Quick wintry update – 1050 am

…Large uncertainty with possible winter storm Sunday and Monday…

…Ice storm possible…

Cold, dry air will start moving into Alabama at low levels Saturday afternoon, as north winds pick up.  Temperatures by Sunday morning will likely be in the mid to upper 20s in central Alabama.  At the same time, an upper-level storm will move out of the SW US into Texas by Sunday morning.

As this upper-level storm approaches the Gulf Coast, a low pressure area will form somewhere in Louisiana or off the Louisiana Coast Sunday, depending on how far south the cold air pushes Saturday night and Sunday morning.  This low pressure area will then move along or near the Gulf Coast Sunday night and Monday. 

Precipitation is not a question with this system.  It will begin to move into west Alabama Sunday, and overspread the state by Sunday night.  As a matter of fact, the precipitation may be heavy, with 0.5 to 1 inch of water equivalent falling.  The key to what type of precipitation falls will be the thermal profiles in the atmosphere Sunday night and Monday.  Generally, the farther south the low tracks, the colder the air is in central Alabama, since we don’t get any southerly flow ahead of it if it stays near or off the coast.

For snow to form aloft, the temperature in the precipitation formation region usually needs to be 23 degrees F or colder, ideally around 14 F.  Clouds are made of water droplets even at temperatures in the teens aloft, and most of these don’t start to freeze until they drop to near 20 degrees.  However, once a snowflake forms, any cloud water droplets it comes in contact with will freeze onto it, making it bigger.  Bottom line…if it is too warm aloft, even if it is a little below freezing, precip can fall to the ground as rain. 

The worst case scenario is that southerly flow at mid-levels, or 4,000  to 10,000 feet) brings in enough warm air there to keep temperatures too warm for snow formation.  Then, the precipitation would fall mainly as rain.  But, if there is enough cold, dry air at the surface such that the cold raindrops evaporating as they fall cool surface temperatures to freezing, causing freezing rain (rain that freezes on contact), we could have some ice accumulation.  Right now, this scenario is what is shown by the latest GFS model run.

Another possibility is that there is just enough cold air aloft for the precipitation to fall as snow.  This is more likely in north Alabama (HSV), but could occur in BHM too.  In this case, 2-3 inches of snow could fall Sunday night.

The third possibility is that the low moves north of the coast, warm air flows in from the Gulf, and we just get a cold rain, with temperatures in the mid to upper 30s.  (Even though lows tend to like to stay near fronts and areas of large temperature changes over short distances like the coast, this inland low was shown by last night’s European model, the one that handled the snow on Christmas so well.)

My gut feeling right now (and I’m not Gibbs on NCIS nor Jack Bauer on 24, so I’m not always right) is that snow is likely from Cullman north.  In central Alabama, freezing rain and snow, with ice accumulations Sunday afternoon, are possible.  I’d give it a 33/33/33 shot (snow/ice/rain).  Farther south, there could be a little freezing rain in Montgomery if the cold temperatures push far enough south, but that seems unlikely. 

As is usually the case in Alabama, the potential winter storm system coming in on Sunday and Monday is very complicated, and to jump out and make a bold prediction 3 days out would be inappropriate.  Also, don’t hang on the subtle changes in every model run, either.  The models are solving very complex equations involving many variables, and some flip-flops are going to occur in what they predict.  We have to sit back and take all this in and give it our best shot, accentuating the uncertainties in the forecast.

Category: Winter Weather