Snow analysis – 408 pm

Snow is moving quickly across Mississippi and into Alabama this afternoon, with reports of light snow as far east as Jasper.  Some of the snow along the leading edge is not reaching the ground (see explanation below), but once snow is over a location on radar for 1 hour or so, the atmosphere moistens up and snow begins to fall.  We’ll probably start to see light snow here in the BHM metro area between 5 and 6 pm.

The snowstorm approaching Alabama has been stronger than the models had predicted over Arkansas, with some places reporting over 6″ of snow there!  So, computer models have to be given less credibility right from the beginning.  However, it is close enough now that we can look at observations (radar, surface, satellite, etc.) and see what’s going on to our west and how it’s changing.

The storm won’t be as strong here as it was in Arkansas.  The sun came out today in Alabama, warming temperatures into the 40s.  Because of this, even once snow reaches the surface, it will take a while for temperatures to drop below freezing due to melting and evaporation of snow.  It can happen in 1-2 hours, but this system is moving fast.

The air here is also very dry aloft.  Take a look at this morning’s sounding from NWS Calera.  The red line is temperature and the green line is dewpoint.  The very dry air causes a large spread between dewpoint and temperature.  So, once snow gets in here, it will take it a while to saturate the atmosphere and make it to the ground.  You may see snow on radar before it falls to the ground.

All that being said, there is a strong upper-level system approaching us, and with the kinds of snow totals we are seeing in Arkansas, there must be some convective snow (like thunderstorms, but along slant) in this system.  That could happen here, making snow accumulations vary widely from one spot to another.  A convective snow storm produced 4″ of snow in one hour in HSV back on January 9.  So, while the overall snow forecast from James in the post above and video (to right) are correct, understand that there may be wild variations.  One spot could get 2″, while someone 25 miles away gets 4″.

Bottom line…snow moving in rapidly, but will have to saturate a dry atmosphere before it reaches the ground.  Then, due to warm temperatures today, it will take another hour or so to get surface temperatures cold enough for significant accumulation.  However, we do expect travel problems early this evening over NW Alabama, spreading SE tonight.

Category: Winter Weather