2Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, OK - U.S.A.
2NOAA/Storm Prediction Center, Norman, OK - U.S.A.
Proximity soundings (within 2 h and 167 km) of derechos (long-lived, widespread damaging convective windstorms) and supercells have been obtained. More than 65 derechos, accompanied by 115 proximity soundings, are identified during the years 1983 to 1993. Owing to the large variability of the synoptic-scale environments associated with derechos, they have been divided into categories: strong forcing, weak forcing, and "hybrid" cases (which are neither weakly nor strongly forced). Nearly 100 supercell proximity soundings have been found for the period 1998 to 2001, subdivided into nontornadic events, tornadic events, and events producing F2 or greater tornadoes.
Derechos occur under a wide range of shear and instability conditions, but the weakly forced situations typically are characterized by large convective available potential instability (CAPE) and relatively weak vertical wind shear. Strongly forced derechos usually have stronger wind shears and lower CAPE than the weakly-forced cases. Most derechos exhibit enhanced storm-relative inflow at low levels. In weakly-forced derechos, this is usually the result of rapid convective system movement, whereas in strongly-forced derechos, storm relative inflow at low levels is heavily influenced by relatively strong low-level windspeeds. "Hybrid" cases collectively look like an average of the weak and strong forcing cases, as might be expected.
Supercells occur in environments that are not all that dissimilar from those that produce derechos, especially derechos occurring with strong synoptic-scale forcing. Although the distinction may not be statistically significant, it appears that derechos have somewhat weaker deep layer (0-6 km AGL) vertical wind shear. Those supercells producing F2 or stronger tornadoes have substantially enhanced 0-1 km (AGL) shear. The Energy-Helicity Index (EHI, proportional to the product of CAPE and helicity) differs substantially between tornadic and non-tornadic supercells. Strongly-forced derechos have a tendency for stronger 0-2 km shear than supercells producing F2 and stronger tornadoes, and are often accompanied by tornadoes, as well as damaging nontornadic winds.
It is not easy to forecast the mode of severe thunderstorm activity on any given day, given conditions that favor severe thunderstorm activity in general. It is possible that the difference between a line of long-lived windstorms and a long-lived, relatively isolated supercell could depend on the details of the ascent that initiates deep convection, as well as subtle differences in the environmental characteristics. Results of this study suggest it might be possible to make probabilistic forecasts of convective mode with some success.