South Dakota ? 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.

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 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.