Organizers of the longest sled dog race in the eastern United States say they are canceling the event due to a lack of snow on the ground. The Can-Am Crown International Sled Dog Races have taken place in northern Maine for more than three decades, and they include a 250-mile event that is the marquee sled dog race in New England. But this year, snowfall has been well below average in Maine, and organizers said Monday that it’s not safe to run the races. It’s one of many cold weather events that has been jeopardized in recent years by increasingly warm winter temperatures in northern parts of the country.