There have been a lot of questions revolving around if the ground is frozen and how cold air temperatures will be for this event. Here is a map of soil temperatures (not pavement or building temps):
So, soil temperatures are in the lower & middle 30s; that means roads and bridges are in that same territory if not a touch warmer. The air temperature is different. It has been below freezing at the Birmingham airport (and in Anniston) for over 12 hours now. Tuscaloosa has been below 32º for about 10 hours, and most of northern Alabama has been in that same category (10-14 hours below freezing).
Because surface temperatures are above freezing, it will take a precipitation a little while to begin icing highways; bridges always ice first because they are surrounded by cold air. There have already been reports of slippery elevated surfaces around Jackson and Vicksburg, Mississippi. Heavier precipitation that falls as sleet or snow will chill the roads faster, so once it begins around you, go ahead and assume that slick spots are likely within 30 minutes of the onset of sleet, freezing rain, or snow. It will not take long to drop the soil & pavement temperatures to the lower 30s and then the 20s. To have significant travel problems, both surface and air temperatures need to be less than 32º F, and we think that will happen tonight through tomorrow.
THE RULE OF THIRDS
Here’s a semi-accurate way to predict how low the temperature will fall once precipitation begins: Subtract the dewpoint from the air temperature. Divide that number by three.
Ta – Td = Approximate Wet Bulb Temperature
That “wet bulb” temperature is extremely important in knowing how long we will stay below freezing. The wet bulb is the temperature that results when evaporative cooling has done its work.
As I mentioned yesterday, the computer models do not handle details very well. Yesterday, they did not forecast a dewpoint of 8º F in Birmingham with a temperature at 23º F.
23 – 8 = 15
15/3 = 5
23 – 5 = 18 (approx. wet bulb)
In that case, the wet bulb would be about 18º F. As we slowly warm into the lower 30s this afternoon, the wet bulb will come up into the 20s. Once precipitation begins, temperatures will hover in the upper 20s to near 30 degrees as the dewpoint rises and the temperature drops toward the wet bulb.
The longer we are below freezing, the more likely it is that 1/4″ to 1/2″ or more of ice accumulations are likely in the Ice Storm Warning and even parts of the Winter Storm Warning areas. Some heavy snow is still very likely; see James’ post below for the latest projection map. Even less than 12 hours away, we still cannot call each town and community down to the inch of accumulation; it’s just not possible. Look at the guidelines we have drawn up here. This is as specific as a forecast can get; after lunchtime today it shifts to nowcasting.
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