Post processed short range ensemble forecasts of severe convective storms

The Storm Prediction Center (SPC) issues both deterministic and probabilistic outlooks of thunderstorms and severe thunderstorms (a severe thunderstorm is a thunderstorm accompanied by large hail (> 0.75”), damaging wind (> 58 mph), and/or one or more tornadoes) for the contiguous United States. These outlooks are issued for time periods covering the next one to eight days. A short-range ensemble forecast technique is described herein that produces 3h probabilistic guidance of one or more severe thunderstorms on a national 40 km grid.

The National Centers for Environmental Prediction (NCEP) short-range ensemble forecast (SREF) system is used to account for both model and initial condition uncertainty of environmental parameters considered important to the development of severe convective storms. A conditional probability of a severe thunderstorm is produced from 21 paired SREF parameters (or layers) such as Pr [CAPE > 500] and Pr [Shear > 30 kts]. Each layer is calibrated using the SPC real-time database of preliminary severe weather reports to determine the frequency of a severe event based on similar environmental conditions over the previous 366 days. The calibration technique for each layer is essentially identical to the methodology described in Bright et al. (2005) to produce calibrated SREF probabilistic thunderstorm guidance. Although more sophisticated statistical techniques may be applied in the future, the final severe thunderstorm probability at each grid point is currently the maximum probability produced by any of the 21 layers.

The unconditional probability of severe weather is obtained by taking the product of the abovementioned conditional severe probability and the SPC SREF calibrated thunderstorm probability (Bright et al. 2005). The unconditional probability of a severe thunderstorm is available for each 3h period through the SREF integration time of 63 hours. Common statistical assumptions extend the probability from a 3h interval to 12h and 24h periods. Verification results reveal skillful and reliable real-time probabilistic severe thunderstorm guidance is obtained from the NCEP SREF through this prediction scheme.

Bright, D.R., M.S. Wandishin, R.E. Jewell, and Steven J. Weiss, 2005: A physically based parameter for lightning prediction and its calibration in ensemble forecasts. Preprints, Conf. on Meteor. Applications of Lightning Data, San Diego, CA, Amer. Meteor. Soc., CD-ROM, 4.3.