During the late autumn and winter, when cold arctic air sweeps across the Great Lakes of North America, snow squalls may form along the lee shores of the Lakes. These squalls can bring locally heavy snowfalls with reduced visibility to a relatively small area. Often, while squalls hit one area, blue skies prevail several kilometres away.
Most lake-effect snowfall occurs, not during the passage of the low pressure cell of a winter storm, but in the strong cold air flow behind the storm's cold front. In areas south of one of the Great Lakes, cold front passage will often be followed by a 24 to 36-hour period of blustery northerly or westerly winds, falling temperatures and persistent flurries of fluffy snow. Often heavy snow squalls accompanied by falling and blowing snow and reduced visibility intermix with brief periods of partly cloudy skies and some blowing snow. When lake-effect snow squalls are well developed, there may be less than 12 hours between the last of them and the more widespread snowfall of the next cyclonic storm system.
Lake-effect snows often add to the miseries of a winter storm's passage. When cyclonic snowstorms pass through areas such as the Prairies/Great Plains, the advancing cold arctic air will quickly clear skies. Although in these situations, temperatures are cold and, if winds are strong, fallen snow can blow or drift, the snowfall is usually over. Not so around the Great Lakes, where it is said: "During the winter, the weather clears up stormy." The cold air picks up substantial moisture as it moves over the Lakes and deposits it as snow inland from the downwind shore. Often, accumulations exceed those deposited during the storm itself. In the more severe lake-effect snow squalls, snowfall accumulations of more that 75 cm (30 in) per day are not uncommon, and fall rates as high as 28 cm (11 in) per hour have been reported. Such severe snowfalls have been termed snowbursts.
Lake-effect snows are not restricted to the Great Lakes shorelines, but are most common and heaviest there. Any large lake may produce lake-effect snow downwind if it remains essentially ice-free. And, although the east and south shores of the Great Lakes are most likely to have lake-effect snowfalls, north and west shorelines can be hit if the air traversing the lake is cold enough and the distance the air passes over open water (known as the fetch) is large enough.
Lake-effect snowstorms are of major economic significance. Over 1.5 million residents of the State of Michigan live in areas which are affected by such storms and their impact on the state's three major economic activities -- industry, recreation and agriculture -- is considerable. The large cities of Buffalo, Syracuse, and Rochester, New York, Cleveland, Ohio, Erie, Pennsylvania and London, Ontario are all located in major lake-effect snowbelts. Lake-effect snowstorms account for countless lost work and school days. They necessitate high expenditures for snow removal, create frequent hazardous driving conditions, and in general, add to the rigours of winter weather besetting all residents of the Great Lakes Region.
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