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Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. The lower layer of air, heated by the lake water, picks up water vapor from the lake and rises through colder air. The vapor then freezes and is deposited on the leeward (downwind) shores.
The same effect also occurs over bodies of saline water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic lift influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.
The areas affected by lake-effect and parallel "ocean-effect" phenomena are called . These include areas east of the Great Lakes in North America, the west coasts of northern Japan, Lake Baikal in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, Adriatic Sea, the North Sea and more.
Lake-effect blizzards are the blizzard-like conditions resulting from lake-effect snow. Under certain conditions, strong winds can accompany lake-effect snows creating blizzard-like conditions; however, the duration of the event is often slightly less than that required for a blizzard warning in both the U.S. and Canada.
If the air temperature is low enough to keep the precipitation frozen, it falls as lake-effect snow. If not, then it falls as lake-effect rain. For lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at an altitude where the air pressure is (roughly vertically) should be lower than the temperature of the air at the surface. Lake-effect occurring when the air at is much colder than the water surface can produce thundersnow, snow showers accompanied by lightning and thunder (caused by larger amounts of energy available from the increased instability), and, on very rare occasions, tornados.
Speed shear is less critical but should be relatively uniform. The wind-speed difference between the surface and vertical height at which the pressure reads should be no greater than so as to prevent the upper portions of the band from shearing off. However, assuming the surface to winds are uniform, a faster overall velocity works to transport moisture more quickly from the water, and the band then travels much farther inland.
Even when precipitation is not produced, cold air passing over warmer water may produce cloud cover. Fast-moving mid-latitude cyclones, known as , often cross the Great Lakes. After the passage of a cold front, winds tend to switch to the northwest, and a frequent pattern is for a long-lasting low-pressure area to form over the Canadian Maritimes, which may pull cold northwestern air across the Great Lakes for a week or more, commonly identified with the negative phase of the North Atlantic Oscillation (NAO). Since the prevailing winter winds tend to be colder than the water for much of the winter, the southeastern shores of the lakes are almost constantly overcast, leading to the use of the term "the Great Gray Funk" as a synonym for winter. These areas allegedly contain populations that suffer from high rates of seasonal affective disorder, a type of psychological depression thought to be caused by lack of light.
The most affected areas include the Upper Peninsula of Michigan; Northern New York and Central New York; particularly the Tug Hill Region, Western New York; Northwestern Pennsylvania; Northeast Ohio; southwestern Ontario and central Ontario; Northeastern Illinois (along the shoreline of Lake Michigan); northwestern and north central Indiana (mostly between Gary and Elkhart); northern Wisconsin (near Lake Superior); and West Michigan.
Lake-effect snows on the Tug Hill plateau (east of Lake Ontario) can frequently set daily records for snowfall in the United States. Tug Hill receives, typically, over of snow each winter. The snowiest portions of the Tug Hill, near the junction of the towns of Montague, Osceola, Redfield, and Worth, average over of snow annually.
From February 3–12, 2007, a lake-effect snow event left of snow in 10 days at North Redfield on the Tug Hill Plateau. Other examples major prolonged lake effect snowstorms on the Tug Hill include December 27, 2001, - January 1, 2002, when of snow fell in six days in Montague, January 10–14, 1997, when of snow fell in five days in North Redfield, and January 15–22, 1940, when over eight feet of snow fell in eight days at Barnes Corners.
Syracuse, New York, directly south of the Tug Hill Plateau, receives significant lake-effect snow from Lake Ontario, and averages of snow per year, which is enough snowfall to be considered one of the "snowiest" large cities in America.
Lake Erie produces a similar effect for a zone stretching from the eastern suburbs of Cleveland through Erie to Buffalo. Remnants of lake-effect snows from Lake Erie have been observed to reach as far south as Garrett County, Maryland, and as far east as Geneva, New York. Because it is not as deep as the other lakes, Erie warms rapidly in the spring and summer, and is frequently the only Great Lake to freeze over in winter. Once frozen, the resulting ice cover alleviates lake-effect snow downwind of the lake. Based on stable isotope evidence from lake sediment coupled with historical records of increasing lake-effect snow, global warming has been predicted to result in a further increase in lake-effect snow.
