Scourge of Jersey

A short-wave trough departing the U.S. east coast spawned numerous storms in the Mid-Atlantic states yesterday. One storm politely waited until crossing the Delaware River to start producing severe wind gusts, toppling numerous power lines and trees, some onto homes and cars.

ProbSevere version 3 (PSv3) had a better handle on this storm than version 2, being consistently 30-40% higher in the 30 minutes prior to the first NWS severe thunderstorm warning being issued. PSv3 is being evaluated by NWS forecasters in the HWT this year, its second year of evaluation.

Figure 1: ProbSevere v3 contours, MRMS MergedReflectivity, and NWS severe weather warnings for a storm in Pennsylvania and New Jersey.

Figure 2: Comparing ProbSevere v3 and v2 shortly before the first warning was issued.

Figure 3: Time series of ProbSevere v3 probabilities, local storm reports, and NWS severe weather warnings for this storm.

At 20:08 UTC (in Figure 2), the top six contributing predictors in ProbSevere v3 were the ENI lightning density (0.76 fl/min/km^2), effective bulk shear (47 kt), 0-3 km lapse rate (7.2 C/km), mid-level azimuthal shear (0.007 /s; moderate), the significant tornado parameter-effective (0.15), and the satellite growth rate (1.5%/min; weak). The machine-learning models in PSv3 are able to better discern complex predictor interactions than the models of PSv2.

Windy in Philly

A modest-looking thunderstorm downed numerous trees and power lines in the Philadelphia area yesterday evening. This storm resided in a high-shear low-CAPE environment (50 kt and 500 J/kg, respectively). 

Figure 1: ProbSevere (storm contours), MRMS MergedRef, and NWS severe weather warnings (yellow polygons) for a storm in the Philadelphia area. 

ProbSevere v3 (PSv3), was able to get a better handle on this storm than v2. PSv3 increased to 28% at 23:40 UTC, the time of the first report. This increase was due to a favorable environment (eff. bulk shear = 53 kt; Meanwind 1-3 km AGL = 41 kt; STP = 0.6, 0-3 km lapse rate = 6.8 C/km) and increasing lightning density (though the storm still had a low flash rate, overall). 

At 23:54 UTC, PSv3 was 41%, with MRMS azimuthal shears increasing modestly. Though the probabilities were somewhat low, PSv3 showed a good improvement over PSv2, which gave probabilities of < 10% almost throughout. 

Forecasters will be able to evaluate PSv3 this spring at the HWT. An offline analysis of PSv3 has found it better calibrated and more skillful that v2, overall. 

Figure 2: Time series of PSv3, PSv2, NWS warnings, and severe local storm reports.