Regional changes in early, middle, and late winter total snowfall distributions are identified over the continental United States in association with warm and cold phases of the El Niño-Southern Oscillation (ENSO). The analysis is primarily motivated by a desire to improve winter season climate forecasts. Original interest in snowfall associated with ENSO was provided by requests for skiing forecasts during the 1997 ENSO warm phase. Geographic regions with internally similar ENSO warm, cold, and neutral phase snowfall distributions are identified using a composite technique. The composites reveal three early winter, five midwinter, and three late winter regions with shifts in the upper-, middle, and lower-quartile seasonal snowfall. The quartile shifts revealed by the composite technique are important for forecasting applications; however, snowfall impact studies rely more on the absolute magnitude of the change in snowfall at individual stations. Potential impacts of the shifts in snowfall distributions associated with ENSO are discussed using the quartile snowfall magnitudes for the stations in the composites. Shifts in regional snowfall distributions are compared to published ENSO winter climate studies, and hypotheses are presented to relate physical processes to the warm, cold, and neutral phase snowfall distributions.

Principal findings include increased snowfall during an ENSO cold phase relative to warm and neutral phases in the northwestern states from early through midwinter, less (more) snowfall during a cold (warm) phase relative to neutral years in the Northeast, and less snowfall (relative to neutral winters) in both warm and cold phases in the Ohio Valley (early winter) and Midwest (midwinter). Combining these snowfall regions with an ever-improving ability to forecast ENSO warm and cold phases will improve seasonal snowfall forecasts. The results should improve mitigation strategies for agencies adversely impacted by ENSO-induced snowfall anomalies.

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