Make sure your fans are working.
Today, temperatures moved up into the upper 70s in western Washington and the 80s in the Willamette Valley (see graphic), but tomorrow temperatures will surge by around 10F, with highs reaching the upper
80s to low 90s in western Washington and way into the 90s in the western Oregon interior. In fact, the Portland NWS office has a heat advisory in place for tomorrow!
Today's high resolution WRF forecast for 5 PM tomorrow, shows the high 80s on the eastern side of the Sound and super warmth from Portland south. It really is a warmer world during summer in the Willamette Valley. And NW Washington, open to the Pacific through the Strait, will be decidedly cooler.
The UW's probcasts system, which makes use of ensembles and lots of statistics, is going for an amazing 92F around the University of Washington on Tuesday (see graphic). This system is generally most skillful during warm-ups like this one.
This heat wave is associated with high pressure building east of the western Washington/Oregon and the development of offshore flow, which brings warm continental air from the interior and downslope flow on the western slopes of the Cascades (downslope air warms by compression).
The offshore (easterly) flow has already started aloft. Here is what we call a time-height cross section of winds (blue barbs) and temperature (red lines). The y-axis is height in pressure (hPa), with 850 (700) hPa about 5000 (10,000) ft. Time in GMT/UTC is on the x-axis with time increasing to the left. Southeasterly flow developed aloft today. You see the low-level northerlies? Very typical when the lowest pressure is south of us and the western interior gets warm.
Warm, dry easterly flow produces very good visibility...and the mountains have been very distinct(see Rainier below).
The forecast surface maps shown below (for 5 PM Monday and Tuesday), indicate that the warming is associated with the northward movement of the "thermal" trough...an elongated area of low pressure that extends northward from CA.
That thermal trough will not be sticking around. Tuesday night it will move east of the Cascades, with cooler air following behind. Take off 10 degrees on Wednesday. And, by the way, July 4th should have excellent weather.