An Examination of "Parallel" and "Transition" Severe Weather/Flood Events

Classical forms of severe weather such as tornadoes, damaging convective wind gusts, and large hail, as well as severe flooding events, all have large societal impacts. On occasion, severe flooding may occur quasi-simultaneously with severe weather. This study examines the connection between severe weather and severe flooding by analyzing both “Parallel” and “Transition” severe weather/flooding events. “Parallel” events are defined as events where traditional forms of severe weather (tornadoes, damaging winds, large hail) are ongoing at the same time during which severe flooding is occurring. Parallel events often occur in conjunction with heavy precipitation supercells. “Transition” events are defined as events where the main hazardous weather threat shifts from traditional forms of severe weather initially, to severe flooding at a later time. These events often occur when discrete supercells grow upscale into a Mesoscale Convective Systems (MCS).

A climatology of these events is currently being constructed using the NCEP Stage IV Precipitation Analysis coupled with Storm Data information to identify flooding situations, and the Storm Prediction Center's storm report archive to identify traditional forms of severe weather. As an initial screening criterion, cases of potential severe flooding are defined objectively as instances when greater than 100 mm of precipitation associated with convective storms falls within a 12 hour period. Severe Weather cases that occur within the same temporal and geographical domain as the severe flooding are classified as “Parallel” events, whereas severe weather cases that occur upstream in time and space of the severe flooding area are considered “Transition” events. The ability to diagnose and predict these high impact events is important from both the scientific and NWS hazardous weather service perspectives, and future work will include identifying similarities and differences in environmental and dynamical conditions which facilitate these two types of events.