Déjà vu all over again and again
Last Sunday and early Monday (April 12 and 13, 2020), severe thunderstorms pummeled much of the Deep South. Later Monday, the severe storm activity headed north, affecting parts of the Middle Atlantic and New England states. There were some 1500 storm reports including more than 200 tornadoes (Fig. 1 and Fig. 2). Note that a significant number of these reports may be deemed duplicates in upcoming weeks, as the reports listing gets a thorough screening.
This weather event also brought heavy rainfall to the area, depositing four to six inches across parts of northern Alabama and more than 2 inches of across many parts of Mississippi, Alabama, and Georgia (Fig. 3). In northern Alabama, rainfall for the event was some three times the usual weekly amount for this time of year.
A nearly identical weather pattern to last weekend was in place today (Fig. 4 and Fig. 5). A stationary front was draped across the southeastern states from east Texas through central Mississippi to southern Georgia (Fig. 6). Along and to the north of this boundary, heavy thunderstorms have erupted. These storms have reinforced the cold air already present, keeping temperatures in the 50’s to lower 60’s late this afternoon. To the south of the front, temperatures have skyrocketed into the 80’s along with brisk southerly winds. The result is a strong zone of low-level convergence along and to the north of the front. When coupled with strong divergence at 35,000 feet (splitting jet stream pattern), the zone of strong upward motion matches thunderstorm activity closely (Fig. 5).
With the frontal zone stretched out west-to-east, and upper-level winds generally blowing strongly from west to east, numerous showers and thunderstorms will be advancing across the same locations this afternoon through much of tonight. Each of the training storms will deposit heavy rainfall atop already saturated ground and rainfall from earlier storms (Fig. 7); this will likely lead to urban, small stream, and flash flooding across northern and central parts of Mississippi, Alabama, and Georgia. Heavy rain could also affect the Carolinas tonight through midday Monday.
Storms today have already dropped heavy rainfall across some locations. Northern parts of Houston saw storms drop between one and two inches of rain within an hour earlier this afternoon. A multi-storm attack across parts of northeast Mississippi saw five to six inches of rainfall by late afternoon. Parts of Central Alabama and northern Louisiana netted about five inches of rain.
Along and to the south of the frontal boundary, where the air is the warmest and the most moisture-laden, severe and tornadic thunderstorms are likely to develop during the nighttime hours.
By Tuesday afternoon, two new weather systems will enter the scene. One, moving in from Canada, will affect the northeast with a risk of severe weather. The second system takes shape over west Texas. It is this system that will, like its predecessors, head eastward and affect the western Gulf states by Tuesday afternoon and the rest of the Gulf states by Wednesday. And there is one more Gulf Coast weather system behind this one, destined to affect the region by week’s end. Both of these will have the potential to bring severe weather and flash flooding.
© 2020 H. Michael Mogil
Originally posted 4/19/20
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