Theme for 2003:
Forecasting Extreme Precipitation in the Sierra Nevada and Implications for the American River Watershed

The purpose of this paper is to connect the dots linking the three topics of this LAR Science Conference: forecasting of extreme flood events, groundwater management and fish and aquatic habitat. The discussion begins with a historical perspective on the events that have driven these topics to the top of the water resource planning and management agenda along the Lower American River. This perspective will lay the groundwork for assessing the substantive connections between the three topics and for outlining what the conference might accomplish with respect to pursuing these connections. The author is Agency Counsel for the Sacramento Area Flood Control Agency (SAFCA). The discussion reflects his experience over the past fifteen years in fashioning a strategic plan for managing the risk of flood damage in the heavily urbanized American River floodplain.

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Influences of upper air characteristics along the coast of California upon wintertime (November-April) precipitation in the Sierra Nevada are investigated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elevation. 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°C. This favored wind direction and temperature signify the importance of both moisture transport and orographic lifting in augmenting precipitation in the region. By utilizing the observed dependency of the precipitation upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture transport. The skill of the model increases with timescale of aggregation, reaching more than 50% variance explained at an aggregation period of 5-7 days. This indicates that upstream air moisture transport can be used to estimate the precipitation totals in the Sierra Nevada region.

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