In early June we had two very hot days, 97 & 98 degrees in Portland, the hottest of summer so far.
We have a repeat coming up tomorrow & Wednesday. I get the feeling these two hot spells will be the “book ends” of Summer 2019.
The weather pattern is the same; a strong upper-level ridge overhead brings us a hot airmass. But the “icing on the cake” is the switch to an offshore wind flow. Today we lost all cooling coming in from the Pacific Ocean; light easterly flow has developed overhead.
Tonight a much stronger easterly flow develops with gusts to 40 mph in the western Gorge and exposed ridges along the west slopes of the Cascades. Expect a that breezy easterly wind to push across most of the metro area by noon, and all the way to the coastline too. Historically this is our hottest weather pattern. Take a look at the surface weather map tomorrow at 5pm from the UW-WRFGFS model. I’ve drawn in the “thermal low” which is the area of lowest pressure. Wind moves toward that low
This is a classic hot weather setup west of the Cascades. By Wednesday at 5pm the thermal trough is shifting over/east of the Cascades. Weak onshore maybe arriving in the central/southern Willamette Valley at that time. That said, often models are a few hours too fast moving that trough east so we’ve kept the high temperature forecast the same for both days.
A big push of ocean air pours inland Wednesday evening and that ends our mini-heatwave. We’ll be somewhere down in the 80s on Thursday and that continues through at least the first part of Labor Day Weekend.
Do you want to experience hot weather at the coastal beaches? That thermal trough position through mid-afternoon Tuesday is perfect for one hot day out there. If easterly flow makes it all the way to the beaches, then it can be just as hot as inland spots. But I suspect the northeast morning wind turns north-northwest mid to late afternoon which should cap off the central coastline around 80 degrees and keep temps below 95 on the north coastline.
Let’s talk clouds and showers. Our typical onshore flow returns Thursday and beyond and we’ll get clouds at times out of that. But what about rain showers? Models are hinting a weak disturbance could bring a few showers overhead during the daytime Thursday. We now have access to the hourly runs of the ECMWF model 4 times a day which is helpful. The 18z run, just like the 12z, showed a few light showers popping up Thursday. So just keep in mind we might see sprinkles here/there on that day.
Chief Meteorologist Mark Nelsen