Use of a new Thunderstorm Potential Index for 12-hour forecasts using mesoscale model data

Severe weather forecasters at the Air Force Weather Agency produce graphical forecasts of thunderstorm areas with expected severity three times each day for dissemination to base weather stations and other DoD customers. These Military Weather Advisory (MWA) forecasts depict aerial coverage of storms expected during the valid forecast period, plus predictions of storm intensity and time periods in each outlined area. Use of a new Thunderstorm Potential Index (TPI) provides forecasters with a “first guess” mesoscale model-based tool depicting areas of general and severe thunderstorms predicted to occur during the MWA valid period. TPI charts are intended to give forecasters an idea of where appropriate MWA thunderstorm areas should be forecast for the upcoming period.

The TPI was originally developed using a “perfect prog” approach with the goal of accurately predicting percent probabilities of local thunderstorm occurrence within a 100km radius of a RAOB location during the 12 hours after RAOB time. 1200 UTC RAOB data from 13 selected locations across the continental United States were collected from March to October of 1990 and 1993. The 13 locations represented diverse climatic regions with a mix of sites from coastal regions of the East Coast and Southeast to the Northern-, Central-, and Southern Plains and Rocky Mountains. Stability indices were calculated for all RAOBs collected and correlated with thunderstorm reports (from surface observations and lightning data) within the 100km radius of the RAOB sites. Three indices (K, LI, and SWEAT) were found to be statistically significant in determining correct yes/no forecasts of thunderstorm occurrence during the forecast periods.

Using multivariate discriminant analysis (using stepwise procedures) to determine the significant indices, a forecast regression equation was derived which produces percent probability of thunderstorm occurrence at a RAOB location. It was found that a threshold value of TPI > 46% maximized the Probability of Detection and minimized the False Alarm Rate. Verification at seven independent locations was completed using RAOB data from April to September of 1996. Comparisons with NGM MOS TSV06 forecasts valid at each verification location produced very favorable results.

With these positive results from the independent verification data set, the TPI was applied to stability indices derived from MM5 output. Contours of TPI=47% have been found to closely match MWA contours for general thunderstorm areas produced by forecasters without knowledge of TPI output. Additionally, TPI threshold values in excess of 70% have been found to correlate with MWA areas of severe thunderstorms. Comparisons of the TPI forecasts and MWA areas over the United States and Europe will be presented showing how the automated contours match up to forecaster-produced MWA thunderstorm areas. Verification results will show performance comparisons of the TPI and MWA using lightning data and severe reports during each forecast period.