What is lake-effect snow?

Lake-effect snow forms when cold air, typically arctic air behind a front, passes over a large body of water that is much warmer than the air, often by twenty degrees Fahrenheit or more. The lake transfers heat and moisture into the lowest layer of the air, which becomes unstable and rises, forming clouds that organize into bands aligned with the wind. When those bands reach the downwind shore and the air is forced up over land, they release snow at rates that can exceed several inches per hour.

The distinguishing feature is how localized it is. Bands are often only a few miles wide, so accumulations can differ by feet across short distances, and a band that stalls over one area for hours produces the extraordinary totals associated with places like Buffalo, the Tug Hill Plateau in New York, and the snowbelts along the eastern shores of Lakes Michigan and Erie. Wind direction determines where the bands land, and forecasters track it closely because a small shift moves the heaviest snow from one set of communities to another.

The season is asymmetric. Lake-effect snow is heaviest in late fall and early winter when the lakes are still warm and the air has turned cold, and it diminishes as the lakes cool and freeze, since ice cover cuts off the moisture supply. Warmer winters with less ice cover extend the season, which is one reason lake-effect totals in some areas have trended upward. The same mechanism produces lake-effect rain in autumn before temperatures fall, and ocean-effect snow along coasts under similar conditions, including on Cape Cod and in Japan, where the Sea of Japan produces some of the heaviest snowfall on Earth.