High wind-producing convective systems over the Northern High Plains

Among Dr. Fujita's most significant contributions were his studies, beginning in the mid-1970's, of non-tornadic convective high wind events or downbursts. The impact of these storms has encouraged continued interest in these events.

Significant high wind-producing convective systems over the Northern High Plains (roughly from the Central Dakotas west to the Rockies) were documented and analyzed for the four convective seasons 1996 through 1999. Over one hundred and fifty convective systems were documented. For these cases, radar data were analyzed in order to classify the morphology of each convective event, and the larger-scale convective environments were analyzed in order to assess the environmental conditions that lead to their formation.

The research to be presented essentially addresses the question: "What are the characteristics of, and differences between the environments which produce severe wind-producing squall lines, bow echoes, supercells, and other weakly-organized convective systems over the Northern High Plains?"

Analyses of this large data set indicate several interesting results. Firstly, of all the storms which were initiated over the Northern High Plains, very few of them (less than 8%) maintained their structure long enough to move out of the Northern High Plains - inferring that the observed diurnal progression of convection east of the Rocky Mountains may be more complicated than previously thought. Secondly, there were distinct differences between the environments which generated high-wind producing supercell storms (all but one of which moved to the Southeast) and severe squall lines (the supercell cases demonstrated much deeper/stronger shear, and were frequently post-frontal). In general, all the severe wind events were characterized by relatively strong low-level shear, with most of the more significant cases associated with weak disturbances moving through the upper-level ridge over the area.

Squall lines were responsible for most high wind events followed by squall line storms developing into bow echoes and line echo wave patterns (LEWP). Classic bow echo evolution, as described by Fujita) and supercell storms were less frequent, but contributed disproportionately to the most violent high wind events.