NEW YORK (REUTERS) - A tropical storm that formed off the North Carolina coast early on Sunday (July 8) was forecast to become a hurricane on Monday, forecasters said, while the remnants of Tropical Storm Beryl looked set to bring heavy rain to Puerto Rico.

Tropical Storm Chris, the third named storm of the Atlantic hurricane season, was about 290km south-southeast of Cape Hatteras in North Carolina with top sustained winds of 85kph, the US National Hurricane Center (NHC) said in an advisory. It was seen staying well off the US coast.

"Swells generated by Chris are expected to increase and affect portions of the coasts of North Carolina and the mid-Atlantic states into early next week," the NHC said, adding that some of the resulting conditions could be life-threatening.

Beryl, which had weakened to a tropical storm from a hurricane on Saturday, degenerated on Sunday into an open wave, according to the NHC.

It said Beryl could reform into a tropical cyclone in a few days when its remnants are forecast to move through the Bahamas.

The National Weather Service (NWS) maintained a hazardous weather outlook for Puerto Rico, the US territory still recovering from the devastation wrought last year by Hurricane Maria, warning of thunderstorms and wind gusts of up to 80kph.

A flash-flood watch remained in effect for Puerto Rico and the US Virgin Islands from late Sunday through Monday evening, NWS San Juan said on Twitter. Tropical storm watches were in effect for Dominica and Guadeloupe.

Puerto Rico Governor Ricardo Rossello met with his Cabinet and weather experts on Sunday to prepare for Beryl's arrival, Rossello's office said in a statement.

A state of emergency was in effect, the statement said.

About 7,000 houses and businesses in Puerto Rico still lack power after Hurricane Maria leveled an electricity grid that was ill-maintained before the storm.

According to a survey by a research team led by Harvard University that was published in May, Maria took the lives of more than 4,600 people on the island.