Catastrophic Flooding in Tennessee

Tennessee was inundated with up to 17 inches of rain in a single day this past weekend. The state experienced substantial flooding and at least 22 people have died in the events.

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.

Observed precipitation totals for the state of Tennessee and the surrounding area on August 21, 2021. Image via NOAA and the National Weather Service.

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.

Mesoscale Precipitation Discussion #0847 chart showing an elevated stationary front and the arrival of a low-level jet. Image via the National Weather Service.

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.

Mesoscale Precipitation Discussion #0849 chart showing areas of high rainfall and increased flash flooding risk. Image via the National Weather Service.

High rainfall was seen throughout Tennessee, Mississippi, Alabama, and Georgia during this time, though the brunt of the storm was concentrated in central Tennessee.

Mesoscale Precipitation Discussion #0851 chart showing areas of high rainfall. Image via National Weather Service.

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.

GOES satellite imagery from around the time of Mesoscale Precipitation Discussion #0851 showing impressive convection over west-central Tennessee.

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.

By Extreme Weather Blog Team

Extreme Weather Blog is a website dedicated to all things weather-related. From the latest in extreme weather news to reputable weather forecasting resources, Extreme Weather Blog is your one-stop-shop for meteorology.

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