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Warm Water and Cold Air

The Science Behind Lake-Effect Snow

Snow storm clouds.

High resolution (Credit: NOAA)

Imagine driving on the New York State Thruway south of Buffalo, N.Y., on a bright winter day as the skies darken and ominous clouds build in the distance. Suddenly, a “wall of white” appears several hundred yards ahead and a blizzard of heavy snowfall obscures everything in your path. You inch forward through the blizzard until you arrive on the other side – where skies once again turn sunny.

Anyone who lives in the Great Lakes region (especially those living along the eastern and southern shorelines) is familiar with this unique type of winter weather. This heavily localized snowfall, known as lake-effect snow, is most common from November to February. The winter weather phenomenon is capable of whiting out large sections of the Great Lakes region, from South Bend, Ind., to Buffalo. Towns and cities at higher elevations can expect even larger amounts of lake-effect snow.

The Recipe for Lake-Effect Storms

Lake-effect snow forms in the winter when cold air masses move over warmer lake waters.  As the warm lake water heats the bottom layer of air, lake moisture evaporates into the cold air. Since warm air is lighter and less dense than cold air, it rises and begins to cool. The moisture that evaporates into the air condenses and forms clouds, and snow begins falling. 

Animation: Radar image of lake-effect snow.

Animation: Radar image of lake-effect snow.

High resolution (Credit: NOAA)

Snow clouds most often form in narrow bands where the size and orientation are determined by the shape of the body of water and the prevailing wind direction. In the most extreme cases, the heaviest bands of snowfall may be 20 to 30 miles wide and extend over 100 miles inland from the lake.

Within the band, snowfall rates may exceed 5 inches an hour and be accompanied by lightning and thunder, a phenomenon known as thundersnow. A band of snow can hover over one location for several hours, dropping several feet of snow; however, 10 to 15 miles on either side of that narrow band skies may be sunny with no snow at all.

Lake-effect snows are not confined to the Great Lakes region, although they are most common and heaviest there. Any large body of water can generate lake-effect snow downwind if it remains free of ice. The Great Salt Lake in Utah produces significant lake-effect snow. There’s also bay-effect snow that forms in the same manner as lake-effect snow, only over the ocean. Cape Cod Bay in Massachusetts and Chesapeake Bay in Maryland and Virginia will occasionally produce bay-effect snow.

More Accurate Snow Predictions Are in the Air

Snow at NEW forecast office.

High resolution (Credit: NOAA)

Detailed forecasts of lake-effect snow squalls are extremely important to the people who live and travel throughout the Great Lakes region. In the past, predicting such intense small storms posed challenges to weather forecasters. Today, research, the availability of high quality data, numerical weather modeling, as well as improved computer capabilities, has produced surprisingly detailed projections of larger lake-effect snow bands.

”Forecasting has come a long way in the past few years, but we still have a lot to learn about the complex interaction of the atmosphere,” said Tom Niziol, meteorologist-in-charge at the National Weather Service office in Buffalo. “As residents enjoy the region’s many winter activities, it’s important to be aware of the hazards of winter weather, including lake-effect snow storms.”

To learn more about lake-effect snow in the Great Lakes region, visit the NWS Buffalo, N.Y., Forecast Office Web site. Or view webcams of the Great Lakes area.

For tips on preparing for snow and icy weather, check out the National Weather Service’s winter weather brochure. NOAA logo.