ProbSevere v3 in west Texas

Severe convection was rather marginal and sparse on 3 May, 2023. However, a few storms of note demonstrated better predictions for ProbSevere v3 (PSv3) over ProbSevere v2 (PSv2). 

A storm in the Texas Panhandle, southwest of Dumas, TX, was in an environment with moderate MLCAPE (1200 J/kg) and low-to-moderate effective bulk shear (30 kt). This storm was around for over an hour before it finally produced severe hail reports (up to 1.5" in diameter). Approximately an hour prior, however, PSv3 was much greater than PSv2; 62% vs. 4% (see Figure 1). In this case, PSv2 was too reliant on the ENI flash rate, which was only 4 fl/min. While lightning is important in PSv3 models, it is less affected by storms with a dearth of lightning than version 2, especially when other parameters show increased severe potential (in this case, MESH and mid-level azshear). 

Figure 1: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warning for a storm near Dumas, TX.

Figure 2: Time series of PSv3 and PSv2 probabilities for the storm near Dumas, TX, which produced 1.5" hail. 

The 00Z sounding from Amarillo, TX (Figure 3) might be able to shed some light on why the MESH was so high (up to 1.6") yet lightning was quite low (~6 fl/min) from both ENI and GLM sensors. In this case, the precipitable water in the column was quite low (about 0.68"). This, combined with the low environmental RH in the mixed phase region of the troposphere probably combined to produce relatively few collisions of liquid and frozen water particles needed to produce charge separation necessary for lightning production. Ice nuclei, on the other hand, seemed to accrete supercooled droplets efficiently, producing large hail stones.

Figure 2: Observed sounding from Amarillo, TX at 00Z on 4 May 2023.

Another storm in southwest Texas, near Pecos, also produced severe hail (1" diameter).  The lightning was also low for this storm, though PSv3 had 51% while PSv2 has 37%. The MRMS azimuthal shear parameters were zero here. This was likely because the average beam height of the lowest tilt of the closest radar was 10-11 km above ground level! The MRMS products compute azimuthal shear between 0-2 km and 3-6 km AGL. The missing azimuthal shear values for this storm reduced the probabilities of severe, but not quite as much in PSv3 as in PSv2. 

Figure 3: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warning for a storm near Pecos, TX.