'Roofs can collapse:' Understanding the lake-effect snow headed for Ontario

Communities around southern Ontario are bracing for intense winter weather as a lake-effect squall pattern threatens to deliver large amounts of snow Thursday night and into the weekend.

According to the Weather Network and Environment Canada, some areas of the Great Lakes region – such as those on the eastern shores of Lake Ontario, Georgian Bay and Lake Erie – could be buried under 40 to 60 centimetres of snow by Sunday.

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"I think the real concern will be down in the Niagara-Welland area in Ontario and, of course, Buffalo (New York), where it's already an emergency," Environment Canada senior climatologist David Phillips told CTVNews.ca in a telephone interview on Thursday. "I mean, roofs can collapse under the weight of all that snow, and you have power outages."

In a snow squall watch issued Wednesday night and updated Thursday, Environment Canada warned road closures could be possible in the hardest hit areas.

"Consider postponing non-essential travel until conditions improve. If you must travel, keep others informed of your schedule and destination and carry an emergency kit and mobile phone," the agency said.

Cities and towns to the east and northeast of Lake Huron, Georgian Bay, Lake Erie, and Lake Ontario are particularly experienced with weather systems like this, Phillips said, citing the lake-effect storm of 2014 that dropped more than five feet of snow over Buffalo. To understand why, he said, it's necessary to understand how lake-effect snow squalls work.

Lake-effect snow storms in Ontario and New York are most common in November and December, Phillips explained, when air in the lowest part of the atmosphere and ground-level temperatures have cooled down, while the water in the Great Lakes is still relatively warm.

As the cold, late-autumn wind passes above the warm water of the lakes, it gathers moisture and energy to create water-laden clouds. Once those lake-effect clouds meet land, they release all that moisture, usually in the form of heavy snowfall.

"So the greater the difference between the air temperature and the water temperature, the more impactful, the more potent and the more lethal the storms will be," Phillips said.

The other key ingredient in the formation of lake-effect snow is the distance wind travels over a warm lake, a factor known as "fetch." The cold winds usually associated with lake-effect snow tend to come from the west and northwest, Phillips said. The more time those winds spend over warm bodies of water like Lake Ontario and Lake Erie, the more moisture they pull.

"If the winds go over the largest fetch of water, then there's more time for that cold air to moderate and pick up the cargoes of moisture that are available to it, just like a sponge over a wet surface," Phillips said.

Both Lake Ontario and Lake Erie are long and narrow, spanning from west to east. This means cold westerly winds that align with the orientation of the lakes have a long fetch with which to gather the moisture that later becomes snow. Phillips said it's the reason why communities situated on the eastern shores of Lake Ontario and Lake Erie – such as Buffalo, Fort Erie, Niagara Falls, Belleville and Kingston – tend to be hit hardest by lake-effect snow.

Finally, Phillips explained, if the winds are persistent and don't waver in their path over the land, that's when communities see sustained, localized snowfall over a period of days.

"A lot of it is like baking a souffle - all the things have to come together to give you that perfect kind of outcome," Phillips said. "And in this case, this is what we've got: warm water, cold air and persistent winds."

Fortunately, lake-effect storms tend to become less common as winter progresses and the lakes freeze, or at least cool down. Phillips said they also don't influence the weather systems that follow.

"One storm does not produce another storm later on," he said. "It doesn't really affect the weather a week or so later."