ProbSevere v3 showing improvement

While the HWT forecasters didn't operate in North Dakota or South Carolina yesterday, I wanted to document several cases where Probsevere v3 (PSv3) was making improved predictions of severe weather, relative to ProbSevere v2 (PSv2).

In eastern North Dakota, very modest shear under a ridge helped these storms to get a little organization. Though PHv3 and PWv3 are rather low, the overall PSv3 probability was 44%, compared to PSv2 of 14%. Soon after the image in Figure 1, golf ball-sized hail was reported. In this storm, the low effective shear and lightning flash rate (12 fl/min) were hurting PSv2, while the MESH, ENI flash rate, low-level lapse rate (8.7 C/km), and decent mid-level azimuthal shear were the top contributors to the probability of severe. 

Figure 1: ProbSevere contours, MRMS MergedReflectivity, and NWS severe weather warnings.

A little further west and about an hour later, another storm showed a nearly 40% difference between PSv3 and PSv2 (50% vs. 13%). Despite a better flash rate and 1" of MESH, PSv2 was again hampered by low effective shear. Similar to the previous storm, the low-level lapse rate, MESH, and flash rate were helping PSv3. But in this storm, a strong satellite growth rate (not displayed in Figure 2) was the 5th leading positive predictor. Soon after the image below, there were two reports of semi-trucks blow over. Interestingly in this storm, the GLM flash rate was very low (1 fl/5 min) compared to the ENI flash rate (34 fl/min). This is another example showing how using data fusion helps create a more robust probabilistic model---when one data source is suspect (for whatever reason), others pick up the slack. We have also seen storms where GLM flash rates are much higher than ENI flash rates.

Figure 2: As Figure 1, but for a second North Dakota storm.

Several storms in South Carolina were showing some good improvement as well. A storm entering western South Carolina was pegged at 35% in PSv3, while only 3% in PSv2, owing to low MESH (0.47 in), low shear (~20 kt), and low/modest ENI flash rate (16 fl/min). In PSv3, a combination of good satellite growth (the growth rate was older, so not displayed in Figure 3 readout below, but still used in the model computations), modest reflectivity-based parameters, and a solid low-level lapse rate (9.1 C/km) combined to produce the 35% probability of severe. Reports of multiple trees down came in soon after this image.

Figure 3:  As Figure 1, but for a storm on the South Carolina /  Georgia border.

Further east, a similar story, with PSv3 13% greater than PSv2 (43% vs 30%). Dozens of trees and powerlines were down from this storm. The probability of wind was much higher than the probability of hail in v3.

Figure 4:  As Figure 1, but for a storm in central South Carolina

An analysis that was provided in the training slides for PSv3 showed that improvement was greatest in the moderate MLCAPE and modest/moderate effective shear regimes, which these storms fit into. One other point to note is that while ProbSevere v2 is often higher on very strong and mature storms, we've seen that quite frequently, ProbSevere v3 ramps up in probability sooner than v2, which we feel is a very important distinction and improvement. While not explicitly shown in the animation in Figure 5, v3 was again a little ahead of v2 in these storms in southwest Texas, where one team of HWT forecasters was working.

Figure 5: An animation of ProbSevere v3 contours, MRMS MergedReflectivity, and NWS severe weather warnings.

ProbSevere v3 less affected by meager flash rates and effective shear

A number of severe storms on May 17, 2023 illustrated improvements of ProbSevere version 3 (PSv3) over version 2 (PSv2). 

Several storms in South Dakota exhibited low total-lightning flash rates (6-10 fl/min). The dearth of lightning greatly diminished the PSv2 probability of severe weather in the next 60 minutes, while not hurting the probabilities for PSv3 nearly as much.

In this storm near Rapid City, SD, PSv3 = 48% while PSv2 = 17%. This was shortly after the first 1-inch diameter hail report. The low effective shear (21 kt) was also diminishing the probability for both models.  The MESH (1.62"), VIL (40 kg/m^2), ENI flash rate (6 fl/min), and lapse rate 0-3 km (9.3 C/km) were the strongest contributors here. One observation we've noticed repeatedly is that total lightning information is still important in PSv3 (even low flash rates), but does adversely diminish the probability of severe to nearly the same degree as PSv2. 

Figure 1: ProbSevere, MRMS MergedRef, and NWS severe thunderstorm warning for a storm near Rapid City, SD.

Further south, the effective shear was a bit more favorable (33 kt). At this time (see Figure 2), PSv3 = 73% and PSv2 = 41%, and a 1-inch diameter hail report was recorded. More hail reports were recorded an hour later. The main difference between this storm and the storm in Figure 1 is that the effective shear was slightly favorable, while the low ENI flash rate was unfavorable in PSv2 (but not so in PSv3).

Figure 2: ProbSevere, MRMS MergedRef, and NWS severe thunderstorm warning for a storm west of Pine Ridge, SD.

Traveling eastward, another severe storm produced a 60-mph wind gust at 21:46 UTC (Figure 3). At this time, PSv3 was 23% higher than PSv2 (though PWv3 was only 11%). The high MESH (1.37") and composite reflectivity (70 dBZ) help compensate for the very low shear (18 kt), while again we see that the low flash rate does not harm PSv3 as much as PSv2. 

Figure 3: ProbSevere, MRMS MergedRef, and NWS severe thunderstorm warning for a storm near Belvidere, SD

Heading to the eastern U.S., numerous trees were reported down at 18:55 UTC in eastern Alabama (Figure 4). At this time, PSv3 = 21% and PSv2 = 2%. The flash rate was decent (25 fl/min), with a MESH of 0.57" and VIL of 31 kg/m^2. The environmental flow, however, was quite poor (eff. bulk shear = 19 kt; mean wind 1-3 km = 16 kt). An enhanced low-level lapse rate (7.5 C/km) and lower wetbulb 0C higher (11.1 kft) also nudged up the probability of severe in PSv3. Even though PSv3 was only 21%, that is a marked improvement over PSv2, and could have helped forecasters identify a possible severe weather threat sooner, while a PSv2 value of 2% would likely be ignored. 

Figure 4: ProbSevere and MRMS MergedRef for a severe storm near Mount Olive, AL.

A storm near Waynesboro, MS was increasing in probability prior to a hail report at 22:08 UTC. At this time, PSv3 was 31% higher than PSv2 (though v3 was highlighting wind as the main threat). The MESH, flash rate, and VIL were the main contributors here, while the paltry effective shear was the top detractor (Figure 5). 

Figure 5: ProbSevere and MRMS MergedRef for a severe storm near Waynesboro, MS.

Finally, in Charleston, SC, this storm produced a hail report and multiple wind reports from 21:40 to 21:50 UTC. At this time, PSv3 = 45% and PSv2 = 17%. Again, the flash rate, MESH, and lapse rate 0-3 km were the top contributors, whereas the lack of satellite growth was the main detractor (Figure 6). 

Figure 6: ProbSevere and MRMS MergedRef for a severe storm over Charleston, SC.

Overall, we've found that PSv3 improves severe weather detection and prediction over its predecessor, often highlighting threats sooner. For mature severe storms with very strong storm attributes, PSv2 is often higher than PSv2, sometimes by 20% or more (e.g., 95% vs. 75%). However, we believe that a little worse performance on the "high end" of storms is a good trade-off for being able to capture severe weather in more marginal (but still impactful) situations, and doing so sooner, overall. 

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.