A powerful storm that is expected to move up the East Coast this weekend was still organizing itself off the Carolinas on Wednesday afternoon. While the storm is expected to intensify between Thursday and Friday, forecasters warned that there was still significant uncertainty about its path and potential impacts.

"The knowns are that there's going to be a strong storm off the New England northeast coast," said Bob Oravec, a senior meteorologist with the National Weather Service. "The only unknowns are the exact details of the track." If the storm tracks to the west, more rain will fall on towns between North Carolina and New England. If it tracks east, many cities will remain dry.

Coastal flooding and erosion are the most certain threats, as the storm is developing during some of the highest tides of the month. The heaviest rains are expected across eastern New England, particularly eastern Massachusetts, including Cape Cod, Rhode Island, and coastal Maine. Cape Cod and the islands offshore could receive five to seven inches of rain. New York City was likely to receive very little rainfall — around an inch over a seven-day period.

Regardless of its path, the storm is expected to weaken gradually on Sunday and Monday. Because it is moving so slowly, however, weather in eastern New England could remain unsettled well into next week. "Even though the storm weakens Sunday and Monday, it's slow to move out," Mr. Oravec said.