Precipitation structure in the Sierra Nevada of California during winter

Influences of upper air characteristics along the coast of California upon wintertime (November-April) precipitation in the Sierra Nevada are investig ated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elev ation. Most precipitation episodes in the region are associated with moist southwesterly winds (coming from the southwest direction) and also tend to occur when the 700-mbar temperature at the upwind direction is close to -2 degrees C. This favored wind direction and temperature signify the importan ce of both moisture transport and orographic lifting in augmenting precipit ation in the region. By utilizing the observed dependency of the precipitat ion upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture tr ansport. The skill of the model increases with timescale of aggregation, re aching more than 50% variance explained at an aggregation period of 5-7 day s. This indicates that upstream air moisture transport can be used to estim ate the precipitation totals in the Sierra Nevada region.