DULUTH – The long stretch of subzero temperatures has rapidly increased ice coverage on Lake Superior, bringing levels closer to the 50-year average following historically low percentages of ice in January.
On Monday nearly 32% of the big lake had ice, according to the National Oceanic and Atmospheric Administration, up from 7% just a week ago.
That's good news for ice anglers, aquatic life that depends on ice cover and shorelines that could use a break from the battering of waves, although the Great Lakes shipping industry prefers low ice for an easier start to the season in late March.
In January, NOAA predicted Lake Superior would peak at 31% ice cover this year, but by the end of the week ice could cover 44% of the lake. Ice typically reaches its annual maximum in late February to early March.
Last year Lake Superior ice topped out at 22.6% on Feb. 17 after near-total coverage in 2019.
Those cycles between high and low coverage are normal, but they are fluctuating more wildly than in years past. On average about half the lake is covered with ice at its maximum extent, though in recent decades ice coverage has been much higher or lower than the average.
Researchers warned that consecutive low ice years could warm the lake, which brings the risk of harmful algae blooms in warm months that choke out aquatic life and can be toxic to humans.
Much of the newest ice has formed on the western half of Lake Superior, which like the rest of Minnesota has been caught in the polar vortex that is finally moving out of the area this week.
"Some notable recent ice growth has occurred between the North and South Shores in the western 'nose' of Superior, within and south of the Apostle Islands, and between Isle Royale and mainland Minnesota/Canada," the National Weather Service's Duluth office wrote Monday. "It's important to remember ice is never 100% safe and that much of this ice is newly formed and can still shift and move."
That warning comes after more than two dozen ice anglers needed rescuing after an ice floe broke off near Duluth's shoreline last week. There were no injuries, but thousands of dollars of equipment was abandoned.
The ice caves at the Apostle Islands National Lakeshore remain closed due to unsafe ice conditions.
January had the second-lowest average ice cover for Lake Superior, according to NOAA records that stretch back to 1973.
January's warm conditions and open water did help bring down historically high lake levels that have increased erosion along shorelines. Lake Superior is about 7 inches lower than it was this time last year, according to the Army Corps of Engineers, and it could fall another 2 inches by March 12. Still, all the Great Lakes aside from Ontario remain "well above" monthly long-term averages, the Corps said.