New storm surge map predicts worst-case scenarios for south La. _lowres


A new storm surge prediction map estimates that a large, very slow moving hurricane could push more than 9 feet of water onto LSU’s campus.

In fact, all of southern Louisiana as far north as Pointe Coupee Parish could face such a storm surge if the conditions are just right, according to a new worst case scenario map released Thursday by the National Hurricane Center Office for Coastal Management.

But before people in Baton Rouge start hiring contractors to elevate their homes, it’s important to know that the chance of that level of flooding happening in Baton Rouge is low. For areas south of interstates 10 and 12, the chance of significant flooding increases, but that depends on the proximity to the Gulf of Mexico and the elevation of the land.

Although alarming at first glance, the map doesn’t reflect the effects of a single storm across the coast of Louisiana. Instead, it shows the worst-case scenario for any particular area along the coast, said Barry Keim, state climatologist.

“That map can never happen (in total),” agreed Ken Graham, meteorologist in charge with the National Weather Service in Slidell.

The scenario in which storm surge could make its way into Baton Rouge would involve storm surge running through Lake Pontchartrain, into Lake Maurepas, up Bayou Manchac and then into low-lying areas of the parish.

Although Lake Pontchartrain is miles from Baton Rouge, it’s not that far away in terms of elevation.

“All of that area is pretty low-lying,” Keim said.

It may seem far-fetched that a Category 3 storm could push that much water that far inland, but just because it hasn’t happened doesn’t mean it couldn’t, Keim said.

“It is conceivable that something like that could happen,” he said.

Graham added that, out of thousands of scenarios run by the map’s creators, there may have been only a handful that showed storm surge getting as far inland as LSU.

“The risk isn’t zero. There is some risk,” Graham said.

The newly released map also shows worst case storm surge scenarios for different categories of storms. The calculations within the map also goes beyond just wind speed.

The map based its finding on thousands of hypothetical storms. In some parts of the country, as many as 60,000 varying storm conditions were used to come up with a map. The variations took into consideration forward speed, direction, size of storm and wind speeds.

“This is the worst case scenario of a number of different storm events,” Keim said.

For example, under a Category 3, there is extensive flooding up through the Atchafalaya River basin that could bring storm surge into parts of Iberville, West Baton Rouge and even Pointe Coupee parishes. That type of storm would be a very large, very slow moving hurricane, Graham said.

“It’s your Isaac on steroids,” Graham said. “Most hurricanes aren’t going to do that.”

The new storm surge map is intended to provide a much clearer picture for coastal residents in Louisiana and around the country to understand the potential flooding risk.

While having the new maps to show potential risk is exciting, it’s what will be done during a storm that could really save lives, Graham said.

When a storm watch is issued — 48 hours before landfall — meteorologists will run the computer model with numerous variations of the storm’s path, size, direction, wide speed and other characteristics.

About an hour after the storm watch is issued, the National Weather Service will be able to put out a worst case scenario map for the particular storm’s surge. So many variables will be put into that computer map that there will be only a 10 percent chance that the water levels portrayed will be exceeded, Graham said.

While it will look similar to the other worst case scenario maps, this one will be specific to the current storm, he said.

In addition, water levels will be portrayed as actual water above ground, unlike previous storm surge maps that used sea level elevations, which at times were hard to translate into actual flooding.

“It’s a huge step forward,” Graham said.

This storm-based mapping was available to forecasters this year, but it hasn’t been needed because Louisiana hasn’t been threatened by a storm.

One drawback to the maps is that it’s difficult to predict what will happen in areas surrounded by levees. Currently, there is no reliable way to estimate flooding potential within the levee systems like those that surround New Orleans or parts of Lafourche Parish, if those levees are overtopped, Graham said. NOAA is currently working on how to color code those areas because there is still flooding risk within those levees.

“We don’t want to cause a false sense of security,” Graham said.

Follow Amy Wold on Twitter, @awold10.