East Tennessee Flash Flood Event of 6 February 2019

Early season thunderstorms produced torrential rainfall and flash flooding across portions of the Cumberland Plateau and central and northern sections of east Tennessee during the evening hours of 6 February 2019. The heavy rainfall caused washed-out and impassable roadways, significant stream flooding, and mud slides across East Tennessee. Big Creek in the town of Rogersville, Tennessee recorded major flooding and approached the record of 12 feet. Rainfall in excess of 3 inches was reported over a 3-hour period in parts of these areas.

This case illustrates the complexities involved in diagnosing flood potential. We examined the synoptic scale features using a top-down approach in order to understand the overall pattern that was a precursor to the event. We studied mesoscale and thermodynamic features to see how they interacted with the synoptic features to enhance the potential of heavy rainfall. We reviewed the range of certainty among the HREF short-term ensemble members leading up to the start of the rainfall to show how these short-term ensembles improved forecaster confidence that flooding was a possibility. Finally, we recognized the importance of being able to diagnose the synoptic and mesoscale pattern that produced the heavy rainfall in order to use this knowledge together with the short-term ensemble guidance to improve messaging for such events.

We found a favorable pattern for convection and heavy rainfall to be in place, featuring a divergent 300mb jet parallel to a quasi-stationary frontal boundary. The jet structure enhanced the frontogenetic forcing as well as the boundary layer moisture transport to maintain convection. The front oriented parallel to the deep layer flow allowed cells to train. The persistent low-level jet pulled unseasonably high moisture levels northward into the stationary front. This led to torrential rainfall within the training cells. The unseasonably high freezing levels of 11,500 feet MSL allowed for favorable warm rain processes producing high rainfall rates. This presentation will evaluate the different factors that caused the flash flood event of 6 February, 2019.