Wednesday, January 27, 2010

Heavy Rain Followed by Possible Ice

Very heavy rain is expected across East Texas over the next 36 hours. A strong winter storm is developing over the southwest and will move across the State tomorrow. The northern end of this storm, from the Red River north, will produce a major winter storm with Freezing rain, sleet, and snow. Severe weather is also possible with this system across the Gulf Coast.

For East Texas it looks like we will mainly see a heavy rain event. The graphic above shows the forecast amount of precipitation 6AM Thursday to 6AM Friday. Notice most of East Texas will have between 1.5" and 2" of rain. The big question for us is will the cold air make it in time for a winter weather event? Right now the answer is no however, there are a few complications to this forecast.

The image above is a forecast Meteogram for Tyler from the 0Z run of the GFS forecast model. The red line is the air temperature with the blue lines indicating wind chill temperatures, think blue from sustained wind and thin blue from wind gusts. Very heavy rain is indicated by the thick green bars towards the right of the graph, time goes from right to left, backwards from what you might expect. Notice the temperatures plunge as the rain moves out. However, this is showing a burst of wet snow, the thick blue bar, Friday afternoon. Now I am not buying into this right now but it does show there is a chance of a few flurries. As the temperatures bottom out Friday night into Saturday morning, notice the precipitation is still in the area, very shallow red, orange, and blue bars. This would be a prolonged period of light freezing drizzle, about 0.02" total amount of precipitation. This would not be a crippling ice storm by any means however, with temperatures in the 20s, we would see a light glaze on area roads and elevated objects. So we will have to watch this very closely.

Now the colors on the Meteogram have the following meaning, green=rain, blue=snow, red=freezing rain, orange=sleet. So it looks as though a lot of that light precip would be flurries and not cause a big problem. However, when you look at the atmospheric sounding, its paints a much different picture. In order to get ice crystals to grow, the temperature needs to be around -10°C or 14°F. You can see in the sounding above the temperature is below freezing the entire column of the atmosphere however, in the saturated area, where precipitation forms, the temperature never reaches that critical level. So this would produce drizzle across the area, liquid precip, that would freeze on contact. Let me say that forecast models have a very hard time handling shallow Arctic air, like this. We could see the low levels of the atmosphere dry out much quicker than indicated just giving dry, cold condition. But if what the models are hinting does occur, we could see freezing drizzle for the northern half of East Texas for as much as 12 hours, and this would cause many travel troubles.

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