Great Lakes Snowstorms

Previously, we looked at the impacts of the Great Lakes on autumn and winter storm systems. The Lakes' effects on stormy weather during this time of year, however, do not always require large low pressure systems to produce stormy weather, for when cold arctic air sweeps across the lake waters out of the eastern sector of arctic high pressure cells, snow showers and heavy snow squalls may form along and inland from the downwind, or lee, shores. Often, these snowfalls will strike a relatively small area, while blue skies prevail several kilometres away. Such snowstorms are called lake-effect snows because they are produced by the contrast in temperature between the overflowing air and the lake waters. When particularly blustery, they are called lake-effect snow squalls.

Lake-effect snows often add to the miseries of a winter storm's passage. When winter storms cross the Prairies/Great Plains, for example, the advancing cold arctic air behind the storm usually clears the skies quickly. Although temperatures are very cold, and, if winds are strong, fallen snow can blow or drift for days, the snowfall is usually over. Not so around the Great Lakes, where it is said: "During the winter, the weather clears up stormy."

During the cold season in the Great Lakes Basin, cold-front passage is often followed by a 24-36-hour period of blustery westerly to northerly winds and falling temperatures. Lake-effect snows form in this strong cold airflow as it crosses a lake toward its southern or eastern shores. Where these winds make landfall, persistent flurries of fluffy snow, intermixed with brief periods of partly cloudy skies and blowing/drifting snow, are common. If the arctic outbreak is severe, heavy snow squalls prevail, accompanied by drifting and blowing snow which reduces visibility. Under certain conditions, less than 12 hours may separate the last of the lake-effect snows and the widespread snowfall of the next storm system.

Lake-Effect Snow Formation

Here is how lake-effect snows form. During the summer season, the Great Lakes absorb large amounts of heat. Because water heats slowly but retains its stored heat for a substantial time, the open waters of the Lakes are much warmer than the arctic air that crosses them during the autumn and winter. This is particularly true for the first cold blasts of winter rushing out of the polar regions, making November and December the prime months for heavy lake-effect snows.

When the cold and relatively dry air characteristic of arctic air masses traverses a lake, the lower portion of the air mass is warmed and moistened. This air thus becomes lighter than the air above it, a condition known to meteorologists as convective instability, and starts to rise, forming cloud bands of stratus, stratocumulus, or cumulus over the water.

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