Rainonsnow

Rainonsnow is a meteorological phenomenon in which liquid precipitation, typically rain, falls onto an existing snowpack. It occurs when warming conditions cause the snow to melt from the surface or when rain falls through a warm layer into a colder snowpack. The interaction often leads to rapid snowmelt and increased runoff, sometimes producing flooding or ice-closure hazards downstream.

Conditions promoting rainonsnow include a combination of a shallow or mid-depth snowpack, air temperatures near or above freezing, and a moist, unstable atmosphere delivering sustained rainfall. Warm air advection, frontal systems, or atmospheric rivers can bring the necessary moisture and warmth. The snowpack’s properties—its depth, density, temperature profile, and water content—significantly influence how much meltwater is produced and how much runoff reaches streams and rivers.

Impacts from rainonsnow vary by region and season. Rapid melting can strain river systems, increase flood risk, and yield ice jams when melted water encounters frozen obstacles. The event can alter snowpack stability, change surface albedo, and affect ecological processes in snow-fed basins. In urban areas, runoff can overwhelm drainage infrastructure and roadways.

Forecasting rainonsnow presents challenges because it depends on precise temperature profiles, timing of precipitation, and snowpack state. Hydrologists use weather models, snowpack measurements, and river stage data to anticipate peak flows and issue advisories. Preparedness measures include monitoring weather forecasts, preserving flood mitigation capacity, and coordinating with transportation networks to manage flood and icing hazards.