A very large snowbelt in the United States exists on the Upper Peninsula of Michigan, near the cities of Houghton, Marquette, and Munising. These areas typically receive of snow each season. For comparison, on the western shore, Duluth, Minnesota receives per season.
Western Michigan, western Northern Lower Michigan, and Northern Indiana can get heavy lake-effect snows as winds pass over Lake Michigan and deposit snows over Muskegon, Traverse City, Grand Rapids, Kalamazoo, New Carlisle, South Bend, and Elkhart, but these snows abate significantly before Lansing or Fort Wayne, Indiana. When winds become northerly or aligned between 330° and 030°, a single band of lake-effect snow may form, which extends down the length of Lake Michigan. This long fetch often produces a very intense, yet localized, area of heavy snowfall, affecting cities such as La Porte and Gary.
Because Southwestern Ontario is surrounded by water on three sides, many parts of Southwestern and Central Ontario get a large part of their winter snow from lake-effect snow. This region is notorious for the whiteouts that can suddenly reduce highway visibility on North America's busiest highway (Ontario Highway 401) from clear to zero. The region most commonly affected spans from Port Stanley in the west, the Bruce Peninsula in the north, Niagara-on-the-Lake to the east, and Fort Erie to the south. The heaviest accumulations usually happen in the Bruce Peninsula, which is between Lake Huron and Georgian Bay. So long as the Great Lakes are not frozen over, the only time the Bruce Peninsula does not get lake-effect snow is when the wind is directly from the south.
On one occasion in December 2016, lake-effect snow fell in central Mississippi from a lake band off Ross Barnett Reservoir.
The West Coast occasionally experiences ocean-effect showers, usually in the form of rain at lower elevations south of about the mouth of the Columbia River. These occur whenever an Arctic air mass from western Canada is drawn westward out over the Pacific Ocean, typically by way of the Fraser Valley, returning shoreward around a center of low pressure. Cold air flowing southwest from the Fraser Valley can also pick up moisture over the Strait of Georgia and Strait of Juan de Fuca, then rise over the northeastern slopes of the Olympic Mountains, producing heavy, localized snow between Port Angeles and Sequim, as well as areas in Kitsap County and the Puget Sound region.
While snow of any type is very rare in Florida, the phenomenon of gulf-effect snow has been observed along the northern coast of the Gulf of Mexico a few times in history. More recently, "ocean-effect" snow occurred on January 24, 2003, when wind off the Atlantic, combined with air temperatures in the 30 °F range, brought snow flurries briefly to the Atlantic Coast of northern Florida seen in the air as far south as Cape Canaveral.
The most populous city in the region, Istanbul, is very prone to lake-effect snow and this weather phenomenon occurs almost every winter, despite winter averages of , comparable to Paris. On multiple occasions, lake-effect snowfall events have lasted for more than a week, and official single-storm snow depth totals have exceeded downtown and around the city. Earlier, unofficial measurements are often higher, due to the relative dearth of sufficiently old weather stations in the region; some sources claim up to of snowfall during the blizzard of March 1987.
Meanwhile, snowfall in mountainous provinces in this region is amplified by orographic effect, often resulting in snowfall of several meters, especially at higher elevations.
The best-known example occurred in January 1987, when record-breaking cold air (associated with an upper low) moved across the North Sea towards the UK. The result was over 2 ft of snow for coastal areas, leading to communities being cut off for over a week. The latest of these events to affect Britain's east coast occurred on November 30, 2017; February 28, 2018; and March 17, 2018; in connection with the 2018 Great Britain and Ireland cold wave. The second event of winter 2017/18 was particularly severe, with up to falling in total over the 27th–28th.
Similarly, northerly winds blowing across the relatively warm waters of the English Channel during cold spells can bring significant snowfall to the French region of Normandy, where snow drifts exceeding 10 ft (3 m) were measured in March 2013.
Warnings about lake-effect snow:
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