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.