An observations-based statistical system for warm-season hourly probabilistic forecasts of low ceiling at the San Francisco International Airport

A computational system that uses statistical equations to forecast hourly p robabilities of marine stratus burnoff (via ceiling) at the San Francisco i nternational Airport for 1-6-h lead times is developed. The system is based entirely upon surface and upper-air observations in the San Francisco Bay Area as predictors. A test of the product on a 3-yr independent sample shows a 6%-21% reduction in the mean square error (mse) compared with persistence "climatology." Th e amount of improvement is noteworthy, considering that a dearth of reliabl e observations exists upstream from the airport. Moreover, the inclusion of important upper-air predictors into the forecast equations can reduce the mse by 3% when compared with a system of equations derived solely from surf ace data. Paired-difference tests reveal that the upper-air data provide th e greatest contribution for valid rimes that are nearest to the data's obse rvational time. Ceiling forecasts are compared to Model Output Statistics (MOS) probabilist ic ceiling forecasts for two different MOS lead times. When forecasts valid at 1500 UTC are verified, 3-h observation-based forecasts result in a 32% reduction in mse over MOS forecasts having a 13- to 15-h lead time. When a more competitive 4- to 6-h MOS lead time is allotted (using an 1800 UTC val id time), 3-h observation-based forecasts result in an 8% reduction in the mse over MOS forecasts. Analysis of the predictive system's performance on the 3-yr independent sam ple reveals that a broad distribution of probabilistic forecasts is produce d, in contrast with forecasts made from persistence climatology, which can offer only a limited probability distribution for each case. Because the pr obabilistic forecasts are shown to be unbiased, it is expected that similar systems designed for operational use would guide users toward more prudent decisions on the implementation or termination of air traffic delay progra ms.