(Updates with new location and increased wind strength)
By Harriet McLeod
CHARLESTON, S.C., July 1 (Reuters) - The first tropical storm of the Atlantic hurricane season formed on Tuesday and was poised to strengthen into a hurricane, forecasters said, threatening to dampen July 4 holiday celebrations along the U.S. East Coast.
Tropical Storm Arthur formed off the central Florida coast and was expected to bring heavy rain, gusty thunderstorms, coastal flooding, rough seas and dangerous rip currents to beaches from Florida to southern New England this week, the Miami-based National Hurricane Center said.
Computer forecast models showed Arthur was not a threat to key oil and gas producing areas in the Gulf of Mexico.
Forecasters and emergency management officials were urging beachgoers to use caution when going into the surf.
"People look at the skinny black (forecast) line and say 'That's 100 miles east, I'm not going to get that storm,'" hurricane center spokesman Dennis Feltgen said.
But he warned, "The impacts of the storm could be over a much wider area."
Low wind shear conditions and warm sea-surface temperatures should allow for steady strengthening, the NHC forecast said. Arthur is expected to become a Category One hurricane by Friday morning, reaching a maximum of 90 mph (145 kph) winds, before losing strength.
On Tuesday evening, the storm was located about 90 miles (145 km) east of Cape Canaveral, Florida, with maximum sustained winds near 50 miles per hour (85 kph).
It was expected to remain offshore and move east of the north Florida coast during the next day or so, and tropical storm and hurricane watches will likely be posted from the Carolinas to Virginia later on Tuesday, the NHC said.
The storm will have the heaviest impact on coastal areas from North Carolina to Cape Cod, Massachusetts, and Long Island, New York, according to meteorologists at AccuWeather.com.
It is too early to tell how close to the coast of North Carolina the storm will get, Feltgen said. Emergency management officials in the state said they were making preparations. (Additional reporting by Koustav Samanta in Bangalore; Editing by David Adams, Jim Loney and Sandra Maler)