Devastation in the Mid-South

Tornadoes wreaked havoc across the U.S. Mid-South on Friday night into Saturday, as an energetic shortwave trough tapped into the abundant moisture and atmospheric instability in the southern U.S. One supercell, persisting for at least 11 hours, spawned a very long tornado (probable path length > 200 miles), generating some of the night's worst damage in Mayfield, Kentucky. Meteorologist Jack Sillin documented the forecast of the supercell's evolution and some preliminary facts (Figure 1). With over 100 tornado deaths, the day was the deadliest since 2011. 

Figure 1: Depiction of NWS forecast evolution of the Quad-State Supercell, by Jack Sillin.

ProbSevere guidance is used by NWS forecasters to aid in warning decision making. A new version of ProbSevere (version 3) is being developed and tested at the University of Wisconsin, honing and improving the probabilistic guidance. For the quad-state supercell, ProbSevere v3 (PSv3) was generally 10-20% higher than PSv2 in the hours before it became severe (Figure 2). For instance, PSv3 was 68% at 22:28 UTC, compared to 46% for PSv2. The top predictors contributing to the higher probability were:

1. 1-3 km mean wind (48 kts)
2. 3-6 km MRMS AzShear (0.011 /s)
   
3. 0-2 km MRMS AzShear (0.012 /s)
4. Effective bulk shear (65 kts)
   
5. Effective SigTor Parameter (1.73)
   
6. Normalized satellite growth rate (2.8%/min; "moderate")

Tornado probabilities were higher for v2 than v3, which we've found to often be the case (i.e., ProbTor v3 is more conservative). However, the trends in both PTv3 and PTv2 matched the trending threat of the supercell well. Users can inspect the storm's trends in probabilities and predictors here. 

Figure 2: Time series of ProbSevere v2 and ProbSevere v3 for the quad-state supercell. Note that severe reports are preliminary. 

Figure 3: ProbSevere v3 contours with MRMS MergedReflectivity at 03:30 UTC and NWS severe active weather warnings, when the tornadic quad-state supercell was in Mayfield, KY.