Probability distributions of rain on snow for northcentral Oregon

Rain-on-snow events can be a major factor in flood damage and can significantly increase water yield in some watersheds of the Pacific Northwest. Winter runoff that occurs on steeply sloping croplands and rangelands can result in serious soil erosion and sediment production. Although the literature documents this type of winter storm event, frequency and magnitude of these events are lacking. We developed algorithms for extracting rain on snow events from daily weather station records and then used them to analyze a 30-year November-through-March record from a weather station in northcentral Oregon. A rain-on-snow event occurs nearly every year, but the probability of a rain greater than 5 mm per day is only about 25%. Rain-on-snow events explainded 57% of yearly peak discharges at a nearby crest gage. Daily precipitation associated with runoff ranged from 5 to 32 mm. Daily precipitation amount and snow depth jointly accounted for 73% of the variance in runoff peaks for rain on snow events.