Sleet at 45 degrees? Expecting sleet when it’s nearly 50?

| February 13, 2012 @ 4:09 pm


Radar shows the beginnings of a large mass of precipitation moving into western Alabama.  The air very dry over Alabama, so most of this precip (especially east of Tuscaloosa) is not reaching the ground.

The temperature in BHM is currently 49 degrees.  But, the dewpoint is 15 degrees.  This means the air has almost no moisture in it at all (it is 0.2% water vapor), but with a temp of 49, the air can hold 0.7% water vapor).  So in every pound of air, the amount of water can be multiplied by 3 before we are saturated.  So, as precipitation falls into a dry air mass like this, the evaporation of water cools the air and yet adds water to the air, raising the dew point.  Where the two meet at equilibrium is the wet bulb temperature.  This is the temperature we should settle out at once it rains/sleets for while. In BHM that temp is about 37.  Here is a map:

And, the air 3,000 feet above the surface is even colder.  So, as some of the warm rain formed up at 5,000 to 10,o00 feet falls initially, it will freeze on the way down in this wet-bulbing process and form sleet.  This has already occurred over parts of NW Alabama.

Fortunately, surface temperatures are expected to stay above freezing (since the wet bulb is above freezing, so no widespread travel problems are expected in central AL.  NWS Huntsville has issued a winter weather advisory, as they have wet bulbs near 34-35, and some of the valleys in NE Alabama, where this morning’s cold air got trapped, or the hilltops, probably have wet bulbs near 32.  So, there may be a few slick spots up in extreme north and northeast Alabama this evening.

Once it starts raining harder overnight and warmer air moves in aloft, it should go to all rain in TCL/BHM/ANB.  But, in NE Alabama, there still could be some icy spots by morning.

Category: Met 101/Weather History, Winter Weather

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