WHERE IS THE RAIN-ON-SNOW ZONE IN THE WEST-CENTRAL WASHINGTON CASCADES? MONTE CARLO SIMULATION OF LARGE STORMS IN THE PACIFIC NORTHWEST

The Pacific Northwest is susceptible to winter storms involving warm rain plus melt­­ing snow. Minor rain-on-snow events can reduce snowpacks, altering water supplies; major ROS triggers most regional episodes of serious flooding and land­sliding.

Re­cog­­ni­tion of ROS has grown during big PNW events, as investigations based on theory, weather re­cords and field studies clarified many aspects of the phe­nomena. ROS seems more important in middle elevations; land use can affect pro­cess­es; climatic shifts are changing its geography and mag­ni­tude. Questions remain: how much does ROS affect the long-term frequency of water input? can we define a peak ROS zone, in some specific area in the Northwest?

Sporadic occurrence hinders study of ROS, but some issues can be addressed by model­ing. A com­put­er pro­gram us­ing probabilistic and deter­min­­istic ele­ments simu­la­tes large “storms” over “1000 yr”, gener­at­ing realizations of weather and snowpack. In an event pre­cip­i­ta­tion falls, snow ac­cum­­u­lates/melts, and water is tracked to the ground. Fre­quen­cy dis­tribu­tions are based on data from Wash­ington’s Cas­cades, some com­bined into func­tions of elev­a­tion and date, so the model can be ap­plied to specific places or gen­er­al­ elevations. Val­i­d­ation focuses on Stam­pede Pass, site of a NWS sta­tion, snow course and SNOTEL on the Cas­cade crest. The model is eval­­u­at­ed by com­par­ing statis­­tics and fre­quen­­cy re­­la­tions of the in­stru­­ment­al record against model realiza­tions, for precip­i­­tation and water to runoff.

The model is used first to evaluate the significance of ROS varying with elev­ation. Over “1000 yr”, snow in some events usually re­duces the water reaching the ground during model storms, more often where common and deep, but sometimes also at moderate elevations. Sev­eral metrics suggest the great­est ROS effect in this re­gion occurs at ~800 m, where rain+­melt en­hance water delivery to the ground during ~20% of major storms and increases the magnitude of the rarer events.