Application of the NCEP/EMC Short Range Ensemble Forecast System (SREF) to Predicting Extreme Precipitation Events

The Environmental Modeling Center (EMC) of the National Centers for Environmental Prediction (NCEP) has developed and is now running routinely a Short Range Ensemble Forecast (SREF) system. The current system is composed of 10 members composed of five members from both the Eta and Regional Spectral Model (RSM) with 48 km horizontal resolution. Initial state perturbations are provided by "breeding", as for the NCEP global ensemble system, but in the context of the respective regional models. Perturbations to physics, as well as initial conditions, and inclusion of additional models are anticipated.

This paper will focus on the application of the SREF system to extreme precipitation events, especially (but not necessarily limited to), the "surprise" heavy snowfall associated with the cyclogenesis that affected the east coast on January 25-26, 2000. The intent is to evaluate the sensitivities of model simulations of the storm and associated sensible weather to the imposed perturbations and address their significance to the cyclone evolution and forecasts of the impending snowfall. A principle issue is the extent ensemble and other model solutions, combined with non-model sources of information, provide a credible threat warranting issuance of appropriate guidance, e.g., "storm watch", to the public and emergency managers. It is argued that when such a threat is foreshadowed and its impact likely would be considerable - i.e., clearly in the envelope of possibilities, but not a certainty in the deterministic sense - it is incumbent upon the National Weather Service and other providers of weather forecasts to convey that information in an appropriate, relevant, and timely manner. Some examples of related issues and approach (e.g., probabilistic forecasts, specifying the most likely outcome with alternative scenarios) will be discussed.