Factors influencing the development and maintenance of heavy-rain-producing convective systems

Recent advances in both computing capacity and numerical weather prediction (NWP) model development have allowed for ensembles of forecasts with explicitly predicted convection to be run in real time. As part of the National Oceanographic and Atmospheric Administration (NOAA) Hazardous Weather Testbed (HWT) Spring Experiment in 2010, the Center for the Analysis and Prediction of Storms (CAPS) ran a 26-member ensemble at 4-km grid spacing, referred to as the Storm-Scale Ensemble Forecast (SSEF). The capabilities of such ensembles, and their representation of forecast uncertainty, in different weather scenarios are not entirely known, however.

In this study, SSEF forecasts from successive days of extreme rainfall in the southern United States will be examined. From 9--11 June 2010, a mesoscale vortex moved from Texas northeastward into Arkansas and was associated with several periods of heavy rainfall that led to flash flooding. During the overnight hours, mesoscale convective systems (MCSs) developed that moved slowly over relatively small areas in south central Texas on 9 June, north Texas on 10 June, and western Arkansas on 11 June. The ability of individual members of the SSEF to predict these MCSs will be examined, as will the representation of mesoscale processes within the ensemble.

The processes favorable for the initiation, organization, and maintenance of these heavy-rain-producing MCSs will be diagnosed by comparing ensemble members with accurate and inaccurate forecasts. In these events, there is a strong sensitivity to the representation of planetary boundary layer (PBL) processes. Differences in daytime PBL evolution determine whether convection initiates during the day, which then influences the overnight heavy rainfall. In all, the ensemble provides clues to the mesoscale processes that are most favorable (or unfavorable) for the development of localized extreme rainfall.