Temperatures were in the negative teens in northwestern Wisconsin and negative single digits in the southwestern corner of the state, said Sarah Marquardt, a National Weather Service meteorologist.
She said temperatures were expected to drop throughout the day, with overnight lows ranging from minus 15 in the Milwaukee area to minus 30 across northwestern Wisconsin.
Tuesday was forecast to be even chillier. Highs temperatures are not expected to rise above zero anywhere in the state, and wind chills could dip as low as minus 50 in the northwestern reaches.
"This is similar to what we had three weeks ago" in terms of life-threatening conditions, Marquardt said. "With wind chills in the minus 30 to minus 40 range, you can get frostbite within 10 minutes on exposed skin."
Schools across the state closed Monday. Some districts, including Milwaukee Public Schools, planned to stay closed Tuesday as well.
The cold snap comes as Wisconsin and much of the Midwest grapples with a shortage of propane, a gas rural residents typically burn for heat. The shortage has created steep hikes in wholesale and retail propane prices.
Walker on Monday announced his administration planned to set aside $8 million to guarantee propane dealers' loans for gas purchases to help ensure enough gas reaches customers.
His administration plans to release another $8.5 million in heating assistance for low-income people's propane bills. Walker officials already have set up a hotline - 1-866-HEATWIS - for applications.
The governor said last week more money wouldn't have any impact and that the problem lies in the supply chain. Asked Monday how the governor squares that with releasing more money, Walker spokesman Tom Evenson said Walker was speaking "about the context of supply and demand."
"The governor was saying last week if we don't have more propane, money isn't going to get you more propane," Evenson said.
Walker also said he planned to ask for federal reviews of propane prices and how exports have affected domestic propane supplies. He said he also will ask Midwestern states to perform a joint review of propane delivery systems.
Republican state Sen. Dale Schultz, who last week called on Walker to spend some of the state's $1 billion surplus on the propane problem, praised the moves.
"We need to have a short-term strategy and we need to have a long-term strategy," Schultz said.
Meanwhile on Monday, Xcel Energy lifted an appeal the utility made Saturday to 46,000 Wisconsin customers to conserve gas after an explosion in Canada ruptured a pipeline and forced two others to shut down for inspection. One of the pipelines resumed operations Sunday.
Marquardt said there's no real warming trend on the horizon for the next few weeks. Temperatures should warm up slightly Wednesday, with highs in the teens and 20s, she said. Thursday and Friday will remain in the low teens. Temperatures might return to the 20s over the weekend, but next week's temperatures will remain below average, she said.
When the weather gets that cold, Don Demet, 57, abandons his informal dress code. The Milwaukee attorney usually wears a sports coat and tie to work, but on Monday he chose a sweater and parka sans tie.
"This has been a lousy year so far," he said, "so I'm hoping we revert to the mean with a nice spring, a nice summer and nice fall."
Richmond reported from Madison, Wis.