It is a rainy, cool Christmas day over BHM, with temperatures in the lower 50s and rain. The rain will continue for another 2 or 3 hours, but will end later this afternoon as the warm front moves north. The rain ending is not a good thing, because that may allow the warm air to come north faster.
We are still concerned about a possible severe weather outbreak tonight over central and south Alabama. A very strong low pressure area is intensifying over Louisiana, and pressures will be falling rapidly over Mississippi over the next few hours. This will cause strong south winds to develop over Alabama once the rain ends, with winds at 3,000 feet 50-70 mph. There will be strong wind gusts at the surface out ahead of the thunderstorms, maybe up to 40 mph.
When the cold front comes through tonight, a line of thunderstorms will move through central Alabama around midnight. The strong winds at 3,000 feet will get brought down to the surface, and trees will get blown down. The bigger concern is ahead of the squall line this evening, especially south of I-20. The wind shear will be tremendous due to the rapidly deepening surface low over Mississippi, and helicity values will be over 400 m2/s2, causing storms to rotate. Individual storms will develop ahead of the line, and these may contain tornadoes. The northward extent of the tornado threat is still uncertain at this point. It seems likely that some tornadoes will occur over SW Alabama. If the warm, moist air gets as far north as TCL and BHM, we could see tornadoes also. It's hard to tell now.
We'll have more this afternoon. Keep an eye on the weather and have a plan.
Dr. Tim Coleman
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