With the advent of NWP models containing explicitly resolved convection comes the potential for new forms of forecast guidance. The latest operational models lack the resolution required to simulate many severe phenomena associated with convection (e.g. large hail, downburst winds, and tornadoes), but they can still provide unique guidance for the occurrence of these phenomena if “extreme” behaviors in simulated storms are strongly correlated with observed severe phenomena. The basis for such guidance is fundamentally different from traditional NWP-based assessments of severe weather potential because it relies on identification of explicit convective phenomena rather than environmental conditions that might support severe thunderstorms. This concept is explored by examining several fields from the 4km NSSL-WRF model to determine their correspondence to observed severe weather reports. Specifically, fields based on simulated model reflectivity, vertical velocity, 10 meter wind speed, and updraft helicity (a diagnostic field to identify supercellular storms) were chosen, as subjective assessments suggested that they were likely to be useful indicators of convection intensity in the model.

The goals of this work are to 1) identify which of these fields are the best predictors of observed severe phenomena and 2) calibrate probabilistic forecasts based on these predictors. To achieve these goals, extreme values of these fields were flagged as "surrogate" severe reports and the correspondence between these surrogate reports and actual observed severe reports was determined. On the basis of this correspondence, a method to generate probabilistic guidance for severe weather was developed. Visual comparisons between this guidance, observed reports, and SPC Outlooks for several cases will be presented. Verification of the guidance during the 2008 severe weather season will also be shown.

last edit: Sun Mar 7 18:32:29 2010 CST , by lmf