Snow analysis – 1155 am

500 mb height, wind, vorticity, every 6 hours starting tonight at 6 pm.

The big system coming in Tuesday and Wednesday is still very complicated, but it seems a little clearer this morning.  During the initial wave of precipitation tonight and Tuesday, the flow pattern sets up so that we will get warm air coming in from the south at low-levels here in Alabama, so it is likely that it will be all rain tonight and Tuesday, when the heaviest precipitation occurs.  A few of the higher elevations of NE Alabama could see the rain mix with snow at times, but for now, it looks like a rain event through Tuesday.

The possibility of snow here will start late Tuesday night and really get going on Wednesday.  By then, the best lift will be to our east.  But, as you can see in the loop above, the 500 mb low (with very cold upper atmospheric temperatures) will move across Alabama on Wednesday.  This will cool the atmosphere in several ways…dynamically due to the low itself, and since the surface low is ahead of the upper low, cold air will start moving in from the NW.  And, with that kind of upper low, there should still be enough lift for light snow on Wednesday, maybe even some convective snow showers. 

We have been stressing the low confidence level in the forecast due to the upper-level low, and that has not changed a whole lot.  But, we feel pretty good about forecasting almost all rain tonight and Tuesday.  On Wednesday, temperatures aloft will cool very rapidly, and any snow falling from aloft will be very cold.  So, as it falls, it will bring a lot of cold down with it, and any melting of that snow will use heat also.  Therefore, we expect the rain to change to snow on Wednesday.  By then, the precipitation should be lighter (total liquid amounts 0.20″ or so on average).  But, higher elevations will get more due to mountain lift. 

Will it accumulate?  In places like BHM and TCL, probably not significantly.  Maybe a dusting to 1/2″, mainly on grassy areas, but temperatures should hold in the mid to upper 30s on Wednesday at mean ground level.  At higher elevations, it could be a completely different story.  The wind will be blowing, so the air will stay mixed, and temperatures in the surface layer will decrease about 1 degree every 200 feet.  So, even on top of mountains like Red Mtn., Shades, and Double Oak, there could be some snow accumulation.  And, in the higher elevations of NE and E Alabama (Sand Mtn., Mt. Cheaha, Straight Mtn., etc.), accumulations up to 3″ are possible. 

Take a look at the NAM model time vs. height section prediction of temperatures for Tue, Wed, and Thu (in degrees C).

The red line is the freezing line.  It drops quickly during the day Wednesday to 950 mb, or about 1,500 ft. above sea level.  With freezing temperatures that close to the ground, snow should fall almost everywhere north of I-20.  But, surface temperatures will likely stay above freezing in most areas from BHM SW, as mentioned above.  The higher the elevation, the colder it will be and the closer you will be to the freezing line aloft. 

Bottom line…this is still a complicated forecast.  But, it looks like the rain will change to snow over north and central Alabama Wednesday.  It may only be 1/2″ or so, mainly in grassy areas, at lower elevations (most of us), but at higher elevations around BHM, and especially in NE Alabama, accumulations are possible.

James will have more details on snow accumulations in his afternoon video, around 300 or 400 pm.

Category: Winter Weather