TRAVERSE CITY, Mich. — For the second consecutive winter, bitter weather threatens to turn the surface of the Great Lakes into a vast, frozen plain.
Nearly 81 percent of the lakes' surface area was covered with ice, the NOAA Great Lakes Environmental Research Laboratory reported Friday. That was down slightly from more than 85 percent the previous day — a glitch that probably happened because strong winds broke apart some ice and created open spots detected by satellites, said George Leshkevich, a physical scientist with the lab in Ann Arbor.
But with forecasts calling for frigid weather at least through the end of the month, the ice cover may keep expanding, he said. It's grown rapidly as temperatures have plunged this month, nearly doubling over the past couple of weeks.
Records show the lakes' most widespread freeze was 94.7 percent in 1979. The ice cover topped out at 92.2 percent last March.
Significant portions of the lakes typically froze over decades ago, Leshkevich said, but the frequency of severe winters has declined since the late 1990s.
"Two almost record-setting years back to back would be very unusual," he said.
One likely explanation for the rapid buildup this month is that 2014's freeze lasted so long — Lake Superior wasn't completely ice-free until June — and summer was so mild that the lakes didn't absorb much heat, he said. "So we started this season with below-water temperatures to begin with."
The ice blanket reaches across more than 90 percent of Lakes Superior, Huron and Erie, while Lakes Michigan and Ontario are more than halfway covered.
It has produced some spectacular visual images, from ice caves along the Lake Michigan shoreline to a glacial buildup making it appear that Niagara Falls had frozen in place.
But it's a headache for the Coast Guard, whose cutters open channels for vessels hauling vital cargo such as heating oil and road salt. The Detroit-based tug Bristol Bay has struggled for days to free the Arthur M. Anderson, a freighter stranded about 70 miles east of Cleveland in ice up to 10 feet thick. The Canadian Coast Guard has dispatched an icebreaker to assist.
The job has taken so much longer than expected that the Bristol Bay's crew ran low on food and had to receive a delivery by helicopter, which lowered supplies in their rescue basket.
Things will get even busier in mid-March, when the shipping season begins for the lakes' regular traffic of vessels carrying iron ore, coal, grain and other bulk cargo.
"We're probably going to be looking at situations like we had last year, where we had to put together convoys — lots of vessels together to make it through," Coast Guard spokesman Lorne Thomas said.
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