Flooding had already reached streets and some homes in coastal New Jersey by Friday night, as a powerful nor’easter began battering the East Coast and forecasters warned of more dangerous conditions through the weekend. In Surf City, waves pushed water across streets and into some homes. Roads were also flooding in Manasquan, about 34 miles farther north. Mayor Mike Mangan said he expected water to remain in the streets for an extended period. Officials had issued a voluntary evacuation notice for some low-lying areas there. The National Weather Service warned of dangerous flooding along the Mid-Atlantic coast, particularly near New Jersey and Delaware. Along the New Jersey and Delaware coasts, wind gusts could reach 60 mph, increasing the threat of downed trees and power lines. Thousands of people across the East Coast were without power Friday night. In Sea Bright, Mayor Brian Kelly said the borough was preparing for gusts of up to 50 mph and street flooding that could reach 6 feet. Officials expected flooding to begin Saturday and worsen around midday. Kelly urged residents in low-lying neighborhoods to consider leaving and said the town’s recreation center would open to evacuees. Kelly compared the potential flooding to effects from 2011’s Tropical Storm Irene, which caused major flooding across the Northeast, while emphasizing that he hoped his assessment would prove too severe. The storm also disrupted travel and events. More than 200 flights to and from airports in Boston, Nantucket, Martha’s Vineyard and New York were canceled Friday, with additional cancellations scheduled for Saturday. Ferry service to Nantucket, Martha’s Vineyard and Block Island faced widespread cancellations because of wind and rough water. The nor’easter was expected to bring its strongest winds to parts of coastal New England. A National Weather Service meteorologist in Norton, Massachusetts, said gusts could reach 60 to 65 mph on Cape Cod, Martha’s Vineyard and Nantucket, compared with about 50 to 55 mph around Boston. With leaves still on trees early in the fall, wind resistance could raise the risk of falling branches, power outages and debris-blocked roads.