A Synoptic Climatology of Combined Severe Weather/Flash Flood Events

Classical forms of severe weather such as tornadoes, damaging convective wind gusts, and large hail, as well as flash flooding events, all have potentially large societal impacts. This impact is further magnified when these hazards occur simultaneously in time and space.  It is a major challenge for operational forecasters to not only accurately predict the development and evolution of such combined events, but also to communicate multiple threats to the public in real time.  This goal of this study is to gain further insight into combined severe weather/flash flooding events across the contiguous United States through the development of a seven-year climatology (2009-2015) of such events.

Combined severe weather/flash flooding cases were identified using reports from the NOAA Storm Data publication. Daily plots of color-coded reports, stratified by hour and event type, were created and used in conjunction with mosaic radar imagery to determine which reports were attributable to a given convective system.  In order to focus on cases affecting mesoscale regions in time/space associated with the same convective system(s), a minimum threshold of 30 severe reports and 10 flash flood/flood reports was applied to identify candidate events. During the seven-year study time period over 150 events were identified. Data from these events have been analyzed using GIS software to extract diurnal, seasonal and geospatial information about the events. Climatological results suggest that combined severe weather/flash flooding events are most common geographically in the Lower Great Plains as well as the Mississippi and Ohio River Valleys. Seasonal and diurnal cycles followed closely to those observed for severe weather, except across the Great Plains and Mississippi Valley, where the peak in flash flooding was generally later at night. 

Once such cases were identified, they were also classified subjectively by convective morphology into seven commonly observed categories, based upon system evolution using archived radar and satellite imagery. Composites of synoptic/mesoscale pattern and environment for each category were created using 13 km RAP/RUC analysis data. Results from this part of the analysis will be presented at the conference.