One more round of heavy rain will drench parts of the saturated South and aggravate flooding before a sweep of drier and colder air provides a break from wet weather later this week.
Enough rain is forecast to fall from Tuesday night to Wednesday night to lead to flash and urban flooding and cause an additional rise on some of the rivers. Flood advisories, watches and warnings are in effect for parts of the southern United States.
|This graphic shows current flash flood watches (dark green) and flood warnings (bright green) across the southern U.S. (AccuWeather)|
The same storm system will renew the risk of severe weather in part of the region.
Large portions of the South have been drowning in rainfall this winter.
"Some locations from central Mississippi to central and northern Alabama, Tennessee, Kentucky, northern Georgia, upstate South Carolina, western and central North Carolina and southwestern Virginia have received twice their average rainfall in 2020," AccuWeather Meteorologist Nicole LoBiondo said.
In some cases, this same magnitude of above-average rainfall extends into late autumn of 2019. For example, since Dec. 1, 2019, Jackson, Mississippi, has received 22 inches of rain with nearly 21 inches in Birmingham, Alabama, and 18 inches in Columbia, South Carolina.
Due to the combination of the ground being saturated and low evaporation rates during the wintertime, much of the rain that has been falling in recent days has been immediately running off into streams and rivers.
In some cases a mere 0.25 of an inch of rain in an hour can be enough to trigger rapid flooding of small streams, according to National Weather Service hydrologists.
|This image shows how much rain is needed to trigger flash flooding as of Tuesday morning, Feb. 11, 2020. (NOAA / Flash Flood Guidance)|
"A second dose of drenching rain will follow in the footsteps of rain that soaked parts of the South from Monday to early Tuesday," LoBiondo stated.
"The rain will spread from parts of Texas and the lower portions of the southern Plains during Tuesday night and early Wednesday to parts of the lower Mississippi, lower Ohio and Tennessee valleys as well as the southern Appalachians during Wednesday afternoon and night," she added.
A general 1-3 inches of rain is forecast from the storm in this swath with an AccuWeather Local StormMax of 6 inches. The StormMax with this particular storm is expected to occur in northeastern Texas and southwestern Arkansas, where the ground is not quite as wet and less prone to rapid flooding. Local amounts to 4 and 5 inches can occur farther east where hourly rainfall can also exceed the threshold of 0.25 of an inch.
According to the Tennessee Valley Authority, rainfall in the Tennessee Valley is more than 400 percent above normal for the year so far.
"River flows and lake levels are elevated at most tributaries and along the main river and will continue rising with this week's additional rainfall," James Everett, senior manager at the TVA River Forecast Center said in a statement.
Spanning Thursday to Friday, a push of colder air will mark an end to the train of rainstorms for the South. Temperatures will be slashed by an average of 10-20 degrees Fahrenheit.
Forecasters say that although the core of this Arctic air will remain over the Midwest and Northeast, it will still slash temperatures. Some areas west of the Appalachians, such as Nashville, Tennessee, may have their lowest temperatures of the season so far.
The air will have much more impact on flooding concerns as a four- to six-day stretch of dry weather soon arrives in the region.
This extended period of dry weather will allow streams to recede. Flooding along some of the larger rivers will take more time to cycle through the flooding process.
The storm from last week was the main catalyst in terms of high water along the river system in the region. Some rivers are forecast to crest even as the latest rainstorm moves through the region. Other rivers may not crest until late next week or in late February due in part to the additional rain this week. This is because the major rivers in the lowlands of the South take many days to respond to flooding after a heavy rain event.
To some extent, officials can mitigate the flow of water along the rivers where there are dams in operation. Such is the case in the Tennessee Valley.
Even after the spell of dry weather that moves in later this week, the region may still be somewhat prone to flooding due to the high water table and the ongoing potential for occasional rainstorms.