Verification of operational cool-season tornado threat-area forecasts from mesoscale NWP and a probabilistic forecast product

Cool-season tornado threat-area forecasts based on numerical weather prediction (NWP) model forecasts of vertical shear, thermodynamic instability, and convergence have been issued twice daily in Australia since the southern winter of 1999. This paper summarises a tiered verification of these operational forecasts for the four cool seasons from 1999-2002 inclusive. These verifications included 27 case studies, detailed model threat-area forecast climatologies, and ingredients verification. It is shown that in most cases useful forecast guidance was provided, that the model climatology has similar spatial and temporal structure to that observed, and that an assessment of the ingredients forecasts by means of contingency tables provides a benchmark objective verification that can be used to monitor product and numerical model forecast performance in predicting these rare, extreme events. A probabilistic threat-area product, based on mesoscale numerical weather prediction model output and verifying radiosonde data for the 2000-2002 seasons, to identify areas where environmental shear and instability are such that there is potential for cool-season tornado occurrence is described. These probabilities are conditioned on the model climatology, and so show a continuous field of probability rather than the earlier categorical threat forecast maps. Testing the system using independent data for the cool season of 2003 shows useful skill.