A white Christmas came to parts of the United States. But in northwest Pennsylvania, the skies really packed a wallop.NPR education reporter Elissa Nadworny went home to Erie for the weekend and found herself in the middle of an epic snowstorm when the lake effect parked a band of snow over the city of 100,000.The storm dumped more than 4 feet of snow in 30 hours – and it's still going."I've shoveled about three times now trying to stay up on it," Tom Fitzpatrick, a 28-year-old back home from Philadelphia, told Elissa as he worked to clear his mother's driveway. "I mean, it's scenic. But from a shoveling standpoint, it's a back-breaker.""It's one for the ages," he said. "It might be the last time I wish for a white Christmas."The storm started Christmas Eve and has since shattered state and local records. The National Weather Service has a lake effect snow warning for the area until Wednesday afternoon. The forecast says there could be an additional 1 or 2 feet of snow, and travel will be difficult or impossible."I've never seen anything like this in my 40 years in Erie, Pa.," said 69-year-old Leonard Grieco, who had some advice. "You can never buy too big a snowblower. Don't go small."Atmospheric scientist Angela Fritz explains in The Washington Post that very specific conditions are required to generate so much snow: