Despite a prolonged deep freeze in early February, the conditions haven't been sufficiently cold enough for long enough this winter to allow Wisconsin's Apostle Islands ice caves to open to the public.

In an update posted Thursday, the Apostle Islands National Lakeshore said although the recent cold snap did bring "increased interest" in the possibility of the caves being opened, the ice access to the caves from Lake Superior is "not safe" for public use.

While temperatures did plunge during the polar vortex earlier this month, it followed a period of unseasonably warm weather across Wisconsin and Minnesota, with many lakes not getting substantial ice until well into December.

"In an ideal year, like 2014 or 2015, we have cold temps for a long time and calm waters as they freeze up," the post from Apostle Islands managers said. "This year, up until two weeks ago, it has been an unseasonably warm winter with minimal ice cover on Lake Superior.

"Remember that on Feb. 4, all we saw was open water at the caves. So while it has been cold, this is still pretty new. The ice which has now frozen up in front of the caves and Meyers Beach is pack ice which was pushed in and frozen in place. This ice can vary significantly in thickness over the whole surface."

"For these reasons, as well as the continued concern about large gatherings during the current pandemic, access to the mainland ice caves on the frozen surface of Lake Superior will remain closed for the 2021 season," it added.

The Upper Midwest is now in a period of warmup, with temperatures in the 40s in parts of the state on Monday, and will stay in the 30s through next week.