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What are 'storm surges' and 'rip currents,' anyway?

If Hurricane Earl stays on its projected track, the biggest problems will be from winds and from what meteorologists warn are rad-sounding but seriously deadly threats: rip currents and storm surges.

Along the East Coast, meteorologists add lifeguards are urging beachgoers to stay out of the water. If you choose to ignore them, they say, you're on your own if you go in above your knees.

In a hurricane advisory, the National Weather Service warns that "rip currents can become life threatening to anyone who enters the surf," adding that it's "highly recommended that if traveling to the beach, you stay out of the water entirely."

So what exactly are storm surges and rip currents?

When a hurricane skitters over the ocean, its winds pile water up higher than sea level. Eventually, this pile of water tumbles ashore a big, monstrous rush. That's a storm surge, and it can cause serious flooding and beach erosion.

The National Weather Service says a rip current, by contrast, defined by speed, not size.

They're caused when waves traveling from deep to shallow water break near the shoreline. That causes the water to swirl in a narrow, fast-moving "circulation cell."

Rip currents typically move at 1 to 2 feet per second, but speeds as high as 8 feet per second have been measured. "This is faster than an Olympic swimmer can sprint," the weather service says.

But both rip currents and storm surges are major hazards.

"A lot of people have no prior knowledge about how to survive in the ocean, so when you throw in a storm like this, it just becomes harder," Noah Rosenthal, a lifeguard supervisor for Lack's Beach Service, tells NBC station WMBF of Myrtle Beach, S.C., where waves as high as 9 feet are expected.