Refining CAM-based Tornado Probability Forecasts Using Storm-inflow and Storm-attribute Information

The 3-km horizontal grid spacing of operationally-run convection-allowing models (CAMs) allows for resolving of some important severe storm processes and the local environment, but not explicit forecasting of tornadoes. Previous studies have identified empirically-derived tornado frequencies that correspond to ranges in values of the Significant Tornado Parameter (STP). The STP accounts for environmental instability, thermodynamic cap, wind speed and directional shear, as well as lifting condensation level of the near-storm environment. This study investigates various methods for extracting a distribution of STP values that characterizes the storm-inflow environment based on CAM output for a suite of over 150 spring severe storm events from two select years. Various thresholds in updraft helicity (UH), used to filter simulated rotating storms, are evaluated as well. For each day and across a sub-CONUS domain, tornado probabilities are generated for combinations of each STP extraction method and UH threshold and compared to tornado probabilities from a practically perfect hindcast.