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A hybrid Frontal-Lake Effect Snowsquall hitting Toronto, Canada during rush hour. Toronto lake effect frontal squall.JPG
A hybrid Frontal-Lake Effect Snowsquall hitting Toronto, Canada during rush hour.

A snowsquall, or snow squall, is a sudden moderately heavy snowfall with blowing snow and strong, gusty surface winds. [1] It is often referred to as a whiteout and is similar to a blizzard but is localized in time or in location and snow accumulations may or may not be significant.



There are two primary types of snowsqualls: lake effect and frontal.

Lake-effect snow

When arctic air moves over large expanses of warmer open waters in winter, convective clouds develop which cause heavy snow showers due to the large amount of moisture available. This occurs southwest of extratropical cyclones, with the curved cyclonic wind flow bringing cold air across the relatively warm Great Lakes which then leads to narrow lake-effect snow bands that can produce significant localized snowfall. [2] Whiteout conditions will affect narrow corridors from shores to inland areas aligned along the prevailing wind direction. [3] [4] This will be enhanced when the moving air mass is uplifted by higher elevations. The name originates from the Great Lakes area of North America, however any body of water can produce them. Regions in lee of oceans, such as the Canadian Maritimes could experience such snowsqualls.

The areas affected by lake-effect snow are called snowbelts and deposition rate of many inches (centimetres) of snow per hour are common in these situations. In order for lake-effect snow to form, the temperature difference between the water and 850 millibars (850 hPa) should be at least 23 °F (13 °C), surface temperature be around the freezing mark, the lake unfrozen, the path over the lake at least 100 kilometres (62 mi) and the directional wind shear with height should be less than 30° from the surface to 850 millibars (850 hPa). [5] Extremely cold air over still warm water in early winter can even produce thundersnow, snow showers accompanied by lightning and thunder.

Radar trace of lake-effect snowsqualls off the Great Lakes from US radars.
Snowsquall20101207 ontario.jpg
Radar image of a strong snowsquall off Lake Huron in December 2010. Over 150 cm of snow from this squall, fell north of London, Ontario. [6]
Lake effect snowsquall york region ontario jan08 03.jpg
A linear single banded snow squall over Southern Ontario

Frontal snowsquall

Frontal snowsquall moving toward Boston, Massachusetts, United States Snowsquall line-Bourrasque neige frontal NOAA.png
Frontal snowsquall moving toward Boston, Massachusetts, United States

A frontal snowsquall is an intense frontal convective line (similar to a squall line), when temperature is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which line passes over as the wind causes intense blowing snow. [3] This type of snowsquall generally lasts less than 30 minutes at any point along its path but the motion of the line can cover large distances. Frontal squalls may form a short distance ahead of the surface cold front or behind the cold front in situations where there are other contributing factors such as dynamic lifting from a deepening low pressure system or a series of trough lines which act similar to a traditional cold frontal passage. In situations where squalls develop post-frontally it is not unusual to have two or three linear squall bands pass in rapid succession only separated by 25 miles (40 kilometers) with each passing the same point in roughly 30 minutes apart.

This is similar to a line of thunderstorms in the summer but the tops of the clouds are only 5,000 to 10,000 feet (1,500 to 3,000 m), often difficult to see on radar. Forecasting these types of events is equivalent to summer severe weather forecast for squall lines: presence of a sharp frontal trough with wind shift and low level jet of more than 30 knots (55.58 km/h). However, the cold dome behind the trough is at 850 mbar instead of a higher level and must be at least -13 °F (-25 °C). The presence of surface moisture from bodies of water or preexisting liquid precipitation is also a significant contributing factor helping to raise the dew point temperature and saturate the boundary layer. This saturate can significantly increase the amount of convective available potential energy leading to deeper vertical growth and higher precipitable water levels increasing the volume of snow which can be produced by the squall. In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow.


Rapidly deteriorating weather conditions during snowsqualls often lead to traffic accidents. Car into ditch bad winter weather hwy 404 south near stouffville rd Jan08.jpg
Rapidly deteriorating weather conditions during snowsqualls often lead to traffic accidents.
Winter conditions on Ontario Highway 401 in Toronto due to a snowsquall. Snowy Higddhway 4012.jpg
Winter conditions on Ontario Highway 401 in Toronto due to a snowsquall.

Both types of snowsqualls are very dangerous for motorists, airplanes, and other travelers; even can be more dangerous than blizzards. [7] The change in conditions is very sudden, with slippery conditions and abrupt loss of visibility due to whiteouts, which often cause multiple-vehicle collisions. In the case of lake-effect snow, heavy amounts of snow can accumulate in short periods of time, possibly causing road closures and paralyzing cities. For instance, on January 9, 2015, a localized, heavy snow squall caused a 193-vehicle pile-up on I-94 highway near Galesburg, Michigan. [8]

See also

Warnings about lake-effect snow:

United States

Related Research Articles

<span class="mw-page-title-main">Lake-effect snow</span> Weather phenomenon

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.

<span class="mw-page-title-main">1993 Storm of the Century</span> Category 5 noreaster in the United States

The 1993 Storm of the Century was a cyclonic storm that formed over the Gulf of Mexico on March 12, 1993. The cold weather, heavy snowfall, high winds and storm surges that the storm brought affected a very large area; at its height, it stretched from Canada to Honduras. The cyclone moved through the Gulf of Mexico and then through the eastern United States before moving on to eastern Canada. It eventually dissipated in the North Atlantic Ocean on March 15.

<span class="mw-page-title-main">Thundersnow</span> Thunderstorm during which there is snowfall

Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is a kind of thunderstorm with snow falling as the primary precipitation instead of rain. It is considered a rare and unusual phenomenon. It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorm, but the top of the cumulonimbus cloud is usually quite low. In addition to snow, graupel or hail may fall as well.

<span class="mw-page-title-main">Alberta clipper</span> Low pressure area weather system common to North America

An Alberta clipper, also known as an Alberta low, Alberta cyclone, Alberta lee cyclone, Canadian clipper, or simply clipper, is a fast-moving low-pressure system that originates in or near the Canadian province of Alberta just east of the Rocky Mountains and tracks east-southeastward across southern Canada and the northern United States to the North Atlantic Ocean.

<span class="mw-page-title-main">Panhandle hook</span>

A panhandle hook is a relatively infrequent winter storm system whose cyclogenesis occurs in the South to southwestern United States from the late fall through winter and into the early spring months. They trek to the northeast on a path towards the Great Lakes region, as the southwesterly jet streams are most prevalent, usually affecting the Midwestern United States and Eastern Canada. Panhandle hooks account for some of the most memorable and deadly blizzards and snowstorms in North America. The name is derived from the region of surface cyclogenesis in the Texas panhandle and Oklahoma panhandle regions. In some winters, there are no panhandle hook storms; in others, there are several.

<span class="mw-page-title-main">Weather front</span> Boundary separating two masses of air of different densities

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably always a wind shift.

<span class="mw-page-title-main">Severe weather terminology (United States)</span> Terminology used by the National Weather Service to describe severe weather in the US

This article describes severe weather terminology used by the National Weather Service (NWS) in the United States. The NWS, a government agency operating as an arm of the National Oceanic and Atmospheric Administration (NOAA) branch of the United States Department of Commerce (DoC), defines precise meanings for nearly all of its weather terms.

<span class="mw-page-title-main">Rainband</span> Cloud and precipitation structure

A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands within tropical cyclones are curved in orientation. Rainbands of tropical cyclones contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

A snow squall warning is a bulletin issued by Environment Canada's Meteorological Service of Canada and the National Weather Service in the United States to warn population of two types of snow events reducing visibility in blowing snow: lake effectsnow squalls and frontal snow squalls.

<span class="mw-page-title-main">Extratropical cyclone</span> Type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to severe gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

Blowing snow is snow lifted from the surface by the wind, at eye level or more, that will reduce visibility. Blowing snow can come from falling snow or snow that already accumulated on the ground but is picked up and blown about by strong winds. It is one of the classic requirements for a blizzard. Its METAR code is BLSN. If the snow remains below 1.8 m (6 ft), it will be called drifting snow. The snow which is being blown about may deposit as snowdrifts.

<span class="mw-page-title-main">Cold front</span> Leading edge of a cooler mass of air

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  5. Jeff Haby. "Lake-effect forecasting". Haby's Weather Forecasting Hints. Retrieved 2009-08-12.
  6. "Snowstorm shuts down London Ontario". CBC News. CBC. December 8, 2010. Retrieved December 13, 2017.
  7. Allison Chinchar, Meteorologist (18 Dec 2021). "What are snow squalls and why they're more dangerous than blizzards". CNN. Retrieved 19 Feb 2022.
  8. "Michigan I-94 Pileup | Firefighter Nation". Firefighter Nation. May 2015. Retrieved 2015-10-09.