Parts of central Tennessee experienced truly torrential downpours and subsequent flooding over the weekend and dozens of people have been confirmed dead.
Indeed, some portions of the state have recorded more than 15 inches of rain in a single day. One town, McEwen, Tennessee, even recorded a total rainfall accumulation of a whopping 17.02 inches in 24 hours on Saturday.
According to the National Weather Service, if confirmed, this would be the new 24-hour rainfall record in the state. This new record would smash the old record of 13.6 inches, which was set in 1982.
As a result of the flooding, Tennessee has seen major structural damage to roads and highways around the state, as well as a loss of electricity to more than 4,200 residents. The flooding has also claimed the lives of at least 22 people.
Weather Behind the Flooding
Flooding is a fairly common occurrence in this part of Tennessee. The region’s many waterways are prone to flooding during periods of heavy rain, but the amount of rainfall that we saw this past weekend in the state is unprecedented in recent years.
The first major warning about the flood potential in the state came from the National Weather Service’s Weather Prediction Center early in the morning on Saturday, August 21. In Mesoscale Precipitation Discussion #0847, forecasters warned of heavy rains and thunderstorms with a risk of flash flooding.
Part of the reason behind the warning was an elevated stationary front that formed in the 850 to 925 mb layer above this part of Tennessee. Forecasters also warned of a strengthening westerly low-level jet that could bring additional warm air and moisture to the region, facilitating further convection.
In the subsequent Mesoscale Precipitation Discussion #0848, Forecasters warned of further heavy thunderstorms with peak rainfall rates of up to 3.5 inches per hour. This line of thunderstorms had more or less aligned itself with a large area of low-level convergence in the surface to 850 mb layers, setting the stage for increased convection.
Furthermore, MLCAPE (a measure of convective potential energy in the mixed layer—can be thought of as a proxy for atmospheric instability), was decently high at the time with measurements of approximately 1000 to 1500 J/kg.
Things only worsened in the next Mesoscale Precipitation Discussion #0849. Additional warm, moist air brought into the region by the low-level westerly jet and the increasing MLCAPE values in the region further supported rainfall rates of around 2 to 4 inches per hour in the region.
High rainfall was seen throughout Tennessee, Mississippi, Alabama, and Georgia during this time, though the brunt of the storm was concentrated in central Tennessee.
The final Mesoscale Precipitation Discussion #0851 of the day related to the region mentioned particularly impressive convection happening to the west of Nashville. High SB CAPE values (1500 to 3500 J/kg), strong bulk sheer (20 to 25 knots), divergence aloft, convergence at the surface, and an inflow of warm, moist air all contributed to the storm.
Forecasters also noted some fantastic examples of anvil cirrus plumes and overshooting tops in the visible satellite imagery, all of which document the exceptionally strong convection that took place in the region during this time and brought major flooding to Tennessee.