Analyzing the Role of Low-Level Forcing in Significant Severe Weather Outbreaks in the Eastern U.S.

Distinguishing the mode of convection prior to development is a challenge that has always existed for forecasters. The Storm Prediction Center routinely issues convective outlooks in 6 categories: no severe, marginal, slight, enhanced, moderate and high risks. These risks are based on expected density of severe weather reports and the magnitude of the potential severe weather, significant or not. Local National Weather Service offices then tailor these convective outlooks for their local forecast areas. Determining whether a severe weather event has the potential to be significant or not, has been especially challenging in the Mid-Atlantic and Northeastern U.S.

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 (Δθ_e) ≥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 Δθ_e were created. It should be noted that the composites represent the maximum Δθ_e 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 Δθ_e and θ_e 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 Δθ_e ≥ 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 Δθ_e and recent examples of significant severe weather events will also be shown.