Forecasting Synoptic and Mesoscale Environments for Tornadoes and Derechos in the Northeast United States

The short lead time associated with issuing a tornado and/or severe thunderstorm warning limits the average forecaster to a quick subjective examination in identifying synoptic and mesoscale conditions associated with tornadoes and severe thunderstorms. This analysis is usually based on assessing a few parameters based on the convective nature of the event. Previous modeling studies of tornadic and severe (e.g. Weisman and Klemp 1982,1984; Stensrud etal., 1997) and theoretical studies (e.g. Davies-Jones 1984; Rotunno and Klemp 1985) have shown relationships between low-level vertical shear and mid-level mesocyclones in supercell thunderstorms. Our study will focus on comparing the synoptic and mesoscale environments that produce significant tornado outbreaks and derechos in the northeastern sector of the United States. It is anticipated that such an analysis will allow us to identify common signatures that may help to distinguish favorable patterns for tornadogenesis and the formation of derechos. A desired outcome of the analysis will be the creation of a set of advanced forecast and warning criteria that can be used operationally in future events.

Our analysis is based on eleven tornado outbreaks and ten derecho events for the last thirty years in the northeast. Preliminary results, using the NCEP/NCAR reanalyis grids from 1973-1998, show that the majority of the tornadoes developed in the equatorward exit region of a strong upper-level jet streak. The majority of these cases occurred beneath an upper-level northwesterly flow. All cases featured a combination of synoptic-scale ascent, upper-level divergence and lower-level instability (CAPE >0). Composite analyses that illustrate the life cycle of tornado and derecho events are in preparation and the results will be presented during the conference.