CHICAGO (AP) — A combination of cold, snow and wind is making travel difficult in parts of the country.

Several Midwestern states have been walloped with up to a foot of new snow as bitterly cold temperatures begin to settle in. Five to seven inches of snow fell overnight in Chicago, where about 1,800 flights have been canceled today. Forecasters say central Illinois, Indiana and Michigan could see up to 10 inches of snow today, with several inches possible in western Tennessee and Kentucky.

Temperatures not seen in years are likely to set records in the coming days across the Midwest, Northeast and South.

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A "polar vortex" air mass will affect nearly half of the continental U.S. starting Sunday and into Monday and Tuesday, with wind chill warnings stretching from Montana to Alabama.

The bitterly cold temperatures already pushed into northern states Sunday morning. The National Weather Service reported a temperature of 9 below zero in Bismarck, N.D., and negative 21 at Duluth, Minn.

Frostbite and hypothermia can set in quickly at 15 to 30 below zero. Medical experts say people should wear hats, face protection and layers of clothing.