(Bloomberg) -- Subtropical Storm Ana has stalled off the U.S. East Coast after forming overnight, arriving more than three weeks before the official start of the Atlantic hurricane season.

The system, about 170 miles (274 kilometers) south- southeast of Myrtle Beach, South Carolina, had sustained winds of 45 miles per hour and is stationary, according to an 8 a.m. advisory from the National Hurricane Center in Miami. A tropical storm watch is in effect from Cape Lookout, North Carolina, to Edisto Beach, South Carolina.

“The main impacts are expected to occur along the immediate coast of Charleston county, where high surf, beach erosion and dangerous rip currents are expected,” according to an earlier advisory. Charleston county runs along the coast of South Carolina.

Coastal areas could receive 2 to 4 inches of rain through the weekend and there may be some flooding along the coast as waters rise from 1 to 2 feet, the hurricane center said.

The atmosphere surrounding Ana isn’t very moist, so the storm won’t produce a lot of rain, said James Franklin, branch chief of the center’s Hurricane Specialist Unit.

Ana is the earliest such a system formed in the Atlantic since April 2003, Franklin said in a conference call with reporters. The official start of the six-month Atlantic season is June 1.

The earliest storm on record fitting the criteria of a named system was on Jan. 3, 1938, Franklin said. The earliest subtropical storm to form in any year was in January 1978.

From 1966 to 2009, the first named storm usually occurred by July 9, according to the center.

--With assistance from Lynn Doan in San Francisco and Naomi Christie in London.

Special Reports /

Subtropical Storm Ana becomes 2015 Atlantic season&rsquo;s first system

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Subtropical Storm Ana becomes 2015 Atlantic season’s first system