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.
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.
Lake-effect occurring when the air at 850 millibars (85 kPa) 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.
[3] 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.
If the directional shear between the body of water and the vertical height at which the pressure measures 700 mb (70 kPa) is between 30° and 60°, weak lake-effect bands are possible.
The wind-speed difference between the surface and vertical height at which the pressure reads 700 mb (70 kPa) should be no greater than 40 knots (74 km/h) so as to prevent the upper portions of the band from shearing off.
However, assuming the surface to 700 mb (70 kPa) winds are uniform, a faster overall velocity works to transport moisture more quickly from the water, and the band then travels much farther inland.
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.
[citation needed] 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.
[13] Lake-effect snows on the Tug Hill plateau (east of Lake Ontario) can frequently set daily records for snowfall in the United States.
[14] The snowiest portions of the Tug Hill, near the junction of the towns of Montague, Osceola, Redfield, and Worth, average over 300 inches (760 cm) of snow annually.
[20] 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.
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.
[30] 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.
[33] Furthermore, cold air, when it arrives to the region, tends to move slowly, creating days and sometimes weeks of occasional lake-effect snowfall.
[35][34][36] Earlier, unofficial measurements are often higher, due to the relative dearth of sufficiently old weather stations in the region; some sources claim up to 4 meters (13 ft; 160 in) of snowfall during the blizzard of March 1987.
[38] Although Fennoscandia is lined with an abundance of lakes, this type of snowfall is rare in these, due to the shallow freshwater freezing early in the cold interiors.
[39] The Sea of Japan creates snowfall in the mountainous western Japanese prefectures of Niigata and Nagano, parts of which are known collectively as snow country (Yukiguni).
[42][43][44][45] In the United Kingdom, easterly winds bringing cold continental air across the North Sea can lead to a similar phenomenon.
[48][49] 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.
[51] 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.
