Americans could be spending Christmas in T-shirts with a freakishly warm winter on the way. A hangover from the summer heatwave will push temperatures above average across much of the country. High temperatures will be driven by warm ocean waters, partly responsible for recent hurricanes, and a La Nina cooling of the Pacific. La Nina, the counterpart of a strong El Nino warming over the past year, can jolt the jet stream and change the Pacific storm season. AccuWeather senior meteorologist Paul Pastelok said: “On the heels of one of the warmest summers on record in the United States, our long-…