Minnesota hailers

Intense storms quickly spun up in southeast Minnesota in the 23:00 to 00:00 UTC hour yesterday, eventually dropping hail with diameters up to 3 inches. The environment was highly sheared (55-60 kt) and the storms straddled a gradient of MLCAPE with values from 300 to 1100 J/kg. 

Figure 1: ProbSevere contours, MRMS MergedRef, and NWS severe weather warnings in southeast Minnesota.

ProbSevere version 3 (PSv3) produced higher probabilities of severe hail sooner than the operational version 2 for a number of these rapidly growing storms. 

The first storm, which dropped 2-inch hail west of New Prague, MN at 23:40 UTC (and later produced numerous severe hail reports near the Mississippi River), had PSv3 probabilities about 20% higher a few minutes before PSv2 did. While a few minutes might not seem like much, it can be crucial during a quickly developing situation. 

Figure 2: Time series of PSv3 and PSv2 for the development phase of a severe thunderstorm near New Prague, MN.

A second storm developing right in the wake of the first one also exhibited higher PSv3 earlier, and maintained a probability ≥ 40% before the first 1-inch report, in Belle Plaine, MN. This storm would also be long-lived and later produce numerous severe hail reports. 

Figure 2: Time series of PSv3 and PSv2 for the development phase of a severe thunderstorm near New Belle Plaine, MN.

The third highlighted storm developed west of the first two and never achieved very high MRMS MESH. However, PSv3 did attain probabilities of 30-40% before the 1-inch report in Norseland, MN, whereas PSv2 was largely under 10%. 

Figure 4: Figure 2: Time series of PSv3 and PSv2 for a severe thunderstorm southwest of Norseland, MN.

For each of these storms, increasing VIL, MergedRef, and ENI lightning density along with the very high effective bulk shear (55-60 kt) enabled PSv3 to produce more accurate guidance. PSv3 is overall much better calibrated than PSv2, meaning probabilities better match the occurrence of events (i.e., reports). In general, the optimal probability thresholds for PSv3 are between 40-60% for hail, wind, and any severe, but between 25-40% for tornado. However, users will still see differences case-to-case based on the meteorological regime they find themselves working.

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. 

ProbHail and ProbWind v3 in Florida

A slow-moving cold front converged with a sea-breeze boundary to produce some strong storms in a weakly-sheared environment. One forecaster at the HWT remarked how the ProbHail values in version 3 (PHv3) seemed more reasonable to him in this environment, compared to ProbHail version 2 (PHv2).

Figure 1: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warnings in the Florida panhandle. The highlighted storm produced severe wind reports

We can see that despite the favorable MESH, satellite growth rate, and thermodynamic parameters in the storm highlighted in Figure 1, PHv3 was only 11% at this time, while ProbWind v3 (PWv3) was 60%. This storm later produced multiple reports of downed trees and power lines. While ProbWind v2 correctly had high probabilities (PWv2 = 81%), PHv2 was heavily overforecasting (PHv2 = 85%). The machine-learning models in ProbSevere v3 (gradient-boosted decision trees) were able to more accurately discriminate between the wind and hail threats in this situation. For this storm, the low effective bulk shear (12 kt), high wetbulb 0C height (13.4 kft) and high PWAT (2.1 in) all detracted from the PHv3 probability. 

Several storms in the Florida panhandle did produce severe hail reports, including the storm highlighted in Figure 2. In general, PHv3 was in the 25-40% range for storms that produced hail in this environment, and perhaps could help forecasters identify hail threats compared to other storms in the area not producing hail, which had PHv3 probabilities in the 0-15% range. The higher MESH (1.5 in) and slightly better effective shear (19 kt) contributed to higher PHv3 for this storm.

Figure 2: ProbSevere contours, MRMS MergedRef, and NWS severe thunderstorm warnings in the Florida panhandle. The highlighted storm produced severe hail and wind reports.