White Christmas Analysis

Here’s the deal, after looking at some of the 12 UTC models.  Over the next couple of days, northerly winds blowing out of a big, cold high pressure area centered near Minnesota will push cold air back in here, with temperatures falling into the upper 20s tonight, and only making it into the lower 40s tomorrow and Friday (Christmas Eve). 

Then, two upper-level disturbances will approach Alabama by Christmas morning.  The important one for us is right now over southern California, and will move east.  The other one comes down from the north.  According to the NAM, European, and GFS, the southern disturbance will help develop some sort of wave of low pressure in the Gulf Friday night, and bring precipitation to parts of central and south Alabama.  The heaviest amounts may wind up being farther south…and if the cold air gets a good push, Montgomery could see snow, more than BHM.  The GFS keeps it too warm for any accumulating snow on Christmas Day, while the other models indicate something.  Here are the snowfall projections for BHM from each model: 

NAM:  1.25″, GFS:  0, European:  1.5″, Canadian 0.25″, NOGAPS: 0

Big spread.  It is still 3 days away, and a lot has to go right for it to snow in Alabama.  The two disturbances could separate enough that the southern one is too far south, and the northern one is too far north, giving TN and KY a good snow, south AL and south GA some snow or rain, and us a few flurries.  Or, the southern one could be strong enough to get a low going on the Gulf Coast (which helps the disturbance itself grow…called baroclinic instability), and we could get 1″ here in TCL/BHM/ANB/GAD.   There is also the factor of cold advection (cold air moving in).  It tends to cause downward motion, possibly negating the upward motion from the disturbance, so we need to get the cold air in here by late Friday.  That’s why TN/KY snow looks very confident…the cold air already be there for sure, so no cold air advection to cancel out lift. 

Honestly, I don’t know exactly what will happen, and I teach Atmospheric Dynamics.  This is a complicated system, with multiple distrbances and wind maxima aloft.  These computer models are calculating Newton’s laws of motion, thermodynamics, and radiative transfer laws, at thousands of spots across the northern Hemisphere, every 1 to 5 minutes.  A lot can go wrong. 

Backing up from the models (at least mainly) and looking at this more simply, from pure meteorology…we have an approaching upper-level trough, and a likely frontal zone somewhere near the Gulf Coast.  If the cold air can push on down to the Gulf Coast, setting up a stronger thermal zone (baroclinic instability), we should get 1″ of snow (or more).  If not, we’ll be looking at a dusting or so.  I don’t see too much to hold the cold air back from the coast, given cold ground temperatures and north winds, so right now I’d guess 1/2 to 1″ in BHM, mainly falling on Christmas day.  But, that could change.

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