SIOUX FALLS, S.D. (AP) — Temperatures crashed to record lows well below zero Monday as a huge mass of arctic air blustered southward across the Midwest and West, keeping people indoors and leading some cities to open shelters.

The cold and remnants of the weekend blizzard that accompanied it closed hundreds of schools from the northern Plains to the Great Lakes states.

The St. Francis House shelter for the homeless in Sioux Falls, S.D., where Monday’s low was minus 11, was a lifesaver, said Richard Byrd.

“I would be probably huddled up right now under a bunch of blankets because this was my only alternative. If it wasn’t for the St. Francis house, I’d be in a scary, scary situation,” he said.

Major highways in Minnesota, and North and South Dakota reopened after Sunday’s blizzard dropped as much as 14 inches of snow, but hundreds of schools were closed in the three states.

Monday morning lows in North Dakota included minus 18 in Bismarck, with a wind chill factor of minus 42. The smaller communities of Bowbells and Berthold reported wind chill factors at around 50 below.

In Williston, N.D., Penny Groth acknowledged: “It’s just darn cold right now.” She said the Gramma Sharon’s Family Restaurant she owns had been closed since Saturday night because of the snow and cold.

Monday’s cold was an abrupt change for many areas. Illinois had unseasonable warmth Sunday with temperatures in the 50s, but Monday morning lows were in the single digits across the northern part of the state. Rockford had a low of just 3, and 20 mph wind made it feel like minus 18, the National Weather Service said.

“Right now it looks like it’s about 14 degrees,” cashier Mike Kanise said Monday afternoon as he checked a weather station at Hannel’s Amaco truck stop in Jacksonville, Ill. “Feels like it’s minus 1.”

Hundreds of Illinois schools were closed because of ice-covered roads. More schools were closed in Michigan, where northern areas had blizzard conditions as wind gusting to more than 50 mph caused whiteouts and generated wind chills as low as 30 below.

The storm and ensuing cold have been blamed for at least nine deaths. An avalanche in Colorado claimed the life of a Ski Patrol member, and exposure probably killed an 87-year-old man found outside his Montana nursing home. Weather-related car accidents were responsible for two deaths each in Minnesota and Missouri. Three traffic deaths were attributed to the weather in Oklahoma.

In Wyoming, authorities on Sunday morning rescued two men stranded overnight on an island in a reservoir where they had been fishing. The freezing cold and high winds prevented them from returning to shore.

Thermometers read 31 below Monday in Glasgow, Mont., and the wind chill was 45 below, the Weather Service said. The Texas Panhandle had lows in the single digits, and Goodland, Kan., registered a record low of minus 10.

Record lows Monday included minus 19 in Denver, where the previous Dec. 15 record of minus 6 was set in 1951; and minus 16 at Sidney, Neb.