Wednesday, July 20, 2011

The Ridge Continues...

As most people in the central US are well aware, it has been extremely hot and humid for the past several days.  Model forecasts indicate that we'll be staying this way for a little while longer, too.

This morning's 500mb analysis shows the ridge that has been promoting these hot, stagnant conditions rather clearly:
GFS 500mb analysis at 12Z, July 20, 2011.
There is pronounced ridging with the main axis of the ridge centered over the Mississippi River valley.  Troughing is evident over the Pacific Northwest, where cloudy skies and cooler-than-normal temperatures offer one of the few places in the country to escape the heat.

It looks like that trough area in the Pacific northwest will try to move inland this week, causing some fluctuations in the structure of the ridge.  Here's the GFS 48-hour forecast for 500mb heights on Friday morning:
GFS 48-hour forecast of 500mb heights and winds at 12Z, July 22, 2011.
The sharpness of the ridge is diminished and the trough over the Pacific northwest seems to be deepening and making progress inland.  However, this progress appers to be shortlived.  Here's the 96-hour GFS forecast for 12Z Sunday:
GFS 96-hour forecast of 500 mb heights and winds for 12Z, July 24, 2011.
It looks like a shortwave is forecast to advance across the northern tier of the US.  This somewhat modulates the ridge strength, but doesn't dig into the southern plains or southeast where it would really have to go to do anything about the ridge.  The resulting 500mb pattern looks very "zonal"--there's a lot of east-west motion as opposed to strong north-south motion.  Also, the iso-height lines (the dark contours) run mostly east-west.  This is all typical of a "zonal" pattern.  Zonal patterns are highly unpredictable, particularly at forecast times that are far into the future.  Since small, shortwave ridges and troughs can make all the difference in precipitation (and temperature) forecasts, a pattern that's very "flat" is susceptible to small perturbations.  Thus, this forecast is very uncertain.

Still, looking further into the future, there's enough confidence that the Climate Prediction Center outlook for 8-14 days from now still has above-normal temperatures forecast for pretty much everywhere in the continental United States except for the Pacific Northwest:
CPC 8-14 day temperature probabilities as of July 20, 2011.
Another function of this ridge is to direct storm systems and moisture flow in a clockwise manner around the periphery of the ridge.  With the ridge centered over the southen plains and lower Mississippi valley, it's no surprise that that area (in the middle of the ridge) would be expecting below normal precipitation.  In contrast, areas around the perimeter of the ridge will have increased chances of seeing storms and probably have higher chances of precipitation.  This explains the CPC's 8-14 day precipitation outlook:
CPC 8-14 day precipitation probability as of July 20, 2011.
In then center of the ridge (Oklahoma and Texas) the CPC is calling for below normal precipitation.  However, on the periphery of the ridge from Arizona through the Front Range of the Rockies and into the northern plains, the probability for precipitation is above normal.  Unfortunately, this is the same sort of pattern we have been seeing for the last several days.  Places that really need the moisture (like Texas and Oklahoma) still don't look to get much of any as long as that ridge is in place.  However, places that have been repeatedly hit by MCSs over the past few days (like the Dakotas, Minnesota and northern Wisconsin) will probably continue to see rounds of storms.

So, in summary, it looks like more of the same for the next few days, even weeks.  The secret to getting out of the heat?  Head to Seattle... 

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