Post-Processing of Canadian Regional-Scale NWP to Develop First-Guess Forecasts of Thunderstorm and Severe Weather Threat Areas

Post-processing of NWP data to provide guidance for forecasters is a widely-used practice within weather services across the globe. In Environment Canada, recent emphasis has been placed on modernizing the suite of operational guidance related to mesoscale convection and severe weather. As part of this effort, experimental forecast products are also being explored to support forecasts of thunderstorm and severe weather threat areas. Such area-based forecast products are expected to form the basis of convection-related “first-guess” meteorological objects within next-generation production systems under development.

Building on earlier efforts to develop thunderstorm area forecasts, output from Environment Canada's Regional Deterministic Prediction System (RDPS; 10-km horizontal grid spacing) model has been calibrated to observed lightning from the 2013 convective season (1 May to 30 Sep 2013). The calibration utilizes four simple “pre-storm environment” parameters associated with convective initiation that are familiar to forecasters and easily relate to their own analysis-diagnosis-prognosis process. Calibration thresholds are uniquely determined for each forecast hour in the “day-1” diurnal cycle (T+1 to T+24 h starting at 12 UTC). During the 2014 convective season, updateable calibration methods were also tested. These “running” calibration systems utilized calibration periods of 5, 10, 20, and 30 days with thresholds updated daily based on correspondence between model forecast parameters and lightning observations during the previous prescribed number of days. Verification results from the seasonal and running calibration methods for the 2014 convective season will be compared and contrasted.

As an extension of forecast thunderstorm areas, experimental forecasts of first-guess severe weather threat areas are also being tested. In these forecasts, areas are categorized by severe weather threat or storm mode based on selected parameters and thresholds. Two forecasts are generated; a conditional severe weather forecast based on environmental stability and kinematic parameters, and an occurrence forecast utilizing the 30% and greater probabilities from the seasonal calibrated forecast. The occurrence forecast has been verified against severe weather event reports and actual severe weather watches issued by Environment Canada. Results suggest that, even with a non-convection-allowing NWP model, a reasonable first-guess severe weather threat area can be generated. This may be used as a visualization tool for the forecaster, a means of providing “heads-up” alerts for potential areas of interest, or even as the basis of a forecaster-modified severe weather watch area.