One upgrade is an improved geographical regionalization method for the probabilities and potential, which prevents regional boundary discontinuities that occasionally appear in the operational LAMP lightning forecasts and permits more efficient computer processing. Another upgrade is predictor inclusion of high resolution MOS lightning probabilities based on the National Centers for Environmental Prediction (NCEP) North American Mesoscale Prediction System (NAM), which complements lower resolution NCEP Global-Forecast-System-based MOS lightning probability input used in both models.
The new lightning potential product consists of three levels of lightning threat (low, medium, and high, plus the no default), each of which is specified from the lightning probabilities by applying an objectively-derived probability exceedance threshold. The thresholding-categorization procedure maximizes the Critical Success Index (CSI) for low, medium, and high potential combined (a yes/no lightning re-categorization) subject to a bias constrained to be near 2.7 (for every 27 predicted events just 10 are observed, i.e., a low threshold probability results in a strong overforecast). Similarly, for medium and high potential combined, the prescribed bias is about 1.1 (about the same as with the operational yes/no lightning forecasts), and for high potential the bias is about 0.4 (a high threshold probability results in a strong underforecast). Thus, the bias for each potential level is fixed (standardized), whereas the threshold probabilities (along with the probability ranges) vary with forecast projection, geographical location, season, or time of day.
Comparative scoring of operational versus new lightning probabilities shows a clear improvement in Brier skill score with the latter, especially for forecast projections beyond about 6 hours. The new probabilities also show improved sharpness and geographical focus, which is seen from subjective examination of individual cases. Also, CSI scores for medium and high lightning potential combined are higher (better) than CSIs for the operational yes/no lightning forecasts. The improved forecast performance is attributed primarily to the supplemental predictor input from the NAM MOS probabilities.