GFS-Based MOS Opaque Sky Cover and Ceiling Height Guidance for the Contiguous United States, Alaska, Hawaii, and Puerto Rico

Sky cover and ceiling height are important weather elements in public weather forecasting, and they are also of special interest to the aviation industry. The Meteorological Development Laboratory of the National Weather Service (NWS) has developed a statistical guidance package of opaque sky cover and ceiling height by applying the Model Output Statistics (MOS) technique to the National Center for Environmental Prediction's Global Forecast System model output. Sky cover is defined by the NWS as the cloud amount in eighths covering the celestial dome observed from the ground surface, and ceiling height is the height above the earth's surface of the lowest non-surface based cloud layer when the sky cover is reported as broken or overcast.

A hybrid approach was adopted for the sky cover redevelopment to produce more skillful guidance. Where developmental data were adequate, equations were developed for single stations; equations were regionalized, otherwise. Ceiling height development used only the regionalized approach. The MOS equations were developed to calculate the probabilities of five sky cover and eight ceiling height categories. Categorical forecasts were generated from the probabilities by using probability thresholds. These thresholds were obtained from the developmental sample and were designed to maximize the threat score while keeping bias within a narrow range around unity.

The new MOS sky cover guidance provides 3-h forecasts from 6 to 192 hours after the initial model runs at 0000 and 1200 UTC, and from 6 to 84 hours at 0600 and 1800 UTC. Ceiling height guidance is available every 3 hours from 6 to 84 hours at all 4 cycles. Both sky cover and ceiling height guidance are available for the contiguous United States, Alaska, Hawaii, and Puerto Rico. Test results on independent data show significant improvement in forecast skill by the new MOS opaque sky cover guidance over the current operational one. We attribute this progress mainly to the use of the hybrid approach in the development of the sky cover forecast equations.