Tropical Storm Bonnie, the first of the year to threaten the United States, is expected to reach the South Carolina coast on Saturday evening or early Sunday, the National Hurricane Center said after upgrading the system from a tropical depression.

Bonnie, coming four days before the official start of the Atlantic hurricane season, was on course to make landfall in a warning area in South Carolina between the Savannah River and the Little River Inlet.

It was packing sustained winds of up to near 65 kilometres per hour, the centre said on its website.

Tropical storms are defined as a cyclonic weather systems packing winds with sustained surface speeds ranging from 63 to 119 km/h.

The current threat marks the second such weather system of 2016, following one that grew into Hurricane Alex in the far eastern Atlantic in January, according to the Hurricane Center. But Alex, a rare wintertime storm that threatened the Azores island group far off the coast of Portugal, never came near the United States.

Earlier this week, U.S. meteorologists predicted an increase in the number of named storms this hurricane season — compared with below-average numbers during the past three years.

The National Oceanic and Atmospheric Administration forecast a 70 per cent likelihood of 10 to 16 named storms in the upcoming hurricane season. By comparison, 2015 saw 11 named storms, including four hurricanes, of which two were major, according to federal data.