Evaluating the Sensitivity of Convection Forecasts with Large Synoptic Forcing During MPEX

Although the ingredients that lead to a significant severe weather outbreak over the Southern Great Plains have been known for a long period of time (i.e., deep trough over the Rockies, moist southerly flow in the Plains, etc.), what is not as clear is how uncertainty in these components during the morning before the outbreak can contribute to errors in the subsequent convection forecast that afternoon. In particular, it is not clear what is the relative contribution of uncertainty associated with the upstream trough, dry line, or moist southerly flow ahead of the dry line on the convective forecast. For this study, we apply ensemble-based sensitivity analysis to 3-km resolution Weather Research and Forecasting (WRF) ensemble model forecasts that were run during the 2013 Mesoscale Predictability Experiment (MPEX) for two cases (19 and 30 May) characterized by large-scale forcing for severe weather over the Southern Great Plains and similar synoptic-scale flow patterns and quantify how uncertainty in these larger-scale features impact the subsequent convection forecast.

After providing a brief overview of the ensemble-based sensitivity technique, we will use this method to determine how uncertainty in the upstream trough, dry line, southerly return flow, etc. at 1200 UTC impact subsequent forecast metrics related to convective initiation later in the day (i.e., area-average precipitation or vertical kinetic energy). Preliminary analysis suggests that convective forecasts over Oklahoma during the 19 May case are sensitive to the southern edge of the upstream trough over the Rockies, even though this trough passes to the north of Oklahoma in the forecast, and to the amount of poleward moisture advection over central Texas and Oklahoma on the morning before the convective outbreak. For the 30 May case, the largest sensitivity is also associated with the upstream upper trough, with comparatively smaller sensitivity to the lower tropospheric wind and moisture fields. The relationship between the regions of sensitivity and MPEX dropsondes will also be discussed.