Mesoscale Model Ensemble Forecasts of the 3 May 1999 Tornado Outbreak

Abstract

A six-member ensemble is developed in which the ensemble members only vary in their model physical process parameterization schemes. This approach is accomplished by mixing three different convective parameterization schemes with two different planetary boundary layer schemes within the nonhydrostatic Pennsylvania State University?National Center for Atmospheric Research Fifth-Generation Mesoscale Model (MM5). The initial and boundary conditions for each ensemble member are identical and are provided by the National Centers for Environmental Prediction Eta Model forecasts starting from 0000 UTC. Verification of the ensemble predictions against Eta Model analyses over 42 days indicates that, although this ensemble system is underdispersive and imperfect, the ensemble forecasts show some skill in predicting the probability of various severe-weather parameters exceeding selected threshold values. This model physics ensemble allows us to begin exploring the possible uses of ensemble forecasts for severe-weather events. Results from this six-member ensemble forecasting system of the 3 May 1999 tornado outbreak indicate that the ensemble provides a strong signal of two mesoscale-sized regions, one in Oklahoma and Kansas and the other in eastern Nebraska, that have the potential for supporting tornadic supercell thunderstorms. Several of the model forecasts also produce convection in these regions. Tornadic thunderstorm reports are found in both of these areas. This ensemble guidance does not provide any clues as to why the tornadoes in Oklahoma and Kansas were so severe, as compared with those in Nebraska, but it does provide hope that ensembles may be useful for short-range forecasting of severe weather.