Forecasting Extreme Wintertime Precipitation Events in Northern California

Extreme weather events have a potentially large socioeconomic and human impact. With this in mind, the aim of this study is to offer a forecast approach that integrates operational model data with local climatology. Normalized field departures from local climatology can give forecasters guidance as to the rarity of events, and can help identify potentially significant events.

Composites of height, wind, temperature, and moisture fields were created using NCEP reanalysis data from cases of 4-inch or greater precipitation events that occurred in northern California between December and March 1991-2000. The authors then assessed how well the magnitude of the anomaly of various fields (at the point 37.5N and 120W) correlated with precipitation totals in the Sierra Nevada mountains (between 35N and 40N latitude). The parameters investigated were, 850 hPa moisture flux, 700 hPa moisture flux, the u- and v- components of the wind at 850- and 700 hPa, and precipitable water. It was found that 850- and 700 hPa moisture flux anomalies were most highly correlated with heavy rains, whereas, surprisingly, the anomaly in precipitable water had the lowest correlation. In addition, the authors attempted to answer a nagging question: During the 10 year period being studied, how common are the highly anomalous values in 700 hPa heights, 700 hPa moisture flux and 850 hPa moisture flux? A limited number of cases was found to have similar magnitudes of these anomalies. While most of the times identified were associated with rainfall events in excess of 100 mm, a few of the identified times yielded rainfall that only in the 25 to 100 mm range.

It is hoped that anomalies of various forecast parameters can be used to identify potentially significant wintertime precipitation events in northern California. The results so far suggest that anomalies in moisture flux and wind fields at certain levels may offer the most promise.