteorologist Mark Nelsen