On a modified definition of a derecho. Part I: Construction of the definition and quantitative criteria for identifying future derechos over the contiguous U.S.

Abstract

Abstract The definition of a derecho has been modified to require hurricane-force (33+ m s−1) gusts, a staple for destructive wind events in earlier derecho studies. The new proposed definition states that the derecho is a widespread severe windstorm characterized by a family of destructive downbursts containing hurricane-force gusts, and is associated with an extra-tropical, cold-pool-driven Mesoscale Convective System. Derecho identification is performed with the use of an algorithm, rendering this study among the first to identify derechos objectively. Through sensitivity tests and qualitative analyses, we propose that the term derecho be used for thunderstorm wind swaths that have path lengths of at least 400 km, with no more than 1-h or 200-km (temporal or spatial, respectively) gaps allowed between wind reports, with at least five reports of 33+ m s−1 wind gusts, separated by 80 km, required for derecho identification. To account for possible overestimation of wind speeds, at least three 33+ m s−1 thunderstorm wind gust reports, separated by 80 km, need to be measured.