INDIANAPOLIS (AP) — Bitterly cold temperatures gripped much of the nation on Tuesday, testing the mettle of even winter-wise northerners and delivering a shock to those accustomed to far milder weather in the South.The cold has been blamed for at least a dozen deaths, prompted officials to open warming centers in the Deep South and triggering pleas from government officials to check on neighbors, especially those who are elderly, sick or who live alone.In St. Louis, where temperature dipped 30 degrees below normal, Mayor Lyda Krewson warned it was "dangerously cold.""It's important that people look out for anyone in need of shelter," she said.The National Weather Service issued wind chill advisories and freeze warnings covering a vast area, from South Texas to Canada and from Montana through Maine. The arctic blast was blamed for freezing a water tower in Iowa, halting a ferry service in New York and even trapping a swan in a Virginia pond.At the same time, a heatwave swept into the country's northernmost state: Anchorage, Alaska, hit a record high on Tuesday of 45 degrees — at the same time Jacksonville, Florida, was a mere 38 degrees.Indianapolis Public Schools canceled classes after the city tied a record low for the day — set in 1887 — of minus 12 degrees. The northwest Indiana city of Lafayette got down to minus 19, shattering the previous record set in 1979.