Composite Analysis of Heavy-Rain-Producing Elevated Thunderstorms in the MO-KS-OK region of the United States

This study focuses on composite analyses of elevated thunderstorms that produced heavy rainfall in the Springfield, Kansas City, Topeka, Wichita, and Tulsa National Weather Service County Warning Areas (CWA) between 1979 and 2012. These CWAs encompass the region which Colman (1990) found experiences the most elevated thunderstorms in the United States. The events composited were chosen if the U.S. Unified Daily Precipitation Analyses exceeded 2 inches between the months of April and September. Event times were then selected using the NARR 3-hourly accumulated total precipitation for the following three hours. Using compositing software developed at Saint Louis University and NARR 3-hour model data, composites of the event 1) near but prior to, 2) six hours prior to, and 3) twelve hours prior to the heaviest rainfall were generated to analyze the environment conducive for these heavy-rainfall-producing elevated thunderstorms. These results should be useful for forecasting and nowcasting such events. Preliminary results for the Springfield CWA only show twelve hours preceding the event a moist environment with a diffluent 1000-500-hPa thickness field and a maximum of θ_e advection over the region. Six hours prior to the event, the composite cross-section begins to show the development of the direct thermal circulation associated with a 250-hPa jet streak, concurrent with a developing low-level jet near 850 hPa. The low-level jet is serving to advect moisture and warm air over a cooler, stable boundary layer, creating elevated instability. The event-time composite shows a steep increase in vertical velocities due to the forcing of low-level jet parcels into the prominent direct thermal circulation. This upward motion occurs in a saturated, unstable environment and leads to the development of heavy-rain-producing elevated thunderstorms in this area.