Mesoscale Model Ensemble Forecasts of the 3 May 1999 Tornado Outbreak

An ensemble of 32 km mesoscale model forecasts are examined to explore the ability of an ensemble to forecast the atmospheric conditions associated with the tornado outbreak that occurred over Oklahoma and Kansas on 3 May 1999. The ensemble consists of 6 different configurations of the Pennsylvania State University – National Center for Atmospheric Research mesoscale model version 5 (MM5) that are initilized at 0000 UTC 3 May and forecasts made out to 36 h. The configurations used consist of various combinations of the Betts-Miller-Janjic, Grell, and Kain-Fritsch convective schemes, and the Blackadar, Burk-Thompson, and MRF planetary boundary layer schemes.

Results indicate that several of the model versions produce deep convection in central Oklahoma at the correct time. Values of model-derived storm-relative helicity, convective available potential energy, and bulk Richardson number shear from these forecasts are within the ranges for tornadic supercell thunderstorms hypothesized by Stensrud et al. (1997). Curiously, the model forecasts with deep convection in Oklahoma also show a southward extension into central Oklahoma of the zone of higher storm-relative environmental helicity and bulk Richardson number shear. This is strongly suggestive of a nonlinear feedback between the model physics that alters the low-level wind field around convective regions. Other aspects of the model forecasts will also be highlighted at the conference.