(Current wet bulb temperature)
James has been talking this week about the possibility of some freezing rain, or rain that freezes on contact, late tonight and early tomorrow. Rain will be moving in and developing, and the timing of that rain is tough now. Radars are already showing light returns of rain that are not reaching the ground developing south of I-20 from Louisiana to Georgia.
The map above shows the current wet bulb temperature. Since the air is so dry, some of the rain will initially melt, using up heat and causing evaporational cooling (think about how it feels cold to you when you get out of swimming in the summer sometimes even if it’s 85 degrees). The wet-bulb temperature is the temperature where, given no other heat sources, the temperature and dewpoint would meet if rain were evaporated into the air until saturation occurred. As temperatures are only near 40 in central Alabama now, and will fall very slowly tonight under cloud cover (see satellite below, notice gravity waves too), expect wet bulb temperatures at the onset of the light rain somewhere between 28 and 32.
So, right as the precipitation starts, we will likely see a brief period, 2 or 3 hours, of sleet or freezing rain that could cause some icing on bridges and overpasses. The earlier the rain starts, the more of a problem we could have. Right now, though, with the atmosphere so dry, it looks like the rain won’t even start until after midnight, so no travel problems are expected this evening. The main window for potential icy spots is likely between 2 am and noon, depending on what time it starts raining. As rain continues falling from the warmer air aloft (temperatures at 2,000 feet will be above 40), the rain drops will warm the air back above freezing, ending the icing. Most of the rain tomorrow and tomorrow night will just be cold, with temperatures tomorrow afternoon climbing into the upper 30s.
If you are traveling anywhere tomorrow morning, make sure and check the weather before you head out. And, if you do encounter any icy spots on the road, use common sense! Leave plenty of space between you and the cars in front of you, keep speeds slow, and if it feels dangerous, pull over for a while.
This does not look like a major icing event at all, just something that travelers should be watching out for in the morning.
