Thirty miles - that was the margin of error Tuesday night as forecasters eyed a strengthening Tropical Storm Arthur.

Forecasters said the storm will pass the Lowcountry 120 miles out to sea as a hurricane on Thursday. On Tuesday, its tropical storm winds extended 70 miles out from the eye. The storm's peak winds were more than 50 mph Tuesday, according to the National Hurricane Center.

"The (hurricane hunter) flight crew has been bounced around pretty good by strong thunderstorms in that area and has been forced to climb to a higher altitude in order to avoid significant turbulence," said hurricane specialist Stacy Stewart.

National Weather Service, Charleston, meteorologists cautioned about possible tropical storm conditions, strong and damaging winds starting Wednesday night and into Thursday.

But "it still looks like the main impacts will be marine (ocean)," said meteorologist Wendy Sellers, with the Weather Service. "Given the current forecast track it's definitely more of a concern to our north."

Breezy winds, possible rain bands, high surf and rip currents are more likely for Charleston County, she said, although the northern county might be in for somewhat worse conditions. Beach erosion is a concern, but the storm is expected to pass by during a lower tide cycle. The risk of rain showers and thunderstorms is forecast to increase to about 50 percent by Thursday because of the system.

Mark Malsik, severe weather liaison for the S.C. Climate Office, also said he still expected the storm to stay east of the Charleston coast and is more concerned about the upper coast from Georgetown north.

One way or the other, swells were picking up Tuesday along the beaches. Surfers had already hit the waves at Folly Beach, with the swells rolling waist high. The veterans - more adept at riding higher, more powerful waves and dealing with strong currents - were looking for surf to rise Wednesday and "max out" Thursday.

"Head high surf. Everyone around here is chomping at the bit," said Bates Hagood, of Ocean Surf Shop.

"I'll be out there - early," said surfer Nancy Hussey. "Traffic, parking - the 'coconut grapevine' has been active" and she expects a lot of surfers.

The forecast for the Friday start of the Fourth of July weekend returns the Lowcountry to normal summer conditions with a chance of thunderstorms.

Arthur is expected to turn more northward by Wednesday as it parallels the Florida coast. A tropical storm watch has been issued for the eastern coast of central Florida, which could get 1 to 3 inches and isolated pockets of 5 inches of rain.

Offshore, a tropical storm warning has been issued because of high seas.

Andrew Knapp contributed to this report. Reach Bo Petersen at 937-5744, @bopete on twitter or Bo Petersen Reporting on Facebook.