"There is a danger of life-threatening storm surge inundation along the coast of southern and southeastern Louisiana," the National Weather Service said.
Barry is expected to make landfall Saturday as a major storm. As in past hurricanes, the greatest danger may not be the high-speed winds, but the water rushing in from the ocean.
What exactly is storm surge?
The rise in water levels, known as storm surge, happens when winds from a fierce storm push water up and onto shore.
Storm surge can become several feet high, and forecasters have said Barry could bring up to 4-6 feet of storm surge to parts of Louisiana and through the Mississippi-Alabama border, the weather service said.
But that could change if the storm intensifies. Forecasters said the stronger the storm, the stronger the winds and the higher the storm surge will be.
After the ocean water crashes onto land, it can also exacerbate flooding. Rivers and streams that usually drain into the ocean can get clogged farther upstream, forcing water levels to rise. It typically leads to more flooding in rain-soaked areas.
Rising water is the deadliest part of a storm
When a big storm approaches, many people worry about wind and debris. But the water a storm brings kills the most people.
Water accounted for more than 75% of all hurricane-related fatalities in the US from 1963 to 2012, according to a report by the National Hurricane Center. About half of all deaths in hurricanes come from storm surge.
Wind was only responsible for 8% of all deaths.
More than 80% of storm-related fatalities in the past three years have been linked to inland flooding, according to Ken Graham, director of the National Hurricane Center.
"The data is very clear, it's water that's killing people," Graham told CNN affiliate KPLC. "Just because you're not on the coast doesn't mean you're immune to the dangers."
CNN's Dakin Andone, Steve Almasy and Holly Yan contributed to this report.
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