A powerful surge of arctic air is sweeping across the United States as meteorological winter begins, sending temperatures plunging far below seasonal norms and placing much of the country under a deep freeze. Weather officials report that more than 235 million people will experience below-freezing conditions through the end of the week as the cold mass pushes steadily south and east. The early-season outbreak marks one of the most widespread cold events so far this year, fueled by a dip in the j