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Surfers look at the Atlantic Ocean as Tropical Storm Bonnie made landfall in Miami Beach, Florida, July 23, 2010. REUTERS/Joe Skipper

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By Frank McGurty

(Reuters) - Tropical Storm Bonnie, the first of the year to threaten the United States, stalled in the Atlantic before strengthening slightly late on Saturday, its centre expected to cross the South Carolina coast on Sunday night and Monday, the National Hurricane Center said.

Bonnie, coming four days before the official start of the Atlantic hurricane season, triggered a tropical storm warning from South Carolina's Savannah River, near Hilton Head Island, and the Little River Inlet.

It was packing maximum sustained winds nearing 45 miles per hour (72 kph), with higher gusts, the centre said at 2 a.m. EDT Sunday (0600 GMT) on its website. The centre upgraded the system from a tropical depression on Saturday.

Tropical-storm conditions, featuring winds extending out 70 miles (110 km) from Bonnie's centre mainly to the northwest, were expected within the next 12 hours, with some weakening expected to begin late Sunday.

Bonnie's centre is forecast to cross the South Carolina coast Sunday night and Monday as the storm moves slowly northward on Sunday at about 6 mph (9 kph), the centre added.

The system will bring up to six inches of rainfall from central and eastern South Carolina to the Georgia border and life-threatening surf and rip currents to the Atlantic beaches during the long Memorial Day weekend, the unofficial start of the summer vacation season.

Tropical storms are cyclonic weather systems packing winds with sustained surface speeds ranging from 39 to 73 miles per hour (63 to 117 kph).

The centre of the system was about 100 miles (165 km) south-southeast of Charleston, South Carolina, the federal agency said.

Little change in intensity is expected before the storm's centre moves near or onto the South Carolina coast later on Sunday before a forecast weakening to a tropical depression late Monday morning.

The formation of Bonnie marks the second such weather system of 2016, following one that grew into Hurricane Alex in the far eastern Atlantic in January.

Heavy rains could fall from the South Carolina-Georgia border to southeastern North Carolina.

Tidal storm surge flooding of one to two feet (30 to 60 cm) above ground level is possible in the area, along with isolated tornadoes.

U.S. meteorologists have predicted an increase in the number of named storms this hurricane season after the number was below average during the past three years.

(Additional reporting by Alex Dobuzinskis in Los Angeles, Ian Simpson in Washington and Chris Michaud in New York; Editing by Steve Orlofsky, W Simon and David Goodman)

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