Forecasting Tornado Outbreaks vs. Derecho in the Northeast 

Data and Methodology  

Forecasting Tornado Outbreaks vs. Derechos in the Northeast 

Introduction

Data and Methodology

Patterns

Storm Relative Wind Flow

References

For this study, 11 tornado and 10 derecho events were chosen, all in the warm season (May through September). The tornado cases were required to include a strong or violent tornado (F2 or greater) or at least 6 weak (F0 or F1) tornadoes. Nine of the eleven tornado cases contained at least two strong tornadoes.  For the derechos, the criteria determined by Johns and Hirt (1987) was used: 

•  There must be an area of concentrated reports consisting of convectively induced wind damage and/or convective gusts greater than 26 m s^-1 (50 kt) with a major axis length of at least 400 km (250 nm).

• The reports within this area must also exhibit a nonrandom pattern of occurrence and must show a pattern of chronological progression either as a singular swath (progressive) or as a series of swaths (serial).

• Within this area there must be at least three reports, separated by 64 km (40 nm) or more, of either F1 damage and/or convective gusts of 33 m s^-1 (65 kt) or greater.

• Finally, no more than 3 h can elapse between successive wind damage events. Derecho storm structure was verified by radar data.