Tropical Storm Isaac let Tampa and the GOP convention mostly off the hook, but New Orleans and the Gulf Coast may not be so lucky, forecasters say. The sprawling system, now slowly strengthening in the Gulf of Mexico, is expected to become a Category 1 or Category 2 hurricane by the time it makes landfall Tuesday night.
In an update on Isaac at 5 p.m.Monday, the National Hurricane Center pinpointed the storm as south of Alabama and predicted the coasts of Mississippi and Louisiana would receive the brunt of the impact early Tuesday evening. The storm's current maximum sustained winds have increased to around 70 miles per hour as it continues its slow northwesterly crawl.
Despite being a lesser-magnitude storm than Hurricane Katrina, which was a Category 3 storm at landfall, Isaac is already nearly as large. Tropical storm-force winds radiate 205 miles out from Isaac's center, while Katrina's extended 230 miles out, according to Jeff Masters, co-founder and director of meteorolgy at Weatherunderground.
The storm's slow speed and huge width will likely mean several days of rain on the Gulf Coast and a strong storm surge despite relatively weak wind speeds, Masters says.
"We're used to seeing hurricanes come ashore and be gone in a few hours. This storm will be pounding the coast for two days," Masters says. "You're going to have very high amounts of rain, in some places 20 inches of it. I don't care how good your drain is that's going to cause problems."
The sheer volume of rain could cause freshwater flooding, especially because the ground is already saturated from ample rain in recent weeks, says John Schroeder, a professor of atmospheric science at Texas Tech University who has begun taking measurements in Lousiana.
Seperately the storm surge from the Gulf, predicted to be 6 feet to 12 feet high along the coast, will provide the first big test of New Orleans' new levee system, which was upgraded following the massive flooding that wrecked the city during Katrina in 2005.
Damage caused from water (as opposed to wind) could mean extensive, and costly cleanup efforts for the city despite a relatively weak storm, says Alan Rubin, an attorney with Cozen O'Connor and national expert on hurricane relief efforts.
"The wind ones you can consider them like mini-tornadoes. Theyre horrible in terms of what they hit directly but then it's done," Rubin says. "The wet ones are much worse because you can't stop water, once it gets into your home it's destroyed."
Often insurance companies will declare a home completely totaled based on the water line within it, which adds the costs of inspection and tearing down homes to the storm's damage bill. The storm will cause more than $1 billion in damage, Masters predicted.
Isaac is also following a very similar path to Katrina on the latter's seventh anniversary, and has prompted mandatory evacuations in many of the areas struck hardest by it. Louisiana Gov. Bobby Jindal announced Monday he would forego his primetime convention speaking slot Wednesday to preside over storm preparations and recovery.
Seth Cline is a reporter with U.S. News and World Report. Contact him at email@example.com or follow him on Twitter.