Storm surge graphic

The largest storm surge ever recorded in America happened during 2005’s Hurricane Katrina in Pass Christian, Miss.

The Gulf of Mexico inundated the city of about 4,500 people with nearly 28 feet of water above average sea level. Not far away in Biloxi, Miss., a combination of a one-foot tide and 11-foot waves brought a high-water mark of more than 34 feet.

Storm surge was the primary cause of death during Katrina. In fact, between 1963 and 2012, it caused 49 percent of hurricane-related fatalities, according to The Weather Channel.

As Hurricane Irma arrived in the Golden Isles, National Weather Service forecasters in Jacksonville said Sunday the local storm surge could be between 3 and 6 feet.

Measuring storm surge can be tricky. Experts interviewed by The News on Sunday gave slightly differing opinions of where the baseline starts.

Essentially, storm surge is the amount of extra water pushed inland by a hurricane’s winds. It is added to the additional tide that may already be occurring. The baseline starts with mean sea level, which is an average of high and low tides. If high tide is 4 feet above mean sea level, and a storm surge of 3 feet occurs, then the wave height would be 7 feet above mean sea level.

Storm surge is different from storm tide. Storm tide is the combination of storm surge and the normal astronomical tide. That’s the 7 feet, as used as an example above.

“It’s tricky and confusing,” said Spud Woodward, the Brunswick-based director of the Georgia Department of Natural Resources Coastal Resources Division.

“Storm surge is basically the amount of water that would be on top of the normal water level at that time,” he said Sunday. “If it was high tide — let’s say it’s an 8 foot tide — then the storm surge would be 8 feet on top of that.”

No matter how you measure it, storm surges can be the most frightening part of a hurricane, said Christopher Smith, a hydrologist with the U.S. Geological Survey.

“People need to understand that storm surge can be very dangerous,” he said Sunday. “It comes in quickly, with lots of velocity.”

Just how much storm surge happens is determined by the size of the storm, its direction of approach and the wind speed, Smith added.

Nate McGinnis, a meteorologist with the National Weather Service in Jacksonville, echoed Smith.

“People should know the amount of storm surge it would have to be to impact them,” he said. “If you think you might be in danger, check the maps and make sure you’re educated about what it could do in a worst-case scenario.”

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