Predictands

Forecast equations have been developed to predict the probabilities of seven categories of ceiling height and four categories of total cloud amount. The category definitions of both parameters are the same as those used for the NGM MOS forecasts. The definition of ceiling height categories one through seven are: < 200 ft, 200 - 400 ft, 500 - 900 ft, 1000 - 3000 ft, 3100 - 6500 ft, 6600 - 12000 ft, and > 12000 ft. The four total cloud amount category definitions are: overcast, broken, few/scattered, and clear. The AVN MOS ceiling height and total cloud amount equations are now being developed for projections 6 through 72 hours at 3-h intervals after 0000 and 1200 UTC. The observations of ceiling height were obtained from the METAR hourly observation; the cloud amount observations were obtained from the same source complemented by satellite cloud data.

Predictors

The predictors used to develop the forecast equations include AVN model, observation, and climatic predictors. The AVN model predictors were available at 3-h intervals for the 6-h through 72-h forecast equations after either 0000 or 1200 UTC. Most of the AVN model predictors consisted of multiple level and mean level (the mean between 2 levels) values. These parameters included the relative humidity the wind components, wind speed, vertical velocity, relative vorticity, absolute vorticity advection, the lifted condensation level, and relative humidity times vertical velocity. Other model predictors included total precipitable water, 3-h precipitation amount, and the K Index. Observed ceiling height, surface total cloud amount, wind speed and wind direction were used as potential predictors for the 6-h through 24-h forecast equations to incorporate persistence into the equations. These predictors consisted of observations reported 3 hours after 0000 or 1200 UTC (these observations are usually available when the AVN MOS forecasts are run operationally). If the observations are not available, backup equations not requiring observations as predictors are used. Climatic predictors for all projections included the monthly relative frequencies of ceiling height below 1000 ft, and total cloud amount for clear, scattered-broken combined, and overcast.

Developmental Data Sample

Data from the period April 1997 through September 1999 were stratified into two, 6-month seasons: cool (October - March) and warm (April - September). Only cool season results are presently available. Approximately 400 days were used to develop the cool season equations. The test sample consisted of the last 15 days of each month during the period October - March 1998-99.

Verification

A standard set of stations covering six regions in the continental United States, Alaska, and Hawaii and Puerto Rico combined, were used as verification for the test sample period. We used the P-score and the ranked probability score to evaluate the forecasts on independent data. Results indicated that the AVN MOS probabilistic forecasts were either more accurate or at least as accurate as the NGM MOS. These graphs are available under ceiling height and total cloud amount on the Internet at address www.nws.noaa.gov/tdl/synop/results.htm.