The brutally cold Arctic airmass that has gripped most of the Continental U.S. the past few days continues to slowly spread out now. It is a very shallow cold air mass, meaning if one goes up only 3,000 feet or so, the air is not nearly so cold. But, such air masses are very difficult to forecast. They don’t tend to move quickly, but instead spread out as a density current, like water poured on a table.
It looks like the coldest of the air will stay just north and west of our HQ here in Birmingham, but some current temperatures across SEC states include: St. Louis 5, Memphis 19, Austin 31, Nashville 25, New Orleans 35, Birmingham 35, Atlanta 43. See map below. Everything in blue is below freezing. Then there is Cooperstown, ND, where several of us guys went this Summer to the Reagan Minuteman Missile Museum…there it is currently -24 degrees!
But then there is the problem with shallow cold air masses. Ice. And it looks like we will have some over parts of the Southeast Sdunay night through Monday. Winter Storm Watches or Warnings are currently in effect for areas from Texas and Oklahoma to just west of Birmingham, including Dallas, OKC, Little Rock, Memphis, Nashville, and Huntsville.
A pair of upper-level shortwave troughs will trigger a surface low in the Gulf of Mexico Sunday night and Monday, along the sharp temperature gradient between Gulf air and Arctic air.
The forcing from these upper lows, acting in tandem as a classic midlatitude storm system with the surface low in the Gulf, will produce widespread precipitation over the Southeast Sunday night and Monday. Far enough west, in places like OKC, Dallas, Little Rock, and Memphis, this system will produce a huge snowstorm, with some areas getting 6 to 12″ of snow. But, farther east, in places like Louisiana, eastern Mississippi, middle Tennessee, and western Alabama, the cold air is not deep enough for snow to form.
Above see the computer model vertical temperature profile for Muscle Shoals, AL Monday morning. The thick blue line is the freezing line. The red line is the forecast temperature at each height. Note that there is a deep layer of temperatures just above freezing from around 3,000 feet to around 7,500 feet. This means either snow can’t form at all, or if it does form aloft it will melt on the way down. Then, it will either re-freeze just above the ground into sleet (ice pellets) if the cold air layer is deep enough, or it will freeze on contact with the ground (freezing rain), causing a glaze of ice on everything from streets to trees to power lines.
Right now, it looks like the best chance for a significant ice storm, including trees going down and power outages will be from central Louisiana through central and northeastern Mississippi, northern and western Alabama, into middle Tennessee. Some locations could get over 1/2″ of ice accumulation. This would be a bad ice storm if it plays out.
As far as Birmingham goes, right now the computer models, except for one, keep the freezing rain mainly north and west of the city. But I am a bit uneasy with the forecast given the shallow cold air and the tight gradient in temperatures. Birmingham may not be out of the woods…it will be close. There could literally be icing and power outages as close as Jasper, Hamilton, and Northport. We will have to watch this very closely. Either way, several more days of very cold air are on the way for the Southeast…at least into Wednesday…when another ice storm could happen, but more likely a bit farther north.
Dr. Tim Coleman
Consulting Meteorologist
Coleman and Knupp, LLC
Coleman Knupp and Dice Weather Blog 