Toronto residents clear snow after a winter storm, Dec. 27 2012.Fernando Morales/The Globe and Mail
A major storm from Texas landed on Toronto Thursday, with the city readying snowplows, airlines warning of flight delays and the rest of the country making jokes about calling in the army.
Toronto's reputation as a snow-shy city was triggered by then-mayor Mel Lastman's request for military assistance in 1999, but it is also compounded by its geography, which often turns forecasted snow into rain or spares the city from so-called lake-effect snow.
Lake-effect snow is produced when cold northern winds blow over a lake's warm water surface. That snow is carried by band-like streamer clouds that can stretch a hundred kilometres, says University of Toronto's atmosphere physicist Kent Moore.
Lake effect and streamer clouds are the cause of the snow-belt conditions that affect large swaths of southern Ontario and Upper New York State in winter.
However, since Toronto is upwind from Lake Ontario, lake-effect snow that comes from that body of water ends up falling on the American side, on cities like Rochester or Syracuse. Buffalo's heavy snows are also a lake-effect by-product, but from Lake Erie.
Toronto sometimes gets the tail end of lake-effect snow from Lake Huron, but most of that has already fallen further west, on municipalities such as Goderich, London or Owen Sound.
"Toronto has one of the least amounts of snow in an average winter of any major city or town in southern Ontario. We're too far away from Lake Huron and Georgian Bay to get much influence from the lake effect," said Environment Canada meteorologist Geoff Coulson.
Prof. Moore said are streamers are erratic and the reason why motorists in Ontario can face clear conditions in one area, followed by sudden, blinding snowsqualls blowing a few kilometres away.
"They can be more difficult to forecast because even a slight variation in that surface wind flow can influence who's going to get the most snow," Mr. Coulson said. "There are also lake-effect events where the wind will lock in and so the lake-effect snow band will lock in over one area over a long period of time, giving huge amounts of snow over one area and much smaller amounts away from that."
Bigger storms are easier to forecast, Prof. Moore said, however there is another twist for Toronto. Lake Ontario stores heat from the summer, making Toronto's climate milder.
"In terms of temperature, we tend to flirt with the freezing mark, more so than areas further north," Mr. Coulson said. "So the challenge we face when a large system is approaching, is that a slight change in the track of the system means that we get warmer and get rain, or perhaps freezing rain.
"Toronto often gets right in that boundary where these types of precipitations could occur."
The coming storm might fit that picture.
After passing through Illinois, the Texas low will pass over southern Ontario, where it expected to dump up to 25 centimetres of snow.
Mr. Coulson said, however, it might be a mix of precipitations – snow, icy pellets and freezing rain.
By late Friday, the storm will arrive in the Maritimes, weakened but potentially merging with another storm system that is coming up the east coast from North Carolina, Mr. Coulson said.
Toronto gets an average of 115 centimetres of snow each winter. However only 42 centimetres fell last winter and 31 centimetres so far this winter.