Tornadic Panhandle storm emblematic of the differences between ProbTor v3 and v2

A powerful storm that saw it's genesis in northeast New Mexico and it's demise in south-central Oklahoma, spanned nearly 11 hours. It produced tornado reports near the Oklahoma-Texas border in the Panhandle region, as well as numerous gigantic hail (5" diameter) and wind reports (up to 75 mph) (Figure 1).

Figure 1: ProbSevere v3 contours (outer contour is colored by ProbTor v3), MRMS MergedReflectivity, and NWS severe weather warnings for a supercell in the OK/TX Panhandles.

This storm clearly demonstrates some important differences between ProbTor version 3 (PTv3) and version 2 (PTv2). Early on in the storm's lifecycle, PTv3 was much greater than PTv2. At 21:24 UTC (see Figure 2), PTv3 = 34%, vs. PTv2 = 1%. The first tornado report was at 21:51 UTC, when PTv2 finally increased to 14% (PTv2 later increased to about 70%). Looking into this further, we found several things contributing to the elevated PTv3 and much diminished PTv2:

• In PTv2, which uses RAP, the SRH 0-1 km AGL was only 50 m2/s2, vs. 100 m2/s2 in the HRRR, which PTv3 utilizes.
• Similarly, PTv2 strongly relies on the mean wind in the 1-3 km AGL layer, which was only 16 kt. This also was dampening the PTv3 probability.
• The 0-2 km azimuthal shear (i.e., low-level storm rotation) was modest. This was not enough to overcome the poor low-level kinematics in PTv2 (until much later), while at the same time, it was not too harsh on PTv3.
• The top contributing predictors for PTv3 were the 0-2 km azimuthal shear, the effective bulk shear (~ 65 kt), the very high MESH (3.43") and surprisingly, a strong low-level laspe rate (8.8 C/km). 

Figure 2: ProbSevere v3 contours (outer contour is colored by ProbTor v3), MRMS MergedReflectivity, and NWS severe weather warnings for the tornadic supercell. Note that PTv3 is 34% here, while PTv2 was only 1%. 

Later on, as the azimuthal shear went off the charts (see Figure 3), PTv2 was much higher than PTv3, but also much more erratic. PTv3 consistently remained in the 30-40% range for awhile, despite very high azshear. This indicates that the PTv3 model has learned that azshear can be quite noisy (due to things such as radar sidelobe contamination), and it learned not to overemphasize the radar-based rotation predictors.

The PTv3 model correctly ramped up probabilities to 30-35% when rotation increased in a decent environment (from about 20:20 to 21:00 UTC), but hedged when rotation (as observed by MRMS azshear) was very high, as evidenced by probabilities below 40%. As of now, there were no other tornado reports after 22 UTC, so this seems like a sensible hedge (the storm was tornado-warned until 02:15 UTC).

In practice, we hope that forecasters find value in earlier ramp-ups in the probability of tornado, while understanding the uncertainty of using scalar-based predictors leads to lower probabilities overall, compared to PTv2 (and fewer erratic swings). We hope that using image-based methods will improve the guidance even further. 

Interactive version of the plots in Figure 3. 

Figure 3: Time series of PSv2 and PSv3 probabilities and certain predictors for this storm.