Swimmers will face treacherous surf and increased rip current danger through Thursday as Tropical Storm Bertha spins offshore.

Monday night a woman nearly drowned in Atlantic City when she was pulled out to sea by a rip current, a narrow channel of fast-moving water that pull swimmers away from shore.

Officials say the incident underlines the importance of swimming at guarded beaches. Monday's incident happened at about 7 p.m. off South Carolina Avenue. Lifeguards in Atlantic City were on duty until 6 p.m.

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"When it's sunny and the water is warm, it's easy to get lost in the wonderfulness of the ocean," said National Weather Service meteorologist Walter Drag. "You've still got to be cognizant of safety and that's where lifeguards come into play."

Atlantic City Beach Patrol Chief Rod Aluise said his guards were reporting seas of five feet during high tide Tuesday afternoon, as the NWS downgraded Bertha to a tropical storm. The conditions forced the beach patrol to restrict bathing to waste-deep while banning boogie boards, he said.

"The seas were a lot bigger this afternoon with a lot of rip currents," he said, "but we expect it to diminish as the storm moves away from us."

Mike Gorse, another NWS meteorologist, said waves are expected to build several feet Tuesday night, with the rip current danger gradually improving Wednesday and particularly Thursday, as Bertha moves to the northeast. 

"The wave action itself won't be terribly high as the system is pretty small and weak," he said. "It's also tracking several hundred miles east of our coast."

But the relatively calm waves can be deceptive.

Jon Miller, a Stevens Institute of Technology professor who tracks rip currents, said storms like Bertha can be more dangerous because they lead to complacency among swimmers.

"With a storm like Sandy, the waves are so big you don't see the public out in the water because it looks too dangerous for them," he said. "Bertha looks like the waves are small enough that people feel comfortable heading out."

Rip currents can occur at any time offshore, but they are exacerbated by the wave and wind action of offshore storms.

Miller said such storms create a "pulsing push of water" that enters the surf zone along the beach.

"The water then has to find its way out," he said. "Rip channels are usually the way the water makes its way out."

Meanwhile, Miller said, the waves tend to arrive perpendicular to the shoreline, increasing the intensity of rip currents. Such currents are also affected by the geography of the shoreline as they tend to occur more often around jetties and piers and can even be affected by beach nourishment projects.

Drag said rip currents can also form when water is piled up against sand bars. The best advice, he said, is to always swim at guarded beaches.

"The lifeguards know about rip currents and they'll keep an eye on you," he said. "If it's a nice night and you swim at an unguarded beach, the people there probably aren't trained to rescue you."

As of Tuesday afternoon, the NWS upgraded the rip current risk along much of the Jersey Shore to high. A small craft advisory had also been issued for boaters due to seas between 3-5 feet.

If caught in a rip current, call for help and do not struggle against the current. As soon as possible, swim parallel to the shoreline until it's possible to swim back to the beach.

Miller will be out surveying Bertha's impacts off Long Branch on Wednesday and Thursday. He's also working on a smart phone app that could be used to track rip currents, but it has been difficult recruiting beach patrols to submit the necessary data.

"Most of the reports we get are when there's a rescue," he said. "The idea of the app is to have a system where we'll know more precisely when rip currents are occurring."

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