FIRST BULLETIN ON THIS SUBJECT

This bulletin, written by Ronald M. Reap of the Techniques Development Laboratory, describes the new NGM based 6-h probability forecasts of nonconvective clear-air-turbulence (CAT) for the contiguous U.S. that have been produced daily in a non-operational mode for the National Centers for Environmental Prediction's (NCEP's) Aviation Weather Center (AWC) and Hydrometeorological Prediction Center (HPC) since August 16, 1995. The probability 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 (PIRITS) from the AWC's data archives. This TPB is the first to describe the new forecast products for predicting CAT. 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 AMMETS and SIGMETS which are inflight advisories for use by the aviation cominunity.

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 give the unconditional probability of category 3 (light-to- moderate) and category 5 (moderate-to-severe) or greater CAT occurring in 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.




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