Previous studies were prompted by multiple past scenarios since 2003 where significant severe weather was predicted but little to no severe weather (and in some cases no convection) occurred. Those studies identified atmospheric features that distinguish the occurrence of significant (hail > 2 inches in diameter, straight-line winds of ≥ 32 ms-1 (65 kt) and/or tornadoes of EF2 magnitude or greater) severe weather outbreaks (including derechos) versus marginal or no severe. It was determined that progressive 850 hPa and 500 hPa vorticity centers and 850 hPa equivalent potential temperature change (Δθe) ≥25 K in 12 hours identified the strong synoptic scale low-level and midlevel forcing that supported significant severe weather events. Other important features include significant instability associated with steep midlevel lapse rates (4-layer lifted indices exceeding -4 C) and an 850 hPa wind maximum ≥40kt.
In this study, 28 significant severe weather events from 1953 to present were analyzed and composites of 850 hPa Δθe were created. It should be noted that the composites represent the maximum Δθe within 6 hours of severe weather reports, since the 850 hPa density discontinuity is often displaced from local and mesoscale surface features in complex terrain. It should also be noted that depictions of Δθe and θe gradients differ depending on the resolution of the data set being used. The North American Regional Reanalysis (NARR) dataset, with 0.3° (32 km) resolution was used in this study.
The composites determined that Δθe ≥ 20K over a distance of 400 km (250 miles) represented the synoptic scale density discontinuity associated with significant severe weather events. Separate composites were created for the Northeastern U.S., Mid-Atlantic U.S. and for derechos. Time lagged composites of Δθe and recent examples of significant severe weather events will also be shown.