US weather forecasters warned Thursday of a subtropical storm in the Atlantic that is expected to bring unseasonably heavy rains to the southeastern United States.

Subtropical storm Ana, which unusually comes ahead of the June start of the annual hurricane season, was located 170 miles (280 kilometers) southeast of South Carolina and had sustained winds of about 45 mph (75 kph), the National Hurricane Center said.

Ana was expected to produce up to four inches (10 centimeters) of rain over eastern portions of North Carolina and South Carolina through the weekend, the center said.

This year's hurricane season, which runs from June 1 to November 30, is shaping up to be one of the least active since the mid-20th century due to the El Nino phenomenon, which generates conditions adverse to the formation of hurricanes, according to experts at the University of Colorado.

The 2014 season was relatively calm, with only six of eight tropical storms reaching hurricane strength.