Forecast Skill of Synoptic Conditions Associated with Santa Ana Winds in Southern California

Abstract

Santa Ana winds (SAW) are synoptically driven mesoscale winds observed in Southern California usually during late fall and winter. Because of the complex topography of the region, SAW episodes can sometimes be extremely intense and pose significant environmental hazards, especially during wildfire incidents. A simple set of criteria was used to identify synoptic-scale conditions associated with SAW events in the NCEP?Department of Energy (DOE) reanalysis. SAW events start in late summer and early fall, peak in December?January, and decrease by early spring. The typical duration of SAW conditions is 1?3 days, although extreme cases can last more than 5 days. SAW events exhibit large interannual variations and possible mechanisms responsible for trends and low-frequency variations need further study. A climate run of the NCEP Climate Forecast System (CFS) model showed good agreement and generally small differences with the observed climatological characteristics of SAW conditions. Nonprobabilistic and probabilistic forecasts of synoptic-scale conditions associated with SAW were derived from NCEP CFS reforecasts. The CFS model exhibits small systematic biases in sea level pressure and surface winds in the range of a 1?4-week lead time. Several skill measures indicate that nonprobabilistic forecasts of SAW conditions are typically skillful to about a 6?7-day lead time and large interannual variations are observed. NCEP CFS reforecasts were also applied to derive probabilistic forecasts of synoptic conditions during SAW events and indicate skills to about a 6-day lead time.