Monday, December 20, 2010

A low with no support

Looking at today's national radar mosaic, we can see that heavy rains are continuing in southern California.  However, what's new is this big swath of light precipitation (think snow) over much of the northern plains and the upper midwest.
Fig 1 -- NEXRAD base refelctivity mosaic for 1738Z, Dec. 20, 2010. From
Some media outlets are calling this particular storm an "Alberta clipper".  But, take a look at the current surface map--does it really look like this is coming out of Alberta?
Fig 2 -- Contoured pressure and shaded moisture at the surface at 17Z, Dec. 20, 2010. From the HOOT website.
The primary low pressure center seems to be over eastern Colorado or western Kansas.  That's a good spot for what we call lee-side cyclogenesis, not necessarily an Alberta Clipper type storm (though lee cyclogenesis plays a role in the formation of those, too).  When strong westerly winds aloft are forced over a mountain range, the air is more or less "compressed" between the mountain tops and the tropopause as it moves over the highest peaks.  It then "expands" again on the other side as it comes down the slopes.  When air expands like that, its pressure falls (you have the same amount of air taking  up more space, so the internal pressure is smaller).  We see this all the time on weather charts--the eastern downslope side of the Rockies is a favored location for storm development.  There's also another factor about how any vorticity is going to be stretched as it descends down the mountain slopes as well, and stretching vorticity increases the relative vorticity and you get another reason why lows "spin up" on the lee side of mountains.  But that's the subject for another blog post...

Of course, because we do a pressure correction to mean sea level in our surface pressure maps, sometimes that messes up our pressure field in the mountains.  So just to be sure the low we're seeing above is an actual surface low and not just some natural error due to there being lower pressure at higher terrain, let's look at a surface pressure change map for the past three hours.
Fig 3 -- Surface pressure changes (contoured) over the previous three hours at 17Z, Dec. 20, 2010.  From the College of DuPage website.
We see a broad area of weak pressure falls over the central plains with a rather strongly-concentrated area of pressure rises over eastern Wyoming.  This couplet of pressure rises and pressure falls does suggest a low pressure center halfway in between, right where we expected it to be.  It also suggests the low is moving in a direction connecting the two maxima in pressure rises and falls, so this would imply the low is moving more or less ESE across Kansas.  What's interesting, however, is the difference in magnitude between the pressure rises and the pressure falls.  The maximum pressure falls only seem to be at most 1 mb in the last 3 hours.  However, there are places behind the low where the pressure has risen 5 mb in the last 3 hours (according to these contours).  If there is more pressure rising behind the low than falling ahead of the low, it suggests that this low is weakening--that in the future it won't be as deep as it used to be.  What could be causing this?  Let's check our upper-air support.
Fig 4 -- 300 mb wind and geopotential height analysis at 12Z, Dec. 20, 2010.  From the HOOT website.
Remember our surface low was probably around northern Colorado about this time (12Z).  Well, that puts it almost right underneath that large ridge whose axis stretches down the high plains.  Ridges are usually associated with high pressure and large-scale subsidence--NOT good for low pressure formation at the surface.  And the jet streaks?  If anything, the low pressure center was under the right exit region of that more-or-less straight jet streak in the western US (the same jet streak we saw helping to usher in all that moisture to California the other day...).  The right exit region of a jet streak is normally associated with convergence aloft.  Convergence aloft leads to downward motion and an increase in pressure at the surface.  No wonder our low is weakening--it has virtually no upper-air support.

But maybe conditions will improve?  Let's look at the 24 hour NAM forecast for 300mb winds.
Fig 5 -- 24 hour forecast from the NAM model of 300mb winds and geopotential height at 12Z, Dec. 21, 2010.  From the HOOT website
If anything, the situation has become even worse--there's now a broad ridge across basically the entire country with an anti-cyclonically curved jet streak stretching from the southwest and into the northern plains.  The only favored location for cyclogenesis in this pattern would be off the cost of California somewhere.  Sure, such strong westerly winds over the Rockies may spin up a few surface lows over the high plains under these conditions for the reasons explained above.  But the chances of these lows intensifying or bringing strong winds and really heavy snows is pretty low.  As further evidence of that, here's the 24 hour forecast of surface pressure from the NAM:
Fig 6 -- 24 hour forecast from the NAM of mean sea level pressure (contoured) , temperature (shaded) and winds for 12Z, Dec. 21, 2010.  From the HOOT website.
What happened to our low?  The main part of it went south into eastern Oklahoma (probably due simply to conservation of potential vorticity--another topic that deserves its own blog post).  However, note how it hasn't really strengthened at all.  Our minimum sea-level pressures right now are around 1000mb in the center of the low--here, 24 hours later, the minimum pressure contour is 1012 mb.  No strengthening there.  Some surface troughing did spread north in the model into northern Minnesota and Wisconsin.  It looks like there may be some interaction with the upper air pattern trying to spin something up there (after all, that is under an "exit" region of a jet--though I think the jet is curved the wrong way for this) but it's difficult to really tell.

So yes---there will be some snow with this low as it slowly moves out over the plains.  But winter storm warnings for heavy snow are the worst you'll see.  No blizzard warnings or high wind warnings or anything like that.  This low simply has no support for it to grow...

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