Thundersnow is a relatively rare occurrence that can indicate unusually heavy rates of snowfall. Presence of thundersnow requires a significant increase in snowfall forecasts, and crucial amendments to aviation forecasts. Lead times for forecasting thundersnow can vary greatly, but most often are twelve hours or less.

On 30 December 2000, a significant snowstorm formed off the Mid Atlantic Coast, and tracked into the Northeastern U.S. Careful analysis of satellite, radar, upper air and lightning data provided six to twelve hours of lead time for forecasting thundersnow in the northeastern U.S., where over two feet of snow fell locally. Thundersnow fell in some locations for more than four consecutive hours.

Thundersnow is often the product of conditional symmetric instability, since convective instability has rarely been observed within a column completely below freezing. However, the thundersnow event of 30 December 2000 may have been unique, as an observed radiosonde thermodynamic profile from the Upton, NY upper air station, was completely below freezing, and showed positive Convective Available Potential Energy between 830 Mb and 610 Mb.

The atmospheric processes contributing to the extreme thundersnow event were evaluated through careful analysis of satellite, upper air, radar, and lightning data, including thermodynamic profiles.