The wind chill could drop to 30 degrees below zero in parts of Montana Wednesday.
AccuWeather meteorologist Tom Kines says the jet stream is much farther south than normal, allowing the cold air to push in from the Arctic and drop temperatures by 20 to 40 degrees below normal levels.
Areas of Montana and the Dakotas are forecast to reach lows in the minus-20s.
The storm hit the northern Rockies on Monday and Tuesday, dumping up to 2 feet of snow in the mountains and in Yellowstone National Park.
A four-vehicle crash in central Montana killed 21-year-old Chelsea Stanfield of Great Falls. Authorities said Stanfield was driving too quickly for the icy conditions.