Monday, December 19, 2011

A quick note on the southern plains blizzard

Heavy rain is falling over central Oklahoma, southern Kansas and north Texas right now as a powerful surface low is deepening over the Texas Panhandle.  Here's the latest map of surface observations from the area.

There's a lot going on with this map, so I brought it into Powerpoint and have done a quick analysis of what I feel are the key features going on here.
This is a classic pattern--you can see that a large area of moderate to heavy snow has developed to the northwest of the surface low where winds are out of the north at 30-35 knots sustained.  Definitely a winter storm.  As this low deepens and moves eastward, points further east are also going to start feeling the winter punch.

So what is driving the development of this cyclone?  Notice that I've drawn a cold front on the eastern side of the map heading off to the northeast.  This cold front is associated with another surface low pressure center way up over central Quebec in Canada.  You can see that in this surface map over the entire continental US from this afternoon.

This cold front is associated with (like all fronts) a baroclinic zone--a zone where there is a large horizontal gradient in temperature.  Along this cold frontal boundary (which runs from northern Michgan down through the Chicago area and back toward the developing surface low in Texas), there is indeed a temperature gradient--from the low 50s in central Illinois back down to the low 30s in Iowa.  That colder air to the north behind the front is associated with a strong high pressure center, here analyzed over the Dakotas.

Remember that surface cyclones in mid-latitudes (like those we see over us) rely on horizontal temperature gradients as their source of strength.  They feed off of these baroclinic zones.  All they need is something to get them going, some sort of lifting mechanism to help support pressure falls at the surface.  And--what do you know--we have a shortwave trough aloft that's moving out of the desert southwest today (as per this 18Z RUC analysis this afternoon at 500mb):

So we have a shortwave trough aloft moving over a low-level baroclinic zone (that pre-existing cold front associated with the weakening low in Canada).  Excellent ingredients to spin up a cyclone.  Notice that there is a small, cyclonically curved jet streak on the southern and eastern side of this low aloft.  The exit regions of such jet streaks are favorable places for upward vertical motion, which will help lower the pressure at the surface.  Notice that the exit region of this jet streak is right over the location where the surface low is deepening over north Texas.  This is not a coincidence...

All of the major models are in pretty good agreement that the surface low will move eastward over Oklahoma over the next 24 hours (at least, the GFS, ECMWF and the NAM).  And, fortunately for our forecasters, that looks to be what is happening.  Here's a map of the previous 3-hour pressure changes.  You can see that there's a concentrated area of pressure falls in central and eastern Oklahoma.  Since the pressure will tend to fall as a low pressure center approaches, this gives us a good indication that the low is headed that way:

Going back to the surface map above over then entire United States, you can see that the high pressure over the Dakotas associated with the cold air behind that dying cold front is helping to generate a strong pressure gradient between the high pressure center and the low pressure center in Texas.  This strong pressure gradient is helping to really speed up those winds on the northern side of the low, contributing to the blizzard-like conditions.

So, we have a big weather event on our hands.  People in Oklahoma, Kansas and Texas should heed all blizzard and winter weather warnings and avoid travel if at all possible.  I'll probably post again tomorrow on this storm as it continues to move east into the Mississippi Valley.  A complex forecast for precipitation type is setting up across the upper midwest as a variety of factors come together to make forecasting difficult.  It will be interesting to watch.

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