BOSTON (AP) — Rip currents are one of the coast’s greatest dangers and account for the most beach rescues every year. Hurricane Erin is bringing the potentially deadly currents to the East Coast this week.

About 100 people drown from rip currents along U.S. beaches each year, according to the United States Lifesaving Association. And more than 80 percent of beach rescues annually involve rip currents.

Already this year, there have been at least 27 people killed from rip currents in U.S. waters, according to the National Weather Service.

Hurricane Erin brought evacuations to North Carolina’s Outer Banks on Tuesday, a day after dozens of swimmers needed to be rescued from rip currents near Wilmington, North Carolina. Authorities warned the storm could bring rip currents from Florida to New England.

Here are some things to know about rip currents:

What is a rip current?

Rip currents are narrow columns of water flowing rapidly away from the beach, like a swift stream within the ocean. They don’t pull swimmers under water, but can carry them out a fair distance from shore.

Low spots along the beach, or areas near jetties or piers, are often where rip currents form. They can be connected to stormy weather but also sometimes occur during sunny days. They can be hard to detect because the surface water often appears calm.

The current can flow as swiftly as eight feet per second (3.2 meters per second), faster than even a strong swimmer can overcome, according to the National Oceanic and Atmospheric Administration.

“If you’re caught in one and you try to swim straight in, you’re not going to be able to,” said Daniel Barnickel of Palm Beach County Ocean Rescue.

New England could be rip currents’ next target

Norton, Mass.-based meteorologist Bryce Williams said those on the New England coast can expect to see elevated wave heights, rough surf and increased rip currents as Erin makes its way north.

New England will start seeing increasing wave activity over the next day. Rising waves will become more noticeable on Wednesday, but Thursday is expected to see the bulk of the impact.

Nantucket is closest to Erin’s path and is likely to see the strongest winds, gusting about 25 to 35 miles per hour at peak with waves potentially reaching a height of 10-13 feet. On a calm day, waves generally rise to about 1-3 feet.

In waters farther out into the ocean, wave heights could rise as high as 13-16 feet and winds may gust up to 45 miles per hour.

“We’re just encouraging folks, especially along the southern-facing beaches, to just be cognizant if the water is too rough and dangerous, that they would stay out,” Williams said. “With those large waves and the rough surf, that does increase the currents of these rip currents, which could pull people out before they even realize it.”

Tips on how to escape rip currents

The most frequent advice from beach rescue teams and weather forecasters is to not panic and to look for a chance to swim parallel to the shore until the swimmer is out of the rip current’s grip. It will eventually dissipate but might leave the swimmer out in deeper water.

It’s nearly impossible to fight the current directly. Many swimmers who get in trouble tire themselves out trying to get back to the beach, lifeguards say. If possible, it’s best to swim near a lifeguard station.

Warning systems exist for rip currents and other hazards

Flags with different colors are used to warn beachgoers of various hazards.

Three flags warn of surf and rip current conditions. Red means a high hazard, yellow means a moderate threat and green means low danger. There’s also purple for dangerous sea life, such as jellyfish, and double red when a beach is closed for any reason.

The National Weather Service posts rip current risks on its websites around the coasts and has developed a computer model that can predict when conditions exist that may lead to their formation up to six days in advance for the U.S. East and Gulf Coasts, Puerto Rico, Hawaii and Guam.

Should someone attempt a rip current rescue?

It can be dangerous to try to rescue someone caught in a rip current, officials say. Often the people trying to perform the rescue can get into trouble themselves.

It’s best to find a lifeguard, if there is one, or call 911 if a struggling swimmer is spotted. People on shore can also try to tell the person to swim parallel to shore.

“Never swim alone. And always make sure that there’s an adult. And make sure that you don’t overestimate your abilities. Know your limits,” Barnickel said.