Estimating precipitation in the central Cascades of Washington

Abstract

Precipitation in the central Cascades of Washington correlates well over 1966-96 with wind and moisture in twice-daily upper-air soundings at a radiosonde station near the Pacific coast, 225 km away. A simple model estimates precipitation by using the component of the wind roughly normal to the north-south range of the Cascade Mountains, which it raises to a power and scales by the relative humidity. Values at 850 mb are taken as an index of the total moisture flux. Thresholds are imposed for the wind component and the relative humidity to reduce the likelihood of estimating precipitation from weak onshore flow on dry days. A split-sample analysis indicates that the model parameters are highly robust against sampling error. This moisture flux model estimates precipitation over 5-day periods with the coefficient of determination r2 ≈ 0.55, which increase for precipitation aggregated over 30-day periods to 0.75 and decreases to 0.30 for daily precipitation. Results over a 4-month period in the winter of 1996/97 were comparable to those from an advanced mesoscale precipitation model. The model estimates water-year (October-September) runoff for the period 1966-96 from two drainage basins in the region with r2 ≈ 0.8, and on 1 May forecasts subsequent May-September runoff with r2 ≈ 0.4.