Rain-on-snow avalanches: forecasting the return to stability

Mid-winter rain-on-snow often results in avalanches that threaten the Milford Road between TeAnau and Milford Sound on the southwest coast of new Zealand. Avalanche activity is typically widespread minutes after the onset of rain; the timing of these immediate avalanches can be predicted with accuracy better than an hour from meteorological forecasts of the transition from snow to rain. The avalanche potential can remain high for several hours after rain starts. Accurate prediction of these delayed avalanches is complicated by difficulties defining how the stress and strength of the snowpack evolves during infiltration of liquid water. During continued rainfall, avalanche activity usually decreases after 24 hours; at Milford we have not observed avalanches more than 36 hours after the onset of rain. The return to stability occurs when drainage through the snowpack has been established; the evolution of snow stability can be tracked by monitoring outflow from the snowpack. Observations of waterfall activity provide information about the state of the snowpack: lack of waterfalls during rain events indicates drainage is not yet established, while active waterfalls implies that drainage is established. Measurements of outflow from an array of spatially distributed lysimeters located near the start zones provides additional information. The time between the onset of rain and first outflow varies depending on the rate of rainfall as well as the stratigraphy and temperature of the snowpack. A large flux of water infiltrates through isothermal, homogeneous snow faster than a small flux through cold, layered snow.