Lake effect snow

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 up by the lake water, picks up water vapor from the lake and rises up through the colder air above. 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 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 snow are called snowbelts. These include areas east of the Great Lakes in North America, the west coasts of northern Japan, the Kamchatka Peninsula in Russia, and areas near the Great Salt Lake, Black Sea, Caspian Sea, Baltic Sea, Adriatic Sea, and North Sea.

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 US 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).

Formation

Some key elements are required to form lake-effect precipitation and which determine its characteristics: instability, fetch, wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover.

Instability

A temperature difference of (or as past researchers have estimated: between 15 and 25 °C) between the lake temperature and the height in the atmosphere (about at which barometric pressure measures ) provides for absolute instability and allows vigorous heat and moisture transportation vertically. Atmospheric lapse rate and convective depth are directly affected by both the mesoscale lake environment and the synoptic environment; a deeper convective depth with increasingly steep lapse rates and a suitable moisture level allow for thicker, taller lake-effect precipitation clouds and naturally a much greater precipitation rate.

Fetch

The distance that an air mass travels over a body of water is called fetch. Because most lakes are irregular in shape, different angular degrees of travel yield different distances; typically, a fetch of at least is required to produce lake-effect precipitation. Generally, the larger the fetch, the more precipitation produced. Larger fetches provide the boundary layer with more time to become saturated with water vapor and for heat energy to move from the water to the air. As the air mass reaches the other side of the lake, the engine of rising and cooling water vapor pans itself out in the form of condensation and falls as snow, usually within of the lake, but sometimes up to about .

Wind shear

Directional shear is one of the most important factors governing the development of squalls; environments with weak directional shear typically produce more intense squalls than those with higher shear levels. If directional shear between the surface and the height in the atmosphere at which the barometric pressure measures is greater than 60°, nothing more than flurries can be expected. If the directional shear between the body of water and the vertical height at which the pressure measures is between 30° and 60°, weak lake-effect bands are possible. In environments where the shear is less than 30°, strong, well organized bands can be expected.

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.

Upstream moisture

A lower upstream relative humidity lake effect makes condensation, clouds, and precipitation more difficult to form. The opposite is true if the upstream moisture has a high relative humidity, allowing lake-effect condensation, cloud, and precipitation to form more readily and in a greater quantity.

Upwind lakes

Any large body of water upwind impacts lake-effect precipitation to the lee of a downwind lake by adding moisture or pre-existing lake-effect bands, which can reintensify over the downwind lake. Upwind lakes do not always lead to an increase of precipitation downwind.

Synoptic forcing

Vorticity advection aloft and large upscale ascent help increase mixing and the convective depth, while cold air advection lowers the temperature and increases instability.

Orography and topography

Typically, lake-effect precipitation increases with elevation to the lee of the lake as topographic forcing squeezes out precipitation and dries out the squall much faster.

Snow and ice cover

As a lake gradually freezes over, its ability to produce lake-effect precipitation decreases for two reasons. Firstly, the open ice-free liquid surface area of the lake shrinks. This reduces fetch distances. Secondly, the water temperature nears freezing, reducing overall latent heat energy available to produce squalls. To end the production of lake-effect precipitation, a complete freeze is often not necessary.

Even when precipitation is not produced, cold air passing over warmer water may produce cloud cover. Fast-moving mid-latitude cyclones, known as Alberta clippers, 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.

Great Lakes region

United States

Cold winds in the winter typically prevail from the northwest in the Great Lakes region, producing the most dramatic lake-effect snowfalls on the southern and eastern shores of the Great Lakes. This lake effect results in much greater snowfall amounts on the southern and eastern shores compared to the northern and western shores of the Great Lakes.

The most affected areas include the Upper Peninsula of Michigan; Central New York; Western New York; Northwestern Pennsylvania; Northeastern 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. Tug Hill in New York's North Country region has the second-most snow amounts of any nonmountainous location within the continental U.S., trailing only the Upper Peninsula, which can average over of snow per year.

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. In February 2007, a prolonged lake-effect snow event left of snow on the Tug Hill Plateau. 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.

A small amount of lake-effect snow from the Finger Lakes falls in upstate New York, as well. If the wind blows almost the entire length of either Cayuga Lake or Seneca Lake, Ithaca or Watkins Glen, respectively, can have a small lake-effect snowstorm.

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. Lake Superior and Lake Huron rarely freeze because of their size and depth; hence, lake-effect snow can fall continually in the Upper Peninsula and Canadian snowbelts during the winter. Main areas of the Upper Peninsula snow belt include the Keweenaw Peninsula and Baraga, Marquette and Alger Counties, where Lake Superior contributes to lake-effect snow, making them a prominent part of the Midwestern snow belt. Records of of snow or more have been set in many communities in this area. The Keweenaw Peninsula averages more snowfall than almost anywhere in the United States—more than anywhere east of the Mississippi River and most of all nonmountainous regions of the continental United States. Because of the howling storms across Lake Superior, which cause dramatic amounts of precipitation, the lake-effect snow is said to make the Keweenaw Peninsula the snowiest place east of the Rockies. Only one official weather station exists in this region. Located in Hancock, Michigan; this station averages well over per year. Farther north in the peninsula, lake-effect snow can occur with any wind direction. The road commission in Keweenaw County, Michigan, collects unofficial data in a community called Delaware, and it strictly follows the guidelines set forth by the National Weather Service. This station averages over per season. Even farther north, a ski resort called Mount Bohemia receives an unofficial annual average of . Herman, Michigan, averages of snow every year. Lake-effect snow can cause blinding whiteouts in just minutes, and some storms can last days.

