Lake-effect snow

Lake-effect snow develops when a cold air mass flows across a lake whose surface is warmer than the air. Heat and water vapour pass from the lake into the lower atmosphere. The moist air rises, cools and forms clouds; if temperatures remain low enough, precipitation falls as snow downwind of the lake. The process is most effective when cold air passes over open water for a substantial distance, allowing it to collect heat and moisture. When a lake freezes over, the supply of moisture and heat from its surface is reduced, and lake-effect snowfall may weaken or end.

Lake-effect snow often occurs in narrow bands rather than as an even blanket. Wind direction determines the path of the air across the lake and where the resulting band reaches land. A persistent wind aligned with a lake's long axis can help produce a focused band; changes in wind direction can shift the heaviest snowfall to a different area. As a result, one community may experience intense snow while places nearby receive far less. Topography and the arrangement of the lakes and shorelines also affect where snowfall accumulates.

The Great Lakes are a prominent setting for lake-effect snow because cold continental air can travel over their relatively warm waters in late autumn and winter. Snowbelts commonly occur downwind of the lakes, including parts of Michigan, Ontario and areas east and southeast of Lakes Erie and Ontario. The timing and amount vary with wind, air and water temperatures, and the extent of open water. Localized bands can produce hazardous travel conditions, rapid accumulation and substantial differences in snow totals over short distances.

Forecasting the exact location and intensity of a lake-effect band is challenging because the bands can be narrow and sensitive to small changes in wind and lake conditions. NOAA researchers have described difficulties in observing winter lake temperatures and ice, as well as representing fine-scale bands in forecast models. Lake-effect snow can bring high snowfall rates and sharply localized disruptions to transport and daily life. Forecasters therefore combine atmospheric observations, lake information and numerical model guidance to assess where bands may form and move.