U.S. natural gas futures surged the most since late October as one energy trader warned that "winter isn't over," with another wave of cold air set to spill into the eastern half of the Lower 48 early next week. The upcoming cold blast arrives even as much of the U.S. enjoys a brief warm-up following an early-month polar vortex that plunged large parts of the U.S. East into Arctic-like conditions. "Winter isn't over yet. Here are forecast anomalies for this time next week showing a large area of unseasonably chilly temperatures across the major…