From: SCOTT NEUMAN, NPR
Published August 30, 2012 06:21 AM

Isaac packs a punch to the Gulf Coast

Isaac might not be in the same league as Hurricane Katrina