A wind storm moved across central Alabama last night, causing hundreds of trees to fall, damage to some homes, and power outages to at least 17,000 Alabama Power customers. Some wind gusts reported by NWS and FAA include Tuscaloosa 50 mph, Tuscaloosa courthouse roof 74 mph, Fairfield 43 mph, BHM airport 48 mph. Looking at the radar pictures above, it doesn't look much like a storm. But, the strong winds last night were caused by a series of rare weather events. Take a look at the pressure trace from Brent, Alabama (in Bibb County) from last night and this morning (6 pm to 2 am CST).
The air pressure dropped almost 0.2 inches of mercury, a lot of it (0.05", or 1.7 mb) in about 15 minutes. The atmosphere can not adjust to rapid drops in pressure like this in the usual way, so high winds occur.
It appears that the winds started out due to a "wake low" over Mississippi, or an area of low pressure that starts near the back edge of a rain area. Warm air aloft behind the rain area and cold air at the surface due to rain creates a jet of air flowing in behind the rain. This air then gets forced downward, causing warming aloft. Warm air is less dense, so the pressure at the surface drops, and air starts flowing in toward the low pressure.
We have had several wake lows over Alabama in the past few years that caused wind damage. One occurred on Feb 22, 1998, one on Dec 20, 2007, and one on Apr 13, 2009. However, even though this event started as a wake low, it looks like that is not what came through central Alabama. Below shows a radar picture (right) and cross-section through the radar picture looking into the wake low (left) from each of the three listed events.
First of all, last night there was no significant rainfall. Secondly, there was no cold air near the surface. Third, there was no precipitation aloft. The lack of these two factors would prevent the rear-inflow jet that likely creates the wake low. Finally, there was no evidence of a significant inflow jet on radar.
So, what caused the high winds over central Alabama? After a preliminary analysis this morning, I can't be 100% confident, but it looks like a unique type of gravity wave called a solitary wave.
A solitary wave is a wave that moves along a layer of stable, slightly cool, dense air near the ground. We had one of those in place last night, as shown by the balloon data from 6 pm at the NWS office at the Shelby County Airport.
If something like the air descending in a wake low pushes down on the stable air near the ground, it warms the air, making it less dense. This lighter air aloft causes the pressure to fall at the ground. Air then blows toward the low pressure.
The wave then moves due to various processes, described in the three drawings shown below.
It is the sinking of the air layers (noted by the horizontal lines) that causes warming and lower pressure at the surface. The wind then blows in from the right, and pulls air down from aloft ahead of the wave. This then warms the air ahead of the wave and pulls air away from that area, both dropping the pressure ahead of the wave. So, the low pressure and the wave move toward that area, while the area where the wave started (1) gets filled in by air coming in from the side.
We will get more data tomorrow and later on this event, but it was a rare event either way. Hopefully, everyone is OK.
Dr. Tim Coleman
UAH Research Meteorology
Fox 6 Severe Weather Expert
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