Towns around the Great Lakes are used to snow. Why did this storm bring so much?

Even more snow could be on the way for places in the Great Lakes region that are still digging out after days of storms caused deadly wrecks, collapsed a barn on top of 100 cows, and buried some towns under nearly 6 feet.

While cleanup was still taking place Dec. 3 around parts of western New York, Pennsylvania, Ohio, and Michigan – where some places saw just an inch or two of snow while others were hit with several feet – areas in New York were still seeing snowfall and a lake-effect snow warning remained through much of Dec. 3. Some spots could get another 4 to 8 inches of snow, according to the National Weather Service.

Layers of snow piled on top of an Ohio high school caused its roof to partially collapse while the building was closed over the weekend. School officials near Ashtabula found more damage on Dec. 2, saying it would take weeks to repair and that they were making plans to move classes elsewhere.

More than five feet of snow blanketed the area east of Cleveland along Lake Erie and more was expected later in the week, with a winter storm watch in place from the weather service beginning the night of Dec. 4 into Dec. 6.

Todd Brainard used a roof rake to scrape several feet of snow from the top of his house in North Perry, Ohio.

“I just don’t want to take the chance of having the roof cave in on my kids or wife or any one of us,” he said Dec. 3. “A lot of people haven’t seen this amount of snow in a long time.”

Ohio Gov. Mike DeWine declared a state of emergency in four counties on Dec. 3, noting that more snow squalls and high winds were expected in the coming days.

In neighboring western Pennsylvania, another 3 to 9 inches could fall from late Dec. 3 through the morning of Dec. 4. Many school districts in the western part of the state remained closed Dec. 3 after several days of lake-effect snow.

Members of the Pennsylvania National Guard were using Humvees and tactical vehicles to transport health care workers and those needing medical care while also rescuing stranded drivers during the past four days, according to a spokesperson, Maj. Travis Mueller.

In Erie, Pennsylvania, which was buried under several feet of snow, more than 200 cars were left abandoned in the snow, hampering efforts to clear streets. Even for a city accustomed to lake-effect snow, the storm was “unprecedented” in its intensity, said the city’s assistant fire chief, Gregory Purchase.

When towns along the Great Lakes get buried in drifts of blowing snow, like several have over the past few days, weather experts start talking about the “lake effect."

Lake-effect snow often occurs in relatively narrow bands that dump copious amounts of snow. The weather phenomenon can drastically increase snowfall totals, and it may slam one area and leave another just miles away untouched.

Lake effect, explained

In the United States, the lake effect typically begins when cold air – often from Canada – blows in over the Great Lakes’ warmer waters.

Warming air from the lakes then pushes the moisture in the sky higher into a zone most conducive to snowfall because of its temperature. That creates clouds capable of dumping lots of precipitation downwind, said Phillip Pandolfo, a meteorologist in the National Weather Service’s office in Buffalo, New York.

Most of the moisture needed for lake-effect snow does not actually come from the lakes, but rather from cold air that blows over them.

“It’s a common misconception that the lakes are a tremendous source of moisture,” Pandolfo said. “In practice, we actually need the air to actually have enough moisture in it before it really starts going over the lakes.”

Clouds form, snow falls

With the right conditions, the rising, moisture-laden air causes clouds to form that could bring “some really intense snowfall rates,” Pandolfo said.

The results typically are thin bands of clouds that can produce heavy snowfall — 2 to 3 inches (5 to 8 centimeters) per hour and sometimes more. And because the bands are narrow, towns near each other could see significant differences in snowfall totals.

Forecasting lake-effect snow can be difficult; slight changes in wind direction can have a major impact on where the heaviest snow falls, according to the weather service.

This story was reported by The Associated Press. AP writers John Seewer, Mark Scolforo, Michael Rubinkam, Becky Bohrer, and Dylan Lovan contributed to this report.