WASHINGTON - A "polar vortex" thrust the eastern two-thirds of the United States into frigid conditions, with even the southernmost states also facing wind-chill warnings."The coldest temperatures in almost two decades" spread into the northern and central parts of the country, packing gusty winds that produced "life-threatening wind-chill values as low as 60 degrees below zero," the National Weather Service said. The extreme temperatures and blustery winds produced by the arctic cyclone created dangerously cold wind chills as far south as central Florida and Brownsville, Texas, the weather service said."This arctic air mass will affect the eastern two-thirds of the country as a sharp cold front moves toward the East Coast," it said. "The cold temperatures will remain in place through midweek before a warming trend begins." Weather Channel meteorologist Kevin Roth said the low temperatures were caused by a "polar vortex" moving down from the arctic. Temperatures may be so cold in some places water mains could burst, weather service meteorologist Bob Oravec told the Wall Street Journal. Cars also may not start due to the arctic freeze."Anyone who has to be outside in this type of weather is going to suffer," he said. Hundreds of school districts, from North Dakota, Minnesota and Wisconsin east to Ohio and as far south as northern Texas, were closed Monday because of the brutal cold. Universities that rarely close for winter weather -- including Northwestern in Chicago and suburban Evanston, Michigan State in East Lansing, Ohio State in Columbus and DePauw in Greencastle, Indiana - were also shut down. "This is shaping up to be one of the worst winter storms in Indiana history," DePauw public safety Director Angie Nally said on the university website. Southern universities, including the University of Texas at Arlington near Dallas, also closed because of the weather, which produced ice overnight.More than 1,900 flights were canceled as of early Monday, including almost half the flights at Chicago's O'Hare International Airport and more than half at Cleveland Hopkins, said flight-tracking website FlightAware.com, which monitors cancellations due to weather, and mechanical and other problems.About 3,700 flights were canceled Sunday, leading to disruptions at scores of airports, including Philadelphia, Indianapolis, St. Louis, Detroit, Newark, New Jersey, and New York's Kennedy International Airport. Amtrak said the severe weather would limit service to and from Chicago, a major hub. Alternate passenger transportation will not be available, the intercity train service said. Snow preceded the arctic air, dropping a foot or more in some areas of Michigan, Illinois and Missouri in the Midwest and several inches in Kentucky and Tennessee in the east south-central region.