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Question

What is lake effect snow and how does it form? 

Answer

Lake effect snows are discrete, narrow bands of snow, often characterized by intense accumulation and dangerously low visibility. The bands occur throughout the Great Lakes region, but also can occur in other areas such as the Great Salt Lake in Utah. Parts of the New England coast can experience a similar ocean effect snow coming off the Atlantic.

Lake effect snow commonly occurs in late fall and winter as cold air passes over the unfrozen, relatively warm waters of the lake, transferring warmth and moisture into the lower portions of the atmosphere. As this warm air rises and cools, the moisture condenses, forming clouds and ultimately snow on the downwind (or leeward) sides of the lake.

A unique aspect of lake effect snow is the extreme variability that can occur in time and space. It is not uncommon for sunny skies at a particular location to be quickly replaced by blinding, wind-driven snowfall in a matter of minutes. One of the more challenging aspects of forecasting lake effect snow is determining exactly where the heaviest snow will occur because even small changes in wind direction can significantly alter the areas affected.

If wind conditions remain the same, lake effect snow storms can produce nearly continuous snow in a small geographic area for an extended period of time. This helps explain why snowfall amounts can vary so widely from location to location -- ranging from trace amounts to a couple feet over a short distance (several miles).

NOAA’s National Weather Service must consider many factors when forecasting the intensity and location of lake effect snow. Some are associated with the current or predicted meteorological conditions (e.g., air temperature, relative humidity, and winds), while others related to local geography (e.g., elevation and shoreline orientation). When heavy lake effect snows are expected, NOAA issues a variety of Watches, Warnings, and Advisories

This month’s expert:  Bruce B. Smith, Meteorologist, NOAA National Weather Service.

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