The Minneapolis Flash Flood: Meteorological Analysis and Operational Response

Abstract

The evolution of the synoptic- and meso-α-scale meteorological setting for the 23 July 1987 Minneapolis flash flood is described. Analyses of conventional upper-air data, including quasi-geostrophic processes, are employed to identify the large-scale forcing that set the stage for the development of mesoscale convection. Surface mesoanalysis identified a significant outflow boundary and mesohigh-pressure system produced from afternoon thunderstorms over east-central Minnesota and western Wisconsin. This outflow boundary became stationary over Minneapolis-St. Paul (the Twin Cities), providing a convergence zone that acted to focus thunderstorm development. Satellite imagery shows that the thunderstorms associated with the mesoscale convective system (MCS) over Wisconsin developed westward during late afternoon and early evening. Radar reflectivity indicated that rapid cell generation occurred just west of the Twin Cities where the outflow boundary from the Wisconsin MCS intersected a second boundary-layer convergence zone oriented east-northeast to west-southwest. A mesolow pressure system exhibiting a line echo wave pattern (LEWP) developed near this intersection or triple point, about 80 km west of Minneapolis. Rapid cell generation occurred just west of the Minneapolis-St. Paul metroplex, and the cells traversed the same area in a ?train echo? pattern that produced pulsating heavy rains. Operational response to this multifaceted weather system is also examined. It appears that forecasters were over-burdened with the issuance of both severe thunderstorm and tornado warnings and did not have the time to perform the necessary analyses and diagnoses needed to keep abreast of current weather, and to anticipate future developments. In addition, the overriding concern for hail, high winds, and tornados at the expense of overlooking the flash-flood threat may have been partly due to the fact that the operational weather system did not verify flash-flood watches and warnings until 1988. A review of operational procedures suggests that equal emphasis in forecasting severe thunderstorms and forecasting heavy rainfall could improve Weather Service response to future multihazard situations.