Winter brings a variety of outdoor activities including ice skating and ice fishing.
A recent warm-up with temperatures as high as the 70s in the Northeast U.S. may have weakened some of the ice that had previously built up on ponds and lakes.
An AccuWeather.com meteorologist said, the conditions probably will improve as arctic air returns to the region.
"The warmth would have made it slushy," Senior Meteorologist Brian Wimer said. "There would have been some melting. It depends how much ice there was."
The arctic air will be firmly in place into the weekend.
"A lot of places have gotten colder. It would firm up the ice again, I would suspect by Friday and Saturday," Wimer said.
Any ice less than 2 inches thick on lakes and ponds is unsafe, and ice may not be the same thickness all over, officials said.
Two Oklahoma children died earlier this month after falling through ice, media reports said.
Four inches of ice can safely support 200 pounds or one person with gear, while 7 inches can safely support a group of people, or 1,500 pounds, according to the Pennsylvania Fish and Boat Commission.
One car can be supported on 8 inches of ice.
But there have been problems with vehicles falling through ice in Minnesota. The state's Department of Natural Resources issued a warning last week, saying that the ice was not strong enough for vehicles.
Last winter, six people died in Minnesota from breaking through the ice. All the deaths involved either a snowmobile or a vehicle, the department said.