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Lake Erie dipped to 34 degrees Tuesday off Buffalo’s harbor, four degrees below average. The water hasn’t been colder on a Dec. 17 in 18 years.

So what does it mean for those shoveling sidewalks and drivers white-knuckling their way through one lake-effect snow squall after another?

With ice already forming along the lake shoreline, it signals relief may be on the way for those already weary of winter, which, at least according to the calendar, officially begins Saturday.

“When the lake is frozen, that has a negative impact on lake-effect snow,” said Jeff Wood, meteorologist at the Buffalo office of the National Weather Service. “It pretty much is the end of it. It’s like pretty much putting a cover on the lake, essentially.”

The lake’s temperature has fallen 20 degrees since Nov. 1. Ice began forming along the northern shore Nov. 25, weeks earlier than usual.

The lake was at 41 degrees Dec. 1, the coldest the lake has been at that point since 2002.

Those frigid temperatures, while helping to generate lake-effect storms, may also choke them off.

Ice is starting to coat parts of the lake, especially in the shallow western basin.

As of Monday, ice covered almost 19 percent of the lake. As the ice spreads, the incidents and severity of lake-effect snow will wind down.

That’s because the lake-effect machine relies on open water for fuel. Cold air over open, warm lake waters, under the right atmospheric conditions, creates the “lake-effect” phenomenon unique to the Great Lakes.

“Anyone living in a snow belt of the Great Lakes experiences lake-effect snow,” said Keith W. Kompoltowicz, watershed hydrology chief with the U.S. Army Corps of Engineers. “It’s part of the mystique of the Great Lakes, where you can get feet of snow in a short amount of time.”

Unlike in the Buffalo Niagara region, the lake-effect machine churns all winter long in the Tug Hill Plateau on the eastern end of Lake Ontario. Because Ontario is so deep, it doesn’t freeze over, and lake-effect snow isn’t cut off. So Syracuse, Oswego and Watertown typically log much higher seasonal snowfall totals than Western New York.

Less lake-effect snow is one benefit of a frozen Lake Erie. There are others.

“There are often a lot more sunny days,” said Wood, explaining that the evaporation of open lake waters helps to create clouds over Buffalo Niagara in the late fall and early winter. “Although, they tend to be quite cold, too, because you’re losing the radiating effect of the lake.”

Nipping any further evaporation may also help to retain lake levels.

That’s because Lake Erie experiences its biggest drop from evaporation during autumn and relies on spring precipitation to rebuild its water level.

The average date over the last 30 years for when Lake Erie reaches the freezing point is Jan. 21, according to the National Oceanic and Atmospheric Administration.

Last year, the lake dipped to 32 degrees on Jan. 30, and the year before it did not dip to the freezing point at all. Wood said the lake has been classified as unfrozen only five times since 1927.

Whether the lake is “frozen” when it hits the freezing point, or whether it’s covered in ice, is not a matter of semantics for George Leshkevich, a manager at NOAA’s Great Lakes Environmental Research Laboratory in Ann Arbor, Mich.

“The lake is ‘frozen’ when it is completely ice covered,” Leshkevich said.

It’s rare for Lake Erie to attain a 100 percent ice cap, but it comes close.

“Because it’s real shallow in relation to the other lakes,” Leshkevich said, “it loses heat real quickly, and it gets a fairly extensive ice cover during the season.”

Last year, the highest average ice concentration on Lake Erie – 83.7 percent – occurred on Feb. 22. The year before, during the extraordinarily mild winter of 2011-12, only 13.9 percent of the lake ever got covered with ice. That was on Jan. 21. In the preceding three years, ice cover reached 93 percent to 96 percent during early February, according to the NOAA data.

“There is a relationship there,” Leshkevich added. “Certainly, the greater surface area that is ice-covered, the less chance there is of lake-effect snows from evaporation.”

For those already wary of December’s lake-effect snows, news of an icy Lake Erie might seem good. But Mother Nature can turn tricky.

Remember Jan. 28, 1977?

That season, Lake Erie iced over early, too. The water temperature hit the freezing point on Dec. 14, 1976.

Combine that with the fourth snowiest December in Buffalo on record – 60.7 inches – with the snowiest January ever – 68.3 inches. Then came powerful southwest winds, gusting at up to 75 mph.

The infamous “Blizzard of ’77” resulted in about a foot of total accumulation between Jan. 28 and Feb. 1, 1977, at Buffalo International Airport. But much of the snowfall came from existing snow “lying on the frozen surface of Lake Erie being blown into the Buffalo area and redeposited,” according to the Weather Service.

“There’s a whole chain of events that have to happen for that to occur,” Wood said. “But, it doesn’t mean that it can’t happen again.”

email: tpignataro@buffnews.com