PORTLAND, Maine — Plunging temperatures across half the country on Thursday underscored a stark reality for low-income Americans who rely on heating aid: Their dollars aren't going to go as far this winter because of rising energy costs.Forecasters warned people to be wary of hypothermia and frostbite from an arctic blast that's gripping a large swath from the Midwest to the Northeast, where the temperature, without the wind chill factored in, dipped to minus 32 (minus 35 Celsius) on Thursday morning in Watertown, New York.Even before the cold snap, the Department of Energy projected that heating costs were going to track upward this winter, and many people are keeping a wary eye on their fuel tanks to ensure they don't run out.The burden caused by higher prices and higher energy usage is felt by all Americans, especially those who struggle to stay warm.Elizabeth Parker, 88, of Sanford, Maine, said she lives in fear of running out of heating fuel and remains vigilant in monitoring the gauge outside her trailer. She said she is allowed to request a fuel delivery thanks to federal aid, but only when her gauge dips to one-eighth of a tank."I couldn't get along without it," said Parker, who lives with her 93-year-old husband, Robert Parker, along with a cat, a dog and four birds.Prolonged, dangerous cold weather this week has sent advocates for the homeless scrambling to get people off the streets and to bring in extra beds for them. Warming centers also were set up in some locations.