Ice formation on Lake Superior is down this winter, meaning trips to the ice caves at the Apostle Island National Lakeshore are in doubt for the season.

Mike Stewart, meteorologist at the Duluth office of the National Weather Service, said the next three weeks will determine whether the lake will freeze enough to support cave tours, but it's questionable because the lake only had 6 to 8 percent ice coverage on Wednesday.

"With this cold spell we had in the last two weeks we're getting some ice formation," Stewart said. "But it's getting warmer."

The cause for the low ice coverage is simple: warmer weather, Stewart said, adding, "Nothing to be alarmed about."

The current ice coverage is about half what it was a year ago, Stewart said. Two years ago, two-thirds of the lake was covered in ice and by February of that year, 90 percent of the lake was frozen.

That was the year the Bayfield area raked in $44 million from more than 290,000 visitors to the ice caves.

Associated Press - Ap, Star Tribune
FILE - In this Feb. 2, 2014 file photo, people visit the caves at Apostle Islands National Lakeshore in northern Wisconsin, transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest.

Stewart isn't giving up hope completely for the ice caves, which were also accessible last year, though for a shorter period. "We could get another cold spell and get some buildup," Stewart said.

To get ice buildup, the temperature needs to be a steady 15 degrees or lower.

There's also no clear percentage required for cave access because how and where the ice forms depends on wind direction and lake currents, he said. "There's a lot of variables — if you have a north wind, you could get a lot of buildup there," he said, referring to the part of the lake near the caves.

Millions of years before civilization, glaciers sculpted towering sandstone cliffs over Lake Superior. When the weather is cold enough, hikers can make the two-mile round trip to see them.

But the government website for the park said the strong El Niño — which has caused the warmer weather — is expected to continue in the coming year.

Hikers can check ice conditions by calling the Ice Line at 715-779-3397, extension 3.

Rochelle Olson • 612-673-1747

Twitter: @rochelleolson