Imagine getting 40 inches of snow in under two days. Then picture driving a few towns over and finding they didn’t even have to plow. This is the world of lake-effect snow.

That exact scenario could unfold late this week east of lakes Erie and Ontario as the prolific “lake-effect snow machine” gets underway. Downwind of Lake Erie, a general 10 to 20 inches is forecast. East of Lake Ontario, double that — in some locales, more than three feet.

Blizzard warnings are in effect downwind of both lakes. Strong westerly winds gusting up to 60 mph at times will channel building snow streamers to the lakes’ eastern shores while reducing visibility below a quarter mile at times. The gales could also trigger scattered power outages and sculpt snow drifts up to eight feet high.

Early Thursday, the National Weather Service in Buffalo issued a strongly worded statement warning of the “rapid onset of blizzard conditions this morning."

“Winds will rapidly increase and temperatures will drop through mid to late morning, resulting in the development of severe blowing and drifting snow and near zero visibility,” the National Weather Service wrote.

In Watertown, N.Y., the visibility dropped from nine miles to a half-mile as snow abruptly began. Winds at 9 a.m. were gusting over 40 mph; earlier in the morning, they were listed as calm. Blizzard conditions were observed there beginning at 7 a.m. Thursday.

“We’ll have six-hour amounts approaching a foot,” said Jim Mitchell, a meteorologist at the National Weather Service in Buffalo. “Two inches per hour on average, some areas will see three to four inches per hour at times. The heaviest snow will be east of Lake Ontario; the biggest window will be Thursday into early Friday.”

It’s the perfect recipe for a ripping lake-effect band that produces snow. A few claps of thundersnow are even possible.

Strong westerly winds blowing the same direction for more than 36 hours will align with the exact fetch of Lake Ontario, maximizing the amount of time chilly air aloft can remain in contact with the warmer lake waters.

“It’s about wind direction,” Mitchell said. “How long is that wind going to stay in the same orientation? Obviously, if the winds shift, the band is going to waver.”

But a stalled low-pressure system to the north will help an ideal wind pattern become established.

It’s rather late in the season to see such hefty lake-effect snow. In Watertown, for example, the overwhelming majority of its biggest snows historically have occurred before the end of February. But Mother Nature is doing everything in its power to generate a winter wonderland.

Feeding the snow: Lack of lake ice, warm waters

Instrumental in the ongoing snow is the lack of ice over lakes Erie and Ontario. Lake-effect snow can only develop when a lake’s waters are exposed to the atmosphere above it. This year, a significant deficit in ice coverage has allowed the lake-effect snow machine to work for deeper into the season than usual.

“Lake Erie a lot of times shuts off because it freezes, but we usually still get some effects from the lake,” Mitchell said. Among these influences can be frictional convergence, or the development of snow showers enhanced by easier air flow over the smoother lake surface.

“It is well below average,” said Elizabeth Thomas, an assistant professor in the geology department at the University at Buffalo. “It’s low, but it is not unprecedented in this record.” She states that ice coverage records date to the early 1970s.

Frank Marsik, an associate research scientist at the University of Michigan, offered numbers on the ice deficit.

“This year, we’re considerably below normal,” he said. “Last year at this time of year, we were looking at about 70 percent Great Lakes ice coverage. This year, we’re looking at about 10 percent.”

He noted that 2012 and 2017 also featured below-average ice coverage.

“Most of the lakes are running 1 to 2 degrees above where they were last year, and that’s playing a role in ice coverage,” Marsik said. He attributes that to mild temperatures this season.

Thomas said that a warming world in the future could favor more lake-effect precipitation and reduced ice coverage. But not all of what falls will be snow.

“If winters continue to get warmer, we might get more rainfall,” she said. “Though if we continue to get those outbreaks of Arctic air … we could get some pretty severe lake-effect storms.”

Open lakes mean flooding concerns

As winds drive water across the lakes, water is piling up along their eastern shores. Lakeshore flood warnings are in effect, the National Weather Service calling for “flooding along the shoreline … especially in bays and inlets and other low lying shoreline areas.”

This flooding will be heightened by lake-water levels that are running up to a few inches above average this year, thanks to recent precipitation. Marsik said 16 of 24 months in 2018 and 2019 had above-normal precipitation in the southern Great Lakes.

Fluctuating water levels and chaotic swings have proved a headache for coastal residents, some of whom have suffered flooding of their homes or even taken a hit on their property values.

There is even the possibility of a seiche of several feet on the east ends of lakes Erie or Ontario through Friday. That occurs when strong winds or a rapid change in air pressure slosh water from one side of the lake to the other. Water levels could be more than three feet above normal along the eastern shores, while the west end of the lakes may see below normal levels.

A seven-foot seiche accompanied strong westerly winds in Buffalo in October. Toledo, on the opposite end of the lake, saw water levels drop by three or four feet.

As winds ease by late Saturday, conditions should improve on lakes Erie and Ontario and downwind areas.