The O2R-R2O Effort to Improve Tornado Forecasting at NSSL and SPC Using Calibrated CAM Ensemble Guidance

In recent years, convection-allowing model (CAM) ensemble output has been extended from forecasting a general severe convective threat into forecasting individual hazards such as tornadoes. Creating these hazard forecasts requires close collaboration between forecasters and researchers. This work combines a ten-member convection-allowing ensemble that produces daily runs generated by researchers at the National Severe Storms Laboratory and climatological tornado frequencies given a right-moving supercell and a significant tornado parameter (STP) value created by forecasters at the Storm Prediction Center (SPC). Collaboration between these two groups led to a methodology of probabilistic tornado forecasting that extends beyond the “magic threshold” paradigm, which treats exceedance of environmental and/or storm-scale metric thresholds as a direct proxy for tornado occurrence. Rather, the new method incorporates climatological frequencies to create a probability of a tornado given the environmental conditions and existence of modelled storm-scale attributes. Thus, the subsequent probabilities are rooted in the observations and combine NWP and observed climatology. This method reduces over-forecasting seen by other probabilistic tornado forecasting methods, and produces skillful, realistic forecasts. These forecasts were implemented in two ensembles and tested in the 2017 Spring Forecasting Experiment at NOAA’s Hazardous Weather Testbed. Generally, participants favored this methodology over other methodologies that used the “magic threshold” paradigm, and agreed that the forecasts were useful as a first-guess for SPC forecasters. By combining the expertise of researchers and forecasters, a methodology rooted in observations and easily adaptable to multiple CAM systems was created and can be used by SPC forecasters as first-guess tornado guidance.