ProbSevere v3 for a couple of early autumn storms

ProbSevere v3 (PSv3) was able to provide an earlier heads-up and increased lead time to the initial severe hazards in several recent autumn storms on the Southern Great Plains. 

On September 23, there were a number of powerful supercells ravaging east-central Oklahoma. The storm in Figure 1 quickly split into two cells, with the right split producing 2"-diameter hail at 23:55 UTC and 3" hail at 00:15 UTC.

Figure 1: ProbSevere v3, MRMS MergedReflectivity, and NWS severe weather warnings for storms in central Oklahoma.

Figure 2: ProbSevere v3 for a storm in Pottawatomie county at 22:56 UTC.

As this supercell was developing (see Figure 2), PSv3 was 37%, whereas PSv2 was only 3%. Hail appeared to be the main threat. At this time, the very weak lightning signal (only 5 fl/min) and low MESH (0.33") were keeping the PSv2 probability very low. However, the top 5 contributing predictors in PSv3 were:

• Lapse rate 0-3 km (8.4 C/km)
• MLCAPE (3685 J/kg)
• Eff. bulk shear (46 kt)
• Sat growth rate (2.7%/min -- "moderate")
• MRMS MESH (0.33")

ProbSevere v3, compared to v2, is more adept at extracting salient signals in the combined NWP, satellite, radar, and lightning phase space. In the developing stage of this storm, the environment and the satellite were more important than the radar predictors.  

The next evening, it was Texas's turn. One lone supercell in west Texas produced golfball-sized hail and wind reports of 59 and 62 mph (Figure 3).

Figure 3: ProbSevere v3, MRMS MergedReflectivity, and NWS severe weather warnings for a lone supercell in west Texas.

Figure 4: The storm in Figure 3, at 01:44 UTC.

ProbSevere v3 again had a jump on this storm before PSv2. At 01:44 UTC, right after the initial severe wind report, PSv3 was 55% vs. v2's 9% (Figure 4). PSv3 was showing both hail and wind as potential threats. The very weak low-level mean wind (5 kt) was keeping ProbWind v2 low (5%), and the modest flash rate and MESH, and very low hail CAPE (~250 J/kg) were combining for a ProbHail v2 of only 9%.

In contrast, the top contributors in ProbSevere v3 were the modest flash rate (18 fl/min), modest MESH (0.72"), the effective shear (47 kt), and the low-level lapse rate (8.1 C/km). PSv3 was able to integrate the marginal radar signature with favorable NWP data (from HRRR) to provide a better indication of severe probability.

Figure 5 highlights the higher probability of severe in PSv3 well before PSv2 shoots up, which was after the initial wind report. This storm later produced severe hail and another wind report.

Figure 5: Time series comparing PSv3 and PSv2 probabilities during the developing stage of the supcercell in Figures 3 and 4.