Before the Great Lakes freeze over, they are a source of relatively warm water.

In the right conditions, this warmth can be transformed into piles of snow over lakeside cities. This happened with a vengeance this Christmas, nowhere more dramatically than in Erie, Pennsylvania.

A “snow emergency” was announced to cover the Christmas period and it turned out to be a very white Christmas, record-breakingly so. Almost on the stroke of midnight on December 25, the wind suddenly picked up, the visibility dropped, and the snow started.

Christmas Day offered a windchill of -17C, visibility dropping to below 100 metres and constant snow. In short, a blizzard from the lake. Some 86cm of snow fell. This beat the single day snow record by 36cm. By the end of December 26, more than 1.5 metres of snow had fallen, in 48 hours.

This prolific event shattered all previous multi-day snowfall records in Erie dating back to 1893, according to the National Weather Service office in Cleveland. The total snowfall so far this month is 246cm. This December is the city’s snowiest single month on record, blowing past the previous record of 170cm set in December 1989.

The cause of such a dump of snow is the contrast in temperature between the Arctic air blowing across the Lakes and that of the water surface. This contrast makes the air very unstable. Rapid evaporation from the lake into this bitterly cold air creates deepening cloud which, blown onshore, dumps all that evaporated moisture as snow.

The air currently over Manitoba, the Dakotas, US Midwest and Great Lakes is truly cold. Maximum temperatures have been of the order of -25C in Winnipeg, -17C in Minneapolis, and -10C in Detroit.

Western Pennsylvania is not the only area affected by this outbreak of lake effect snow. Northern Wisconsin, western Michigan, the Ohio shore from Cleveland and all the way up Lake Erie and northwestern New York State are vulnerable. As is the extension of the Canadian Province of Ontario, open to the chill breeze from Lake Huron.

The risk of further lake effect snow will carry on, with a brief interruption, into the New Year but ultimately may well be self-defeating. Such Arctic cold will hasten the freezing of the lakes surfaces and so stop the effect.

With additional information from Weather Underground