The Upcoming Week AND What Have We Been Missing?

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I just took a look at the latest model output and it appears that we have a fairly decent week ahead.  In fact, the Climate Prediction Center's 6-10 day forecasts have a different look than we are accustomed to:
Higher odds for warmer than normal in the West (including us), near normal over the middle of the U.S., and cooler in the East.

The persistent West Coast troughing has weakened and we have fairly normal weather for the entire week.  That means dry and temps getting up into the mid-70s.  Just perfect for painting your house (which I just did).

But something has been missing this year.  Something that many of us miss: some good warm spells, with substantially above normal temperatures (like the 80s!! and dare I say it?...the 90s).

Here is the plot of temps versus normal for the last four weeks at Sea Tac:
We got to 80F only twice and never got to 85F.  Most days we  fell short of the normal max.  So why have we been plagued with lack of warm spells?

The answer is that we simply are not getting the right set up--big upper ridge over the west side of the Cascades, surface high pressure to the east, and a thermal trough at low levels moving up the Oregon and Washington coasts.  This is particularly ironic for me, because my student Matt Brewer and I have a project, funded by the U.S. Forest Service to study thermal troughs and to learn how to predict them better.  Thermal troughs are often associated with wildfires.

Want to see what a good thermal trough looks like?  Here is a forecast of one from almost exactly a year ago.  The solid lines are isobars (lines of constant pressure) and the shading denote lower-atmosphere temperatures.  You see the low pressure extending to the WA coast?  Starting over Nevada?  That is it.  And high pressure to the northwest of us over southwestern BC.

We haven't seen such a pattern this year.  Want to see a super thermal trough?  Here is the one with our big heatwave during July 2009:

The low pressure area extended to Vancouver Island!

Thermal troughs, or thermally induces pressure troughs, typically extend up the coast from CA during our heat waves.  They are inevitably associated with offshore flow that keeps the cool marine air away from us.  They are the tail of about a big dog...the dog being large scale high pressure that sets up the offshore flow.

Anyway, lets hope we get a nice thermal trough before the summer is out.  Warm when one wakes up, perfect at night for an outdoor bbq or an evening stroll. 

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