Minnesotans heading out Tuesday morning will face the lowest windchills of the season, so low that the National Weather Service has issued a windchill advisory that is in effect until noon for all of central and southern Minnesota.

“The combination of below zero temperatures and northwest winds of 10 to 20 miles per hour will generate windchills of 25 to 35 degrees below zero,” the weather service said.

As of 6 a.m., Lakeville had the lowest windchill in the metro area at minus 28 degrees. Other cities with a minus 20 degree windchill included St. Paul, Crystal and Eden Prairie. Blaine and South St. Paul reported in at minus 19 followed by Lake Elmo at minus 18.

In greater Minnesota, it ws a tie for coldest windchill between Mahnomen and Fosston at minus 36 degrees. Not far behind was Park Rapids at minus 35 and Hibbing at minus 34. Cities with a windchill of minus 30 or lower included Rochester, Worthington, Staples, Fergus Falls, Alexandria, Duluth, Crookston, International Falls and Bemidji, the weather service said.

Actual air temperatures were in the single digits and teens below zero except at Winona, which was at 3 degrees above zero and was the only reporting station in positive territory.

The bitter cold prompted the weather service to issue a reminder that frostbite and hypothermia “can occur if precautions are not taken.” It also said frostbite can set in as little as 30 minutes.

For those needing a place to escape the cold, the welcome mat is out at Twin Cities Salvation Army seven worship and service centers from 8 a.m. to 4:30 p.m. weekdays. The Harbor Light Center at 1010 Currie Street in downtown Minneapolis will remain open all day and night as a warming center.

“We want people to know they have a place to go to warm up and eat a hot meal,” said Major Jeff Strickler, Twin Cities commander. “This is dangerously cold weather, particularly for people who are experiencing homelessness.”

A number of school districts have delayed the start of classes Tuesday. Among those starting 2 hours late are Tracy Area Public Schools, Comfrey, Windom, Mountain Lake, Mountain Lake Christian School, Red Rock Central, Milroy, Cedar Mountain, Springfield and Central Minnesota Christian School in Prinsburg.

Slick conditions on the roads have led to a number of crashes on metro area freeways. By 6 a.m., MnDOT reported seven active crashes or stalls, including rollover wrecks on southbound I-694 at 10th Street in Oakdale. Other crashes had lanes blocked and temporarily closed on Hwy. 10 near 7th Avenue in Anoka. The lanes reopened just after 6 a.m.

The State Patrol responded to 172 crashes and 121 vehicles that slipped off the road between 4 and 9 p.m. Monday, mainly in the metro and southern part of the state.

Icy roads kept the State Patrol and local police busy Tuesday morning, too. At 7:45 a.m., more than a dozen crashes littered metro area freeways and highways. Some big traffic snarls developed on northbound I-35E from Eagan to downtown St. Paul and on I-35 through Lakeville and into Burnsville. Highways 169 and 100 also were plagued with mishaps that tied up traffic on both routes between 36th Avenue and Hwy. 7.

MnDOT advised motorists to drive slow and expect black ice during Tuesday’s morning commute.

Rock salt, normally used on roads, is less effective below 15 degrees, so roads are treated with liquid calcium chloride, which lowers the freezing temperature of the brine and prevents precipitation from bonding to the road surface.

A slight reprieve from the deep freeze is in store with a high near 17 degrees Wednesday and near 30 degrees Thursday. Winds are also expected to subside, but the warmth will bring a chance of snow and freezing drizzle Wednesday night into Thursday.

By Saturday, the frigid air returns with highs in the single digits above zero and lows in the single digits below zero. A low of minus 10 is predicted Sunday night, the weather service said.