Above: Seven-day rainfall totals ending at 7 am EST Monday, February 10, 2020. This map is produced from a blend of hourly surface observations with radar- and satellite-derived amounts. Image credit: NOAA/NWS/AHPS.
Several large Southern cities, including Birmingham and Atlanta, are facing a major risk of flash flooding from late Monday into Tuesday, as near-record midwinter moisture surges into a stubborn frontal zone. At midday Monday, the NOAA/NWS Weather Prediction Center expanded a high risk of excessive rain—a red flag for flash flood danger—across a belt stretching from central Alabama into northern Georgia, valid until Tuesday morning. A lesser but still noteworthy flood threat extends from far east Texas to southern West Virginia.
The biggest concern is a prolonged period of heavy rain that’s expected late Monday and Monday night. As impulses move through a strong jet stream locked in place across the South, they will trigger periods of intense showers and thunderstorms, fueled by low-level winds hauling in moisture from the western Caribbean and western Gulf of Mexico.
The potential rainfall amounts are impressive in themselves; some locations could end up 5” or more in 24 hours. What’s making this an especially concerning flood threat is that soils across the region are already saturated and waterways are swollen after a week of rain that dumped 3” to 6” of rain across many of the same areas (see image at top).
A high risk for excessive rain is a big deal
Many folks who are familiar with the risk categories (from “marginal” to “high”) issued for severe weather threats aren’t as conversant with these excessive-rain outlooks. As Jon Erdman explains in a weather.com feature: “Just as a high risk of severe thunderstorms in a severe thunderstorm outlook from NOAA's Storm Prediction Center indicates higher confidence of a major event such as a tornado outbreak, a high risk excessive rain outlook points out those scenarios that aren't your typical local flash flood event.
“A more typical soaking rain may trigger minor flooding in areas that typically see runoff, such as urban, poor-drainage areas, roads and small streams. These are typically covered by "marginal" or "slight" risks in the WPC outlook. When a high risk is issued, the WPC expects severe, widespread flash flooding, including areas that don't normally experience flash flooding.”
It’s no small thing for WPC to issue a high risk. These events account for a disproportionate number of the deaths and damage associated with flooding in the United States, as noted in the embedded tweet below.
“Antecedent conditions are very wet across the region...with soil saturated, streamflows high, and dams nearing capacity in spots,” noted WPC in its rainfall discussion on Monday. “Also, we are pre-growing season, meaning there is less vegetation to absorb the rainfall. Thus the antecedent conditions suggest that rainfall of the magnitude forecast will cause significant and potentially life-threatening flooding and flash flooding today into tonight."
As of Monday afternoon, flash flood watches were plastered across most of the region of concern, with several flash flood warnings already out in Mississippi and Alabama. A total of 2.49” fell in Tuscaloosa, AL, in the six hours ending at noon CST with similar amounts expected Monday afternoon and evening.
"A considerable flash flood threat is expected to materialize over the next few hours with additional heavy rainfall," noted the NWS/Birmingham office in a flash flood warning for the Tuscaloosa area.