ProbSevere v3 gives sooner "heads up" in South Carolina

Several isolated storms in South Carolina tapped into some better bulk shear, becoming better organized, as well as threatening. ProbSevere v3 (PSv3) highlighted elevated probabilities of severe before version 2 for the three storms shown here. 

Figure 1: Animation of ProbSevere, MRMS MergedRef, and NWS severe weather warnings for several storms in South Carolina yesterday afternoon.

The first storm, northwest of Myrtle Beach, SC (Figures 2 and 3), had PSv3 hovering in the 20-40% range for a while before increasing to 60% and then later to 70%. In the 50 minutes before the official NWS warning, PSv2 was mainly under 10%. The MRMS VIL (32 g/m^2), 0-3 km lapse rate (8.2 C/km) and MRMS 3-6 km AzShear were leading contributors to the PSv3 probability at 19:00 UTC, when PSv3 was about 40% and PSv2 was 8%. 

Figure 2: A storm in eastern SC that downed multiple trees.

Figure 3: PSv3 and PSv2 time series for the storm highlighted in Figure 2.

A second storm, northwest of Charleston, SC, took a while before becoming severe and dropping silver dollar-sized hail. PSv3 remained in the 30-40% range for a while (owing to a favorable environment), while PSv2 was < 10 %. The probabilities in the 30-40% range early on better conveyed the severe threat that this storm would soon exhibit. The VIL (37 g/m^2), 0-3 km lapse rate (8.8 C/km) and the satellite growth rate (moderate) were leading contributors to the enhanced probability of severe early on in this storm's lifetime.

Figure 4: A storm northwest of Charleston, SC, which dropped large hail.

Figure 5: PSv3 and PSv2 time series for the storm highlighted in Figure 4.

A third storm, which followed in the wake of the storm NW of Charleston, SC, also exhibited higher severe probabilities (in the 20-40% range) well before PSv2 latched on to it. This storm went on to produce numerous wind damage reports as well as some large hail. In a similar refrain, the VIL, low-level lapse rate, and satellite growth rate all contributed to the higher probability of severe early on (with the 3-6 km AzShear and composite reflectivity also aiding). The PSv3 models are able to find connections between the observed and environmental predictors in a more robust way, compared to PSv2. 

Figure 6: A third storm in South Carolina, producing numerous wind damage reports.

Figure 7: PSv3 and PSv2 time series for the storm highlighted in Figure 6.

Forecasters at the HWT have been able to use a time series tool in the ProbSevere AWIPSII plug-in. The more accurate ProbSevere v3 models, coupled with instant access to storms' time series history will hopefully aid forecasters in the warning decision-making process. 

Lone severe storm in Georgia

A single severe thunderstorm formed in an environment with marginal effective shear (~35 kts) and MUCAPE (~1200 J/kg) yesterday afternoon in northern Georgia, traveling southward through the eastern suburbs of Atlanta. The storm produced between quarter-sized and golf-ball-sized hailstones and downed multiple trees and power lines. 

Figure 1: ProbSevere contours (pink is > 70%; gray is < 5%), MRMS MergedReflectivity, and GOES-16 visible reflectance.

In the absence of strong environmental forcing, the ProbHail model is largely influenced by the MRMS MESH and ENI total lightning flash rate. The low wet-bulb 0C height (8500 ft) also contributed positively to ProbHail. Even in marginal environments with few storms, ProbSevere can still highlight potential threats. 

The ProbWind model remained fairly low (≤ 40%), owing mainly to a weak 1-3 km mean wind and low-to-moderate MRMS AzShear values. Work is ongoing to improve ProbWind predictions in both wet and dry microburst environments. 

Figure 2: Time series of ProbSevere models for this storm, with corresponding severe reports and NWS warnings. 

Figure 3: Time series of select predictors and the maximum hazard probability (thick red line), with corresponding severe reports and NWS warnings.