Rain on snow: shallow, transient snowpacks with frozen soils

Rain-on-snow events may contribute significantly to annual water yield and flood damage in some Pacific Northwest basins, but the soil and weather conditions required to trigger such events are not documented. Major upland runoff events were observed during the 1980 winter in north central Oregon. While soils were still frozen, high dew point temperatures and high windspeeds associated with the rapid intrusion of a warm, moist Pacific air mass liberated large quantities of heat (of vaporization) at the snow=air interface. This heat source accelerated the snowmelt rate beyond that expected from radiation, sensible heat, and heat content of the rain. Condensation melt accounted for up to 36% of the total hourly melt and averaged 31% for the entire event. Melt rate for the event was as great as 4.00 mm/h (.16 in/h) and averaged 2.1 mm/h (.08 in/h). A combination of snow water-equivalent, air-temperature regime and frozen soil similar to that in 1980 was present during the 1982 winter, but no rain-on-snow or surface runoff occurred.