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 390°, 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 Laporte and Gary.

Lake-effect snow is uncommon in Detroit, Toledo, Milwaukee, and Chicago, because the region's dominant winds are from the northwest, making them upwind from their respective Great Lakes, although they, too, can see lake-effect snow during easterly or northeasterly winds. More frequently, the north side of a low-pressure system picks up more moisture over the lake as it travels east, creating a phenomenon called lake-enhanced precipitation.

File:Buffalo2001-20.jpg, Buffalo, New York, after of snow fell from December 24, 2001, to December 28, 2001
File:Fultonles.jpg, Fulton, New York, after a snowburst dropped of snow over most of Oswego County January 28–31, 2004
File:Snow-removal-cleveland-4.jpg, The Veteran's Day storm of November 9–14, 1996, may be the most severe early-season lake-effect snow storm the Great Lakes has witnessed in the past 50 years. At the height of the storm, over 160,000 customers were without power in Greater Cleveland alone, as the storm produced isolated snowfall tallies approaching .

Ontario, Canada

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.

Toronto and Hamilton are usually spared lake-effect squalls because they are not on the leeward side of Lake Ontario during the dominant northwest winds. Some central and northern portions of the Greater Toronto Area, though, can be affected a few times each year by lake-effect snow from Georgian Bay. Downtown Toronto and Hamilton get most of their lake-effect snow when the wind comes from the southeast or east, over Lake Ontario. Such easterly winds are usually associated with a winter cyclone passing just to the south of the Great Lakes.

When the wind is from the north, the snowbelt runs north-south from Grand Bend to Sarnia and London. Areas such as Lucan and Kincardine have experienced some of the heaviest snowsqualls from Lake Huron in this region. When the wind is slightly more westerly, the snowbelt runs from Tobermory, Owen Sound, and Grand Bend to as far south and east as Arthur, Orangeville and Caledon. This snowbelt often reaches Kitchener and can affect the Halton and Peel regions of the Greater Toronto Area. These northwesterly winds usually also bring snow southeast of Georgian Bay, which can reach beyond Lake Scugog. A westerly wind sends lake-effect streamers east from Owen Sound to Gravenhurst, Barrie, and Orillia, and may even reach as far south and east as York Region in the Greater Toronto Area. When the wind is from the southwest, lake-effect streamers from Lake Huron and Georgian Bay run from Noelville to Sudbury, Gravenhurst, and Algonquin Provincial Park. Winds from this same direction coming over Lake Ontario cause squalls to come ashore from Cobourg through the Belleville area to Kingston and the Thousand Islands, with Prince Edward County being the area most vulnerable to extreme snowfall amounts. Some snow bands can occasionally reach Quebec and Maine, while snow originating from Lake Erie, Lake Ontario, and even Lake Michigan can impact southern Ontario. Easterly winds primarily affect the Niagara Peninsula. Local lake-effect snowsqualls can occasionally occur downwind of Lake Simcoe when the lake is unfrozen, usually in early winter or late fall.

Lake Superior has its own independent snowbelts, affecting Wawa, Sault Ste. Marie, Marathon, the Keweenaw Peninsula in Upper Michigan, and Pukaskwa National Park. Thunder Bay is usually not affected by lake-effect snow, unless it is associated with a winter storm.

Elsewhere in the United States

The southern and southeastern sides of the Great Salt Lake receive significant lake-effect snow. Since the Great Salt Lake never freezes, the lake effect can influence the weather along the Wasatch Front year-round. The lake effect largely contributes to the annual snowfall amounts recorded south and east of the lake, and in average snowfall reaching in the Wasatch Range. The snow, which is often very light and dry because of the semiarid climate, is referred to as the "Greatest Snow on Earth" in the mountains. Lake-effect snow contributes to roughly six to eight snowfalls per year in Salt Lake City, with about 10% of the city's precipitation being contributed by the phenomenon.

Similar snowfall can occur near large inland bays, where it is known as bay-effect snow. Bay-effect snows fall downwind of Delaware Bay, Chesapeake Bay, and Massachusetts Bay when the basic criteria are met, and on rarer occasions along Long Island.

The Finger Lakes of New York are long enough for lake-effect precipitation.

The Texas twin cities of Sherman and Denison are known, in rare instances, to have experienced lake-effect snow from nearby Lake Texoma due to the lake's size (it is the third-largest lake in Texas or along its borders).

On one occasion in December 2016, lake-effect snow fell in central Mississippi from a lake band off Ross Barnett Reservoir.

Oklahoma City even had a band of lake-effect snow off of Lake Hefner

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