ISTANBUL Blizzard warnings have been issued across a wide stretch of the US East Coast as a powerful winter storm threatened to unleash heavy snow, strong winds, and coastal flooding from the Mid-Atlantic to New England. The US National Weather Service said early Saturday that nearly 30 million people were under blizzard warnings from Sunday morning through Monday afternoon, including residents of New York City, where forecasters said snowfall could reach up to 18 inches, with as much as 2 feet