Asnow squall, orsnowsquall, is a sudden moderately heavysnowfall withblowing snow and strong, gusty surfacewinds.^[1] It is often referred to as awhiteout and is similar to ablizzard but is localized intime or inlocation and snow accumulations may or may not be significant.

There are two primary types of snow squalls: lake effect and frontal. Both types can strongly reduce visibilities and sometimes produce heavy snowfall.

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 warmGreat 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 theprevailing wind direction.^[3][4] This will be enhanced when the moving air mass isuplifted by higher elevations. The name originates from theGreat Lakes area ofNorth America, however any body of water can produce them. Regions inlee ofoceans, such as the CanadianMaritimes could experience such snow squalls.

The areas affected by lake-effect snow are calledsnowbelts and deposition rate of manyinches (centimetres) of snow per hour are common in these situations. In order for lake-effect snow to form, thetemperature 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 directionalwind 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 producethundersnow, snow showers accompanied bylightning andthunder.

A frontal snow squall is an intense frontalconvective line (similar to asquall line), whentemperature 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 snow squall 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 lowpressure system or a series oftrough 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 km) 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 (56 km/h; 35 mph). However, the cold dome behind the trough is at 850millibars 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 ofconvective 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 dubbedthundersnow.

Both types of snow squalls are very dangerous formotorists,airplanes, and othertravelers; even can be more dangerous thanblizzards.^[7] The change in conditions is very sudden, with slippery conditions and abrupt loss of visibility due towhiteouts, which often causemultiple-vehicle collisions. In the case oflake-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] On very rare occasions, particularly powerful snow squalls can even becomesupercells and formtornadoes, even if no lightning orthundersnow is present.^[9]

• Convection
• Snow shower

Warnings about lake-effect snow:

United States

• Lake Effect Snow Advisory
• Lake Effect Snow Warning
• Severe weather terminology (United States)
• Snow squall warning

Canada

• Snow Squall Warning
• Severe weather terminology (Canada)

• Winter Weather Awareness by the USNational Weather Service
• Environment Canada - Weather and Meteorology - Hazardous weather-winter hazards
• Video of a snowsquall timelapse while driving on Highway 407 ETR in Greater Toronto