Reports of sleet continue to come in from parts of western and central Alabama, as the leading edge of the precipitation moves in. With the dry air in place at low levels, it will take the precipitation a while to become established, with the steadiest precipitation probably occurring in Birmingham between 8 pm and 4 am. I am concerned that precipitation could be heavier than it looked like before, with BHM getting 0.25″ according to the SREF (a composite of models) and the NAM. Radar composite is attached.
As discussed in the past, there are competing processes going on in these situations. As the precipitation falls (as sleet or rain), evaporational cooling tends to move the temperature toward the wet bulb temperature, which is currently below 32 over the northern half of the state. The melting of any sleet that falls initially will also cool the atmosphere. However, the raindrops that fall bring down some warmer air from aloft, which will tend to raise the temperature. The balance of these processes is almost impossible to predict. We’ll know more when we get the balloon data from the NWS around 7 pm.
Wet bulb temperatures at 4 pm
The models are still not handling this situation well. For example, at El Dorado, Arkansas, where freezing rain has been falling since about 7 am, with ice accumulations and temperatures near 30 all day, the 12 UTC NAM predicted temperatures near 35 most of the day. Similar warm bias in the model exists over Mississippi. The one advantage we have over areas further west is that we got some sunshine today, and temps have warmed into the upper 30s. There was never a chance for that to occur to our west.
Ground temperatures are also cold around here. It has not been above 50 degrees in a week now, and lows dropped to 19 in BHM last night. Not to sound all gloomy about it…I agree with James that this will not be an ice storm. I just think there is a chance for some significant travel problems, and even scattered power outages, in the northern half of Alabama tonight. The good news is that temperatures will rise to near 40 tomorrow, allowing any ice to melt, so this will not be a long-term event.
