Evaluation of Snow Forecasts Provided by the Weather Support to Deicing Decision Making (WSDDM) System

The National Center for Atmospheric Research Weather Support to Deicing Decision Making (WSDDM) system uses real-time snow water equivalent (SWE) data from snow gauges to calibrate radar data. The calibration is done through adjustment of the coefficient in the radar Z-S equation which relates radar reflectivity to snowfall rate (in liquid equivalent). The main output from WSDDM is a 30 min prediction of SWE at a particular gauge site.

The WSDDM system has shown success in the eastern U.S., primarily in New York City and Chicago. This paper reports on an evaluation of the system for New York and for the more mountainous terrain of the Salt Lake City area. The WSDDM system was installed at the Salt Lake City National Weather Service forecast office in 1996. Unlike previous field tests, the Salt Lake system ingests wideband level-II data from the WSR-88D as well as SWE data from 8 snow gauges. Initial performance results were encouraging and showed large fluctuations in the Z-S coefficient with time.

Recent activities have entailed modification of which elevation angles are used in the reflectivity analysis. Data from 20+ winter storms in northern Utah are being used to evaluate the system for the different gauge sites, including the site at the airport. These data have been analyzed using a fixed Z=75S**2 and will be used as a benchmark for the WSDDM system.

Important considerations for this study include the large distance between the WSR-88D's 0.5 degree beam and the airport (~5000 ft) and the shallow nature of Utah's winter storms. Considerations at other sites include beam blockage and varying altitude and range of the gauges.