The extensive episode of derecho-producing convective systems in the United States during May and June 1998: A multi-scale analysis and review

A multi-scale analysis is presented on widespread and long-lived convectively generated windstorms, known as derechos. Analyses of the derecho-producing environments during 15 May-30 June 1998 indicate that this exceptional episode of derechos and derecho groupings (or series) was supported by ingredients (i.e. moisture, instability, and wind shear) that were supplied by the large-scale setting. In particular, the semi-stagnant subtropical ridge and associated capping inversion across the southern tier of the U.S. were important in supplying amplified moisture and instability to derecho-genesis regions through an underrunning process. Regions of preferred derecho formation appeared to correspond to shifts in the overall strength and position of the ridge, illustrating the importance of the ridge in focusing successive organized convection. Initiating mechanisms varied widely and were not restricted to warm-air advection regimes along quasi-stationary boundaries that forecasters often associate with warm-season derecho environments. In several cases, derecho-producing convective systems were generated by tropospheric features not consistent with common conceptual models of derecho environments such as closed lows and strong vorticity maxima. Further, three distinct series types were identified and classified based on their initiating mechanisms. Copyright (C) 2007 Royal Meteorological Society.