July 05, 2010

Upcoming Heat Wave and the Start of Summer

This year it will be very easy to note the start of summer in western Washington---it will be Tuesday, July 7th. We are just about to make the big summer transition and the change will startle you....somewhat like a meteorological slap to the face.

Today, it got to 65 at the airport, when 74F is typical...9 degrees below normal. But for the remainder of the week we will be on the other side of the climatological divide, with temperatures rising into the mid-seventies tomorrow, 80s on Wednesday, and near 90F on Thursday.
Let me show you some of the weather maps and describe a key feature of our heat waves...the west coast thermal trough...an area of low pressure associated with low-level hot air. I have written quite a bit about the thermal trough in scientific papers and in my book, and currently a graduate student of mine is doing a comprehensive study of this key feature of nearly all NW heat waves.
Take a look at surface pressure maps for the next few days. These maps also include near surface winds and low level temperatures (color shading). Temps are in C (20C is 68F, 30C is 86F)
Lets start with this afternoon at 5 PM. Warm over the SW U.S. (red and brown colors!), but yellow (cool) over the NW with higher pressure offshore. There is a huge pressure difference off of N. coastal CA with very high winds...but that is another story. No heat wave for us.

Next tomorrow (Tuesday) at the same time. See the difference, we have gone from yellow to orange. See the tongue of lower pressure moving northward into the Willamette Valley...that is the thermal trough. Why a trough? Because warm temps are associated with less dense air which results in lower surface pressure. Why is the air warming? Two reasons: high pressure aloft is producing general sinking and warming of air (by compression) is the first. And there is easterly flow developing over the Cascades, which produces enhanced sinking (warming) on the western slopes.
Here is Wednesday afternoon: we are now in pinks and the thermal trough is right over us, with easterly flow over the Cascades. We could get to the upper 80s with this pattern, especially over the eastern PS suburbs. Portland will be very, very warm with this....well into the 90s.

And then finally Thursday afternoon....very warm over the whole region. Certainly upper 80s, lower 90s for the PS region away from the cold water. But something subtle is changing...pressure is falling more over eastern Washington. Might the thermal trough jump the Cascade, leading to a cool down on Friday or Saturday? Keep tuned!


  1. Ugh. We jump from June in Juneau to August in LA

  2. Excellent tutorial Professor Mass. :)

    I'm especially interested in the part about how a thermal trough dissipates and the "natural air conditioning" returns with the cool onshore flow at the end of these heat waves.

    Going back to your book for the answer :)

  3. Just a note that Tuesday is the 6th, not the 7th.

  4. Hope your Grad student is including air quality impacts during our summer stagnations. Lot's of profiler data. With hall the heat you don't immediately expect a trapping inversion and yet it's there. Temps aloft drive the whole thing.

  5. This seems like a textbook example of a summer warmup, showing the gradual eastward progression of the thermal trough. If the high is a 'Rex Block", the above normal temperatures could be with us into next week and beyond.

    However, the natural AC system typically shows up during this weather pattern - last summer for that one week was an exception.


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