On a Modified Definition of a Derecho. Part I: Construction of the Definition and Quantitative Criteria for Identifying Future Derechos over the Contiguous United States

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 extratropical, 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 pathlengths 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. Significance Statement In this study, the definition of the term “derecho” has been modified to “a widespread severe windstorm characterized by a family of destructive downbursts containing hurricane-force gusts associated with an extratropical, cold-pool-driven mesoscale convective system.” This proposed definition varies from most previous studies in that there is a stricter requirement for the presence of relatively more widespread significant-severe/hurricane-force wind gusts (33 m s −1 /65 kt/75 mph) and that, unlike many earlier studies, the term derecho is only applied to organized thunderstorm wind swaths developing from cold-pool-driven MCSs. Continued increases in thunderstorm wind reports (including measurements) over the last couple of decades have supported better sampling of rarer, particularly damaging thunderstorm wind events, allowing for a stricter definition based on reports. Quantitative criteria used to identify derechos were revised based on common features associated with the more organized cold-pool-driven MCSs and associated wind swaths over the past 23 years. These criteria will be implemented to preliminarily identify derechos in real time, with such information being readily available for dissemination.