Forecasting Heavy Wintertime Precipitation Events in Southern California (Formerly Paper 9.6)

Southern California receives much of its annual precipitation through heavy rainfall events during the winter season. Given the potentially large socioeconomic impact of flooding that might occur during significant precipitation events, the aim of this study is to ascertain whether anomalies of various forecast parameters might be used to identify heavy rainfall events a few days in advance.

For 34 cases of 4-inch or greater rainfall events that occurred in southern California between December and March 1991-2000, composites of heights, winds, precipitable water, and moisture flux at various standard atmospheric levels were created. These composites were then compared to climatological norms of the same fields to determine the magnitudes of anomalies that were present. The following anomalies were found to occur during heavy wintertime rainfall events over southern California. Heights were significantly below normal off the northern California coast at 700-, 500-, and 250 hPa. Southern California was under the influence of anomalously strong left exit region jet dynamics at 250 hPa, as well as a southwesterly low level jet. In terms of moisture, a positive precipitable water anomaly was centered over southern California, and a positive anomaly in moisture flux at 850 hPa was nosing into the region.

It is hoped that numerical weather prediction models can be used to indicate an imminent threat of significant precipitation by determining whether anomalies in heights, winds, and moisture as outlined above are present. An ensemble approach might provide the best forecast utility.