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Update on Forecast Reasoning
by Jim O'Donnel
Tuesday, February 01, 2011

This is just a quick update to give you all my forecast reasoning after reviewing the latest run of model output for the Thursday night/Friday winter weather event expected in Southeast Texas.  I'm going to refrain from getting too technical here but still try to give you the gist of what the models indicate.


Before I get to the present models, keep in mind what it generally requires for it to snow in Southeast Texas and why it normally doesn't snow here because of the lack of one or more of these necessary ingredients:


1.  An Arctic air mass firmly in place that produces a layer of very cold air near the surface;


2.  Sufficient "depth" of the Arctic air mass where the temperature remains at or below freezing from just above the surface upward for about 8,000 to 10,000 feet in the atmosphere;


3.  Sufficient moisture overriding the layer of cold air at the surface.  This moisture source can either be from the Gulf of Mexico or the Pacific or a combination of both;


4.  No inversion layer (warmer air) aloft.  Warmer air aloft is more favorable for a freezing rain or sleet event than snowfall.  Sometimes, we start out with freezing rain and as colder air aloft "deepens", the precipitation transitions into sleet (ice pellets) and then snow.  Sleet often changes to snow due to evaporative cooling as it initially falls through a layer of air that is above freezing but cools to at or below freezing.


Now to the present situation and what I expect will happen.


What we have today is an unusually deep layer of Arctic air over Texas.  Arctic air masses in Texas are normally quite shallow because they have usually modified a great deal from their source region after traversing the many thousands of miles it takes to get here.  Sometimes, an Arctic air mass is only a few thousand feet deep with much warmer air above the surface cold air.  This is why we normally see freezing rain or sleet rather than snow in Southeast Texas.  Again, today's Arctic air mass is very deep.  The temperature is essentially below freezing from near the surface all the way upward for at least 10,000 feet in the "column" as meteorologists often refer to a vertical profile of the atmosphere.  This depth of cold air should maintain itself over the next few days as shown on "thickness" prognostic charts.


By Thursday, a strongly positive tilted (think of a "/" (slash) vs a "\" (backslash)) upper level trough of low pressure will eject out several upper air disturbances that will move west to east across Texas.  This will draw copious amounts of Pacific moisture into Texas and produce lift (upward vertical motion).  This will act as a triggering mechanism for precipitation to develop.  As the core of the main upper low gets closer, the column will cool even further and the precipitation will increase in coverage and intensity.


Meanwhile, a surface low pressure system will develop in the northern Gulf of Mexico well offshore from the Texas coast.  This surface low will circulate Gulf moisture inland with the Pacific moisture adding the necessary "fuel" for our snow event.  The key is that the surface low must form far enough offshore to keep us in its colder "wraparound" (backside) sector...while remaining close enough to the Texas coast to produce a plentiful moisture supply.  This is why the exact track of the low (give or take 100 miles or so) is quite critical on dictating just how much snow we get...or don't get.


In general terms, the forecast models usually under forecast the intensity and duration of winter events in Southeast Texas.  This was clearly the case during the 2004 Christmas snow event which unexpectedly clobbered parts of the upper Texas coast between Victoria and Galveston with a foot of snow.  This is why forecasting snow in a place that rarely sees it is one part science and about four parts voodoo.  You'd like to be right, but if you're wrong, you can look absolutely ridiculous.


What I look for is run to run model consistency coupled with a real-time scenario that is comparable to previous winter weather events in the local area.  That is why it's worth mentioning again that this current setup reminds me a great deal of the 2004 system.  In this type of pattern, as long as the sufficient depth of cold air remains in place, the higher moisture near the coast will usually favor higher snowfall totals along and just inland from the coast rather over the more inland areas.  That is what I think will happen in this case.  It's essentially the same type of setup as a "nor'easter" along the Atlantic seaboard.


Earlier today, I mentioned 1 to 3 inch accumulations with possible 3 to 6 inch totals in a few places.  I still think that looks plausible based on the latest model run.  But, admittedly, this is the part of my forecast that has the greatest chance of going completely awry.  If the moisture needed is inadequate, we will only get a dusting of snow or an inch at best.  On the other hand, if the moisture is greater than now expected, we could get absolutely hammered again as we did over 6 years ago.  For now, I think the most prudent course of action is to go with something in the middle and that's what I've done.


Please keep in mind one more thing.  If the Gulf of Mexico surface low develops too far west (in other words, just offshore), then snow will probably be confined to northern areas of Southeast Texas...with sleet or a mixture of rain and sleet more likely in the Houston/Galveston area.  I don't expect that will be the case this time but I'm just apprising you of another possible scenario.  Again, it's very tricky to get all of these different ingredients to come together at precisely the same time.


I'll keep you posted on any important new developments as we get closer to the actual event.


Jim O'Donnel 

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