The Determination of Total Snow Water Equivalent Volume Using the NOHRSC SNODAS Snowpack Model on Mount St. Helens, Washington, USA.

Mount St. Helens in Washington State erupted in 1980 with devastating results due to the major eruption and resulting pyroclastic debris flows into the surrounding watersheds. These flows entrained large amounts of the existing snowpack on and around the mountain. With the renewed volcanic activity that began in October 2004, close monitoring of the snowpack is required. The total snowpack snow water equivalent (SWE) volume determined by the Natural Resources Conservation Service (NRCS) is used by other federal agencies to determine the flood size that could be expected from a rapid snowmelt flooding event from possible eruptive activity on the mountain. In the 2006 water year, the National Weather Service (NWS) National Operational Hydrologic Remote Sensing Center (NOHRSC) Snow Data Assimilation System (SNODAS) model was used to determine the total amount of SWE on the mountain. This model was updated in early May from a series of intensive snow surveys undertaken by 7 teams around the mountain. The teams measured SWE, depth, density and temperature in many locations. This additional data was then ingested into the SNODAS model and slight changes in the gridded data and SWE total resulted. The modeled snowpack changed slightly in low and mid elevations, but increased moderately in high elevation areas due to the data from the snow survey in that area. The snow survey improved the modeled snowpack, especially in the high elevation environment. Further improvements in modeled snowpack for areas above existing SNOTEL measurement stations are needed for more accurate snowpack determinations. The overall investigation provided significant validation that the SNODAS model provides a fairly accurate and a much improved comprehensive picture of the snowpack on Mount St. Helens. These techniques could be applied to other volcanoes in the Cascade Mountain Range.