Overview
Beginning Monday, December 21st, an evolving and days-long, large-scale pattern featured a broad 500mb trough across the western U.S. and a western Atlantic surface ridge. This setup placed Alabama within a steady fetch of deep-layer moisture and warm surface air, which included near-record to record high temperatures and persistent wind shear. A surface boundary pushed southward into Central Alabama by Thursday, December 24th. Several bands of moderate to heavy rainfall and embedded storms developed south of this boundary and led to widespread flash flooding across the southeast sections of Central Alabama. From December 23rd to December 24th, many locations received 6-10 inches of rain, with localized higher amounts.
The surface boundary lifted back to the north late on Thursday, December 24th, and stalled over north Alabama on Christmas Day. With storm motions parallel to the boundary, a band of heavy rain moved over the same areas for several hours. Rainfall totals of 4-8 inches occurred over a large area. This resulted in widespread and significant flooding for locations in the northern third of Alabama. Farther south, rain rates within heavy thunderstorms, plus slow storm motions, resulted in flash flooding as well. The excessive runoff caused by the heavy rainfall led to flooding along almost every main stem river in Central Alabama, as well as numerous small creeks and streams.
In addition to the flooding, the combination of instability and shear was conducive for a limited risk of severe thunderstorms south of the boundary during the period of December 23-25. A couple of weak tornadoes touched down in Barbour County on the morning of December 24th. Two additional tornadoes touched down along the I-20 corridor in the late afternoon hours of December 25th...one in Tuscaloosa County and one in Jefferson County. Thunderstorm wind damage was also confirmed in Bibb County on Christmas Day.
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