Tropical Storm Ophelia has gained strength as it churned toward the North Carolina coast, promising a weekend of heavy rain and windy conditions throughout the mid-Atlantic.

Forecasters issued a hurricane watch for parts of eastern North Carolina on Friday. The storm was expected to make landfall in North Carolina on Saturday morning and dump as much as 7 inches (17.7 centimetres) of rain across portions of the state and into southeast Virginia.

The intensifying weather system spun into a tropical storm in the afternoon and by nighttime was producing maximum sustained winds of 70 mph (113 kph), with higher gusts, according to the US National Hurricane Center.

The storm was not expected to gain further strength before landfall and was forecast to weaken afterwards, according to the hurricane centre.

Just before 11 pm, forecasters said Ophelia was about 70 miles (115 kilometres) south of Cape Lookout, North Carolina, and heading north-northwest at 12 mph (19 kph).

Water levels were rising along portions of the North Carolina coast, and a storm surge warning was in effect for some areas. Surges between 3 and 5 feet (0.9 and 1.5 meters) were forecast for parts of the state, the hurricane centre said.

Expecting stronger winds, rainfall

The governors of North Carolina, Virginia and Maryland declared a state of emergency. Some schools closed early as communities prepared for the storm’s arrival, and several weekend events were cancelled.

“We are expecting an extended period of strong winds, heavy rainfall, and elevated tides," Maryland Gov. Wes Moore said in an evening statement.

Nancy Shoemaker and her husband Bob stopped by a waterside park in downtown Annapolis, Maryland's capital, to pick up sandbags to help protect their waterfront home.

Last October, they experienced a big surge of water that came into their yard and even washed some sandbags away.

“We’re hoping it won’t be that way this time,” Nancy Shoemaker said.

“If we have a lot of wind and a lot of surge, it can look like the ocean out there, so that’s a problem.”