Using meteorological observations and model simulations, we show that the catastrophic ice storm of 4-5 December 2002 in the southeast US resulted from an unusual combination of a classic winter storm and a warm anomaly in the western North Atlantic Ocean. At the time of the storm, observations show that the Atlantic sea-surface temperature (SST) near the southeast US coast was about 1.5 C warmer than its multi-year mean. The impact of anomalous SST on the strength of ice storms was evaluated with the Regional Atmospheric Modeling System (RAMS). We find that with a warmer ocean, more snow is converted into freezing rain, while not significantly affecting the inland surface temperature. A cooler ocean produces mostly snowfall and less freezing rain.