A major lake-effect snow event is unfurling downwind of Lake Erie and Lake Ontario, with heavy snow squalls producing up to a metre and a half of snow.

Every year, the US snowbelt is graced with very heavy snow, thanks to cold air being shoved across the Great Lakes during late-autumn and winter - and it is happening right now. But how exactly does it work?

Lake-effect snow is only possible when the lake’s water temperatures are much warmer than the cold air flowing over them, usually from the north or north-west during this time of the year.

This means that the lakes have very little to no ice on them, with abundant moisture is available when a cold air mass moves across the warmer body of water.

When cold winds kick up and flow across the warmer body of water, clouds form and grow. The elevation of the land downwind from the body of water is important too. The more these clouds rise, thanks to the increasing land elevation, the better the instability for snowfall to occur.

Temperatures on land also need to be near or below freezing for any snow to pile up. The heaviest bands can produce 5 - 8cm or more of snow per hour in some cases, causing white-out conditions and dangerous travel conditions.

A burst of cold air riding on a strong north-westerly flow over the very warm Great Lakes is producing ideal conditions for lake-effect snow.

Up to 40cm of snow has already been reported in some areas, with up to 1.5m of snow possible across north-western Pennsylvania and western New York. The phenomenon of thundersnow was also captured on our WeatherRadar below.

New York State Governor Kathy Hochul has declared a State of Emergency for New York as historical lake-effect snow unfolds.

Whilst most common in the US over large lakes, the Baltic Sea can also produce the same phenomenon during this time of year, forming in the exact same way.

In November 2010, up to 76cm of fresh snow fell within a short space of time due to the lake-effect in the Ostholstein district in Germany, and also in the city of Lübeck.