A wintry storm pushing through the Rockies and Midwest is bringing bitterly cold temperatures and treacherous driving conditions blamed in at least six deaths.

The wind chill could drop to 30 degrees below zero in parts of Montana Wednesday while wind chills of minus 20 have already been recorded in the Nebraska Panhandle. Low temperatures in the Denver area were expected to drop below zero over the next several days.

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, AccuWeather meteorologist Tom Kines said.

Areas of Montana and the Dakotas were forecast to reach lows in the minus-20s, while Laramie, Wyo. is expected to see a low temperature of 28 below. The icy arctic blast was expected to be followed by another one later in the week, creating an extended period of cold weather that hasn’t been seen since the late 1990s, meteorologists said.

In California’s Central Valley, temperatures dropped into the upper 20s overnight, not enough to cause any damage to citrus crops. Citrus farmers, however, are anticipating colder temperatures overnight Wednesday and Thursday and are continuing to take precaution, said Bob Blakely, of California Citrus Mutual.

The unrelenting storm has spread about 2 feet of snow and brought daily activities to a grinding halt in northeastern Minnesota. The heavy snow and ice has contributed to hundreds of traffic accidents around the state and was cited in at least four fatal crashes since Monday. The storm closed the University of Minnesota Duluth and most other schools in the area and canceled holiday parties, Salvation Army bell ringing and a lutefisk dinner in Duluth, where more than a foot of snow has fallen over three days.

In Colorado, the American Red Cross opened a warming shelter in a church in Black Forest near Colorado Springs, to help people still recovering from a wildfire in June that destroyed nearly 500 homes and killed two people.

Meanwhile, the snow and cold sent the Denver Broncos indoors to practice and canceled a men’s World Cup downhill training in Beaver Creek. The racers need a clean, slick surface to practice on and will try to squeeze in a training run on Thursday, when the snow should taper off but bitter cold will continue.

Officials warned residents to protect themselves against frostbite if they are going to be outside for any length of time.

“When it gets this cold, you don’t need 30, 40 mile-per-hour winds to get that wind chill down to dangerous levels. All it takes is a little breeze,” Kines said.