An Evaluation of SNODAS for Determining Snow Water Equivalent on Mount St. Helens

When Mount St. Helens erupted on May 18, 1980 melting snow and ice on the mountain enhanced a large debris flood wave from all sides of the mountain, creating exceptional flooding along the Toutle River in southwest Washington. Mount St. Helens became active again in September 2004. During both of these episodes there was considerable concern that another eruption would entrain the existing snowpack resulting in a devastating flood. The US Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), National Water and Climate Center (NWCC) was requested to provide weekly snow water equivalent (SWE) on Mount St. Helens to the National Weather Service Northwest River Forecast Center (NWRFC) and North Pacific Division of the US Army Corps of Engineers for determining potential flood magnitude in the event of an eruption. In the 1980s the snowpack was determined from the NRCS SNOTEL station network located around the mountain. SNODAS uses a physically based, spatially-distributed energy and mass-balance snow model in conjunction with assimilated satellite, airborne and ground-based observations of snow covered area and SWE to determine several snowpack parameters and modeled SWE at a 1 km grid scale national coverage. The SNODAS system is compared to the traditional SNOTEL method of determining the snowpack at three elevation bands and four quadrants on Mount St. Helens. SNODAS provides an improved model of the snowpack SWE around Mount St. Helens.