Usually, the biggest storms on a hot, humid afternoon pop up in the hottest part of the day, and them start to die off as the sun goes down. But, this past Sunday, the storms started popping up in northern Alabama around 5:00 pm, got stronger as we approached sunset, and were still around way after dark.
The storms formed along a boundary separating slightly cooler air to the east of I-65 (due to lingering morning clouds), and warmer eair west of I-65 (more sunshine). Here is the satellite picture from 12:00 pm Sunday:
Storms often develop along boundaries between cooler air and warmer air, even small ones like this, caused by morning clouds, with only a 3 or 4 degree temperature difference. The warm air is less dense than the cold air, so it exerts less pressure, and air flows toward the low pressure on the warm side, providing lift for storms.
Examples where storms do not develop in the heat of the afternoon, but instead really get going around 5:00 or 6:00, is something we are studying at UAH. It's called the afternoon-to-evening transition period (AET). You've probably noticed how the wind tends to die off as the sun starts to go down. This is because turbulence, generated by the sun's heating, calms down, and the higher winds stay aloft. But, if there is a boundary (like the one from Florence to Birmingham on Sunday) where maybe there wasn't quite enough convergence during the day for storms to fire, the increased winds aloft (due to decreased winds at the surface, it all has to balance out) may produce just enough lift to initiate thunderstorms between 4:00 and 7:00 pm.
Dr. Tim Coleman
UAH Research Meteorologist
Fox 6 Severe Weather Expert and Blogger
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