Validating Long-Held Rules of Thumb in Forecasting Snowmelt Runoff in Northern Wyoming

An historic snowpack in the Bighorn Mountains of Wyoming triggered an unprecedented snowmelt runoff season in the summer of 2011. A cool and wet spring, resulting from a persistent strong La Niña, allowed snowpack to linger in these mountains much longer than usual. Snowpack was greatest on the western slope of this range (2300-3750 m MSL), where snow water equivalents (SWEs) were 200% to 300% of normal in mid-June. More seasonal temperatures by late June finally triggered a rapid snowmelt that produced all-time peak flows on small waterways along the canyon-filled west slope of the Bighorns.

For many years government agencies, such as the United States Geological Survey and the National Weather Service, have used rules of thumb to predict when snowmelt would begin to impact area creeks and rivers. Specifically, daytime maximum temperatures in the valleys (1500-1800 m MSL) have long been used to predict the onset of snowmelt runoff. Additionally, minimum temperatures at mountain locations (2400-2900 m MSL) have provided indications of active overnight melting within this prime snowpack region. A comprehensive review of air temperature and SWEs from numerous observing stations in the basins and mountains was conducted to validate the merit of these rules of thumb. Improved correlations between temperature and snowmelt onset, amount, and timing will assist Wyoming hydrologists and forecasters in future years. Of particular interest is improving the ability to forecast anomalous runoff seasons that result during extreme climatological winter patterns.