Surface Boundaries and Severe Weather: Living on the “Edge” in a World of Forecast Uncertainty (Invited Presentation)

Severe storm forecasting, and flash flooding is included in this category consistent with international practice, remains a big challenge despite significant forecasting advances thanks to the advent of the state-of-the-art convection-allowing models (CAMs) and mesoscale ensemble forecast models.  Every spring, the Storm Prediction Center (SPC) conducts a Spring Forecasting Experiment (SFE) through the NCEP NOAA Hazardous Weather Testbed.  In recent years, the SFE has focused on evaluating how real-time, CAM-based, mesoscale forecast models predict convective mode and convective initiation in an effort to address how initial condition uncertainty and model physics uncertainty impact severe storm forecast uncertainty on time scales out to 36 hours.  Results from the SFE reveal that predicting convective mode and the timing of convective initiation properly remains a significant challenge.  Many of these forecast challenges link back to difficulties in representing the structure and evolution of surface boundaries properly in the initial conditions and in the ensemble model forecasts

An important component of the SFE is understanding how forecasters can use CAM-based ensemble forecast model output to craft real-time probabilistic forecasts of various severe weather hazards.  The purpose of this presentation is to provide an overview of the role of human forecasters in the severe storm forecasting process in the face of rapid advances in CAM-based, storm-scale numerical weather prediction.  This overview will be presented from the perspective of someone who has followed the weather daily for decades, who has enjoyed the luxury of not sitting on the forecaster hot seat at the SPC (or the NCEP Weather Prediction Center), and who has made more wrong forecasts than almost anyone alive (just ask my wife).  Illustrative examples of some challenging convection-related weather events will be presented.  Experienced human severe weather forecasters are very cognizant of the role that surface boundaries can play in convective initiation and in determining convective mode.  These experienced human forecasters in many cases can maintain an “edge” on the CAM forecasts by focusing on the structure and evolution of important surface boundaries both prior and subsequent to the onset of deep convection. Mesoscale convective systems (MCSs) that transition from severe weather-producing systems to flash flood-producing systems are especially difficult to predict. Illustrative examples of some of these MCS-related forecast challenges will be shown.