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Hurricane Arthur Whips Up Worries About Rip Currents

Hurricane Arthur's approach means beachgoers will have to watch out for rip currents, which can hit even when there's no storm in sight.

Hurricane Arthur's approach means Fourth of July beachgoers will have to watch out for a potential threat that can loom even if there's nary a storm cloud in sight: life-threatening rip currents.

The National Hurricane Center is forecasting a heightened risk from high waves as well as rip currents, sometimes erroneously referred to as "riptide" or "undertow." A rip current is a phenomenon that involves a narrow stream of water surging out from shore, at speeds that can outdo an Olympic swimmer.

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More than 100 Americans are killed annually by rip currents, according to the United States Lifesaving Association. Florida is the No. 1 state for rip fatalities by a wide margin. The hurricane center says coastal areas of east Florida, Georgia and the Carolinas should be on alert while Hurricane Arthur is on the move.

Amy Godsey, state meteorologist for the Florida Division of Emergency Management, is used to fielding questions about the science of rip currents — and how to stay safe in the surf:

What causes rip currents?

The key factor is an extended period of onshore wind flow, Godsey told NBC News. "A storm really doesn't have to make landfall, and it doesn't have to be a hurricane," she said. "When the wind swirls around a storm, that pushes a lot of water, and that water has to go somewhere."

One result is higher waves. That alone poses enough of a challenge to swimmers. But when the water is pushed to shore from different directions, it can flow back out to sea at speeds of up to 6 mph (9 kilometers an hour), in a channel that can measure less than 30 feet wide, or spread to a width of more than 300 feet (10 to 100 meters). Rip currents don't pull swimmers underwater — they pull them away from shore.

What's the threat?

It can be difficult to recognize a rip current, from the shore or in the surf. If you get caught up in the outward-flowing stream, the most natural response is to try to swim back to shore — but the strength of the current is such that you'd never make it to safety that way.

How can I survive?

To get out of the stream, swim or tread water in a direction parallel to the shore and perpendicular to the water's flow. DON'T PANIC. If the water is shallow enough, get your feet down on the bottom to churn your way out. Call for help if you can't break free. Once you're well away from the rip current, head back to the beach.

If you see someone in trouble, get help from a lifeguard. If there's no lifeguard available, call 911. You can throw the victim something that floats, or yell instructions from shore, but be careful about trying to swim out to the spot. "Many people drown while trying to save someone else from a rip current," the lifesaving association says.

How do I find out about the risk?

The National Weather Service provides regular advisories about rip current risks along coastal areas via the clickable forecast map on its homepage. For example, take a look at the forecast for Jacksonville, Florida. When you're at the beach, watch out for warning flags: A yellow flag means there's a moderate surf risk, and a red flag means there's high risk. The ultimate authorities are the lifeguards on duty. If you're at an unguarded beach, be extra-cautious. "If in doubt, don't go out," the National Weather Service says.

For everything you ever wanted to know about rip currents, check out this guide from the weather service.

Is it safe once the storm has passed?

Not necessarily. "The waves and the rip current threats are usually the first to arrive and the last to leave," Godsey said. "You can have a storm 600 miles away and still get the wave impact from that storm. ... The weather can be beautiful, the skies can be clear and the sun can be out, but there's still that danger in the water. Awareness is the key to beach safety."

I've been through a rip current — now what?

The National Weather Service is collecting cautionary tales from survivors in the United States and the rest of the world. "I am 36 years old and a pretty good swimmer, but today was the scariest day of my life and reminded me to respect the ocean," a New Jersey resident wrote. By sharing your story, you can help others avoid the risks of the rip.