This bulletin, written by Ronald M. Reap of the Techniques Development Laboratory, describes the new NGM-based MOS 6-h aircraft icing index forecasts for the contiguous U.S. that have been produced daily for the National Centers of Environmental Prediction's (NCEP's) Aviation Weather Center (AWC) and Hydrometeorological Prediction Center (HPC) since early 1997.

The forecast equations were developed from forecast fields from the NCEP's Nested Grid Model (NGM) (Hoke et al. 1989), national lightning location data (Krider et al. 1976), and pilot reports (PIREPS) from the AWC's data archives. This TPB is the first to describe the new forecast products for predicting aircraft icing. The forecasts were designed to provide general objective guidance in the 2-26 h timeframe to forecasters at AWC and HPC. AWC is responsible for issuing operational AIRMETS and SIGMETS which are inflight advisories for use by the aviation community.

The new forecasts are valid for the 2-8 h, 8-14 h, 14-20 h, and 20-26 h projections after 0000 and 1200 UTC initial data times. They predict the occurrence of category 4 (light-to-moderate) or greater aircraft icing for grid blocks that are 48-km on a side. Separate forecasts are produced for the high-band (> 15,000 ft AGL) and low-band (< 15,000 ft AGL) regions.