By David Adams
MIAMI (Reuters) - A tropical storm warning has been issued for a portion of North Carolina and South Carolina, the U.S. National Hurricane Center (NHC) said on Friday.
Subtropical storm Ana formed off the southeast coast of the United States with maximum sustained winds of 45 miles per hour (75 km/h), the Miami-based weather forecaster announced late Thursday.
A warning is now in effect for areas from South Santee River, South Carolina to Surf City, North Carolina, it added.
The first named storm of the Atlantic hurricane season had stalled about 180 miles (285 km) south-southeast of Myrtle Beach, South Carolina, but was expected to strengthen slightly as it approached the coast.
Ana, which began drenching the South Carolina coast with heavy rain on Thursday, could reach winds of 50 miles per hour (81 km/h) on Friday the NHC said. It should remain near or over the warm waters of the Gulf Stream, the NHC said, before weakening as it approached the coast.
Ana's formation is the earliest appearance of a named storm in the Atlantic since a previous incarnation of Subtropical Storm Ana on April 20, 2003, said Jeff Masters, chief meteorologist for Weather Underground, a commercial weather service.
The Atlantic hurricane season runs from June 1 through Nov. 30.
Subtropical and tropical storms do not generate very different wind strengths, but tropical storms cause more rain and have the potential to rapidly intensify into hurricanes, said Masters.
"I expect that the worst Ana will be able to do is intensify to a 65-mph tropical storm that brings 4 - 6 inches of rain to the coasts of South Carolina and North Carolina on Saturday and Sunday," he said.
Forecasters with Colorado State University predicted in April that the Atlantic Ocean will see a "well below average" number of hurricanes this season due to cooler Caribbean waters and the El Niño effect.
(Reporting by David Adams in Miami and Kevin Jose in Bengaluru; editing by Chizu Nomiyama, G Crosse)