Verifying WRF ensemble forecasts of updraft helicity

Increased computational power has allowed for numerical weather prediction models to be run with horizontal grid spacings of a few kilometers. Such convection-permitting forecasts are capable of producing reasonable representations of convective development without using a cumulus parameterization scheme. Nevertheless, these forecasts cannot fully resolve localized severe weather phenomena, such as large hail, strong winds, and tornadoes, that occur on horizontal scales of hundreds of meters to a few kilometers. Thus, in order to predict these phenomena, proxies for these hazards must be generated from model-simulated fields. The proxy used in this study was updraft helicity (UH), which predicts updraft rotation that occurs in the mesocyclones of supercell thunderstorms. This study focused on the predictability of mesocyclones because supercell thunderstorms commonly produce severe weather. Forecasts of UH for one severe weather case were generated from a five-member, convection-permitting Weather Research and Forecasting (WRF) ensemble and were verified against a rotation tracks product developed at the National Severe Storms Laboratory. The rotation tracks product is a measure of azimuthal shear derived from the radar radial velocity field. Neighborhood probabilities of the forecast and observed fields exceeding specified thresholds were computed and used to calculate fractions skill score, probability of detection, and probability of false detection.