Changes in the frequency of occurrence of snowfall during El Niño/Southern Oscillation (ENSO) events will be presented for the continental United States. This study is motivated by the need to improve winter season climate forecasts for government agencies (i.e., Dept. of Transportation and Dept. of Energy), winter entertainment facilities, as well as for climatological studies. Data from the United States Historical Climatology Network are utilized. Four hundred and forty two stations having more than 20 years of data containing 15 or more snowfall events in each year are used for a 97-year (1900 – 1997) period of study. Four categories are created for each ENSO phase based on the magnitude of daily snowfall amounts: Light (0 - 52.8 mm], Moderate (52.8 - 152.4 mm], Heavy (152.4 - 304.8 mm], and Extreme (greater than 304.8 mm). Significant differences and percent changes between neutral and cold/warm event ENSO winters are created to show regions with increased or decreased occurrences in each snowfall bin. Statistical tests are applied to provide confidence levels for the identified changes in snowfall frequency.

Results show regions in which there are very large changes in the frequency of occurrence of snowfall between neutral event ENSO winters and cold or warm event ENSO winters. For example, in the Pacific Northwest, there is an increase in occurrences of light, moderate, and heavy snowfalls during the cold event ENSO winter, while there is a decrease during the warm event ENSO phase for light, moderate, heavy, and extreme snowfalls. In other regions, significant changes occur in only one or two snowfall categories.

With these results, government and private companies can mitigate adverse impacts of winter storms during ENSO phases. Winter entertainment facilities, such as ski resorts, may actually benefit from these results. Combined with other winter precipitation studies and the ever-improving ability to forecast each ENSO phase, these results can aid in preparation for winter storms during ENSO winters.