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Why did our snow forecast crash and burn?
by Jim O'Donnel
Saturday, February 05, 2011

If you're a meteorologist in Southeast Texas, your new address is likely a dog house.  Everyone wants to know what the heck happened to the snow and why we got surprised with so much more ice than was originally expected.

 

First, I hope that you understand my explanation below is not intended as an excuse.  The forecast wasn't just a little off...it was drastically wrong.  That's just the plain truth and there's no way to sugar coat it.  I am every bit as guilty as the government and TV forecasters in that regard and I take no pleasure in having to admit how fouled up my projections were.  Like them, I felt we had a very favorable setup for a significant snowfall throughout Southeast Texas and only a very limited area near the immediate coast would receive any significant ice accumulation.  As we all know, ice was the main event over the vast majority of the area and the greatly anticipated snowfall only materialized at a few sparse locations on the far western and northern periphery of our region.

 

Now that I've eaten a healthy dose of crow, I'll try and explain what happened.

 

Early this week, forecast models correctly projected a large and unusually deep Arctic air mass would plunge southward into Texas.  That part of the forecast certainly panned out.  Then, the models suggested moisture aloft from both the Pacific Ocean and Gulf of Mexico would begin to override the frigidly cold air at the surface.  Meanwhile, a potent upper level disturbance over northern Mexico was forecast to move eastward across Texas promoting strong upward lift and creating a triggering mechanism for wintry precipitation to form.  Finally, a surface low pressure area was forecast to develop over the central Gulf of Mexico.   The counterclockwise flow around this feature would help transport even more moisture back into Texas while maintaining a northeasterly component of wind across our area maintaining the cold air mass in place at the surface.  Everything appeared to be falling into place for a significant winter weather event.

 

Accordingly, the forecasts reflected a healthy snowfall with 1 to 4 inch accumulations.  Winter storm warnings were issued and precautions were taken.  Up to 1/4" ice accumulation was forecast along the coast.  While motorists dreaded the prospect of hazardous driving, young children were eagerly looking forward to seeing their first snow.  As late as Thursday evening, this still looked quite plausible.

 

If you followed my forecasts here on the Guidry News Service over the past few days, you may recall that I was concerned about the possibility of a warmer layer of air aloft (called a temperature inversion) nosing into the cold air.  The importance of this cannot be overstated.  While the vertical profile of the atmosphere (called the "column") over us was at or below freezing up to at least 10,000 feet, a small inversion layer that initially developed around 5,000 feet expanded to around 8,000 feet as the night progressed.  Even though the temperatures in this inversion layer were still quite chilly (around 3 to 6 degrees Celsius), they were still above freezing (>0 degrees Celsius) so they were warm enough to melt the snowflakes that had developed higher in the cloud layer.  These melted and partially melted snowflakes then fell toward the earth.  Some of them refroze on the way down becoming ice pellets (sleet).  But, the vast majority made it to the ground entirely in liquid form and then froze on impact with exposed objects.  This is why it's called "freezing rain" or "freezing drizzle".  This, of course, led to an instant nightmare of iced over roads, highways, bridges and overpasses and coated trees and power lines with a heavy glaze of ice resulting in scattered power outages (including for 4 hours at my house).

 

One other important thing happened that wasn't expected to.  The upper air disturbance that was forecast to move eastward toward us abruptly changed course and began moving northeast.  This allowed the coldest core of temperatures aloft and the greatest lift to remain well north and west of us.  Consequently, Dallas/Fort Worth was unexpectedly hammered with 5 inches of snow overnight even though they had only expected about an inch at the most there.  Moderate to heavy snow also occurred from the Hill Country northward up I-35 through Waco and northeastward through the ArkLaTex.  Tonight, snow is falling throughout much of Arkansas eastward into Tennessee.

 

So, why didn't local meteorologists (including me) detect these subtle changes in the atmospheric profile that would have possibly altered the forecast significantly?  One reason is that we simply don't have the best information available when it comes to the upper atmosphere in Southeast Texas.  This is because there is a rather huge gap in the network of United States upper air observations stations.  The closest ones to the Houston/Galveston area are located in Lake Charles and Corpus Christi with additional surrounding stations in Fort Worth, Del Rio and Shreveport.  Thus, local forecasters have to extrapolate what the upper air profile of temperature, humidity, winds, and pressure is in Southeast Texas by using data recorded from these distant stations.  Also keep in mind that weather balloons are only launched twice daily at 6:00 a.m. and 6:00 p.m. so the data for us not only has to be estimated...it can also be markedly different several hours after it was initially recorded.  If the weather is not changing much (as in the summer), this disparity doesn't usually cause much of a problem.  But, during the winter and spring months, the weather is constantly evolving and air masses are in a constant state of transition.  Just when you think it's cold enough aloft...it can unexpectedly warm up.  And when you think there is plenty of moisture aloft...it can unexpectedly dry up.  It is a never ending challenge to get it right on a routine basis but when a rare winter weather event (like today's) is thrown into the mix, it can become nearly impossible to have a handle on everything that's happening in the atmosphere so the forecast can...and sometimes does...crash and burn.

 

Based on the 6:00 p.m. weather balloon soundings from Corpus Christi and Lake Charles Thursday evening, extrapolation suggested a temperature of around 0 degrees Celsius at about 5,000 feet over Southeast Texas based on a reading of -6 degrees Celsius over Corpus Christi and 6 degrees at Lake Charles.  But rarely is the atmosphere perfectly linear...so to speak.  In reality, temperatures aloft were probably closer to about 3 to 6 degrees Celsius over the Houston/Galveston area and this above freezing inversion layer was probably several thousand feet in depth.  That is the reason our precipitation remained primarily in a liquid state (freezing rain) and never transitioned into a frozen state (sleet/snow).

 

What happened today illustrates just how difficult it can be to forecast these winter weather events in Southeast Texas.  Personally, I would rather forecast the movement of a stalled hurricane than predict whether it will snow or not in Houston.  But, you can't be right all of the time and I certainly wasn't this time.  Hopefully, next time, that won't be the case.  My apologies to everyone.

 

Please stay warm and dry!

 

Jim O'Donnel




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