Where Is the Rain-on-Snow Zone in the West-Central Washington Cascades?: Monte Carlo Simulation of Large Storms in the Northwest

Abstract

Rain-on-snow (ROS) occurs when warm, wet air moves into latitudes and/or elevations having vulnerable snowpacks, where it can alter water inputs to infiltration, runoff and erosion. The Pacific Northwest is particularly susceptible: winter storms off the Pacific cause locally heavy rain plus snowmelt almost annually, and disastrous flooding and landsliding intermittently. In maritime mountainous terrain, the effects seem more likely and hydrologically important where warm rains and seasonal snowpacks are liable to coincide, in middle elevations. Several questions arise: (1) In the PNW, does ROS affect the long-term frequency and magnitude of water delivery to the ground, versus total precipitation (liquid and solid), during big storms? Where and how much? (2) If so, can we determine which elevations experience maximum hydrologic effects, the peak ROS zone? Probabilistic characteristics of ROS are difficult to establish because of geographic variability and sporadic occurrence: scattered stations and short observational records make quantitative frequency analysis difficult. These problems dictate a modeling approach, combining semi-random selection of storm properties with physical rules governing snow and water behavior during events. I created a simple computer program to perform Monte Carlo simulation of large storms over 1000-years‖, generating realizations of snowpack and storm-weather conditions; in each event precipitation falls, snow accumulates and/or melts, and water moves to the ground. Frequency distributions are based on data from the Washington Cascades, and the model can be applied to specific sites or generalized elevations. Many of the data sets were based on observations at Stampede Pass, where highiii ACKNOWLEDGEMENTS Although my interest in rain-on-snow was first tickled by the Christmas floods of 1964, it really developed in the late 1970s, when I studied at the University of Washington and worked on several projects demonstrating the power of ROS events to affect hydrologic and geomorphic processes in forested mountains. The first attempt at a dissertation petered out in the late 1980s: the approach, the computers and the investigator all required additional seasoning. I kept up with ROS issues while working at the Washington Department of Natural Resources, and applied some preliminary insights and results to forest-practices regulatory and watershed analysis procedures. The project was rebaptized by the February storm of 1996, leading to my enrollment at Portland State in order to complete the modeling and produce scientifically credible results. As should be expected for a project ~30 years old, many people have contributed to the shape of the work and its products. Dennis Harr of the U.S. Forest Service (Corvallis) and the UW College of Forest Resources, set off a revival in ROS studies among many colleagues and students in the 1970s and ‗80s, particularly concerning interactions with timber harvest across the Northwest; besides major concepts, he provided me with many information source...

Northern Pacific sea-level pressure controls rain-on-snow in North America

“…Rapid snowmelt during ROS events can critically shift the hydrological cycle; as a result, groundwater recharge, river flow patterns, and water quality patterns can deteriorate 8 . These impacts hold great importance in North America, where 85 million people depend on rivers and reservoirs for the transportation and storage of water resources from mountain snow 11,12 , which are susceptible to substantial flooding events caused by ROS 1,2,[13][14][15][16][17] .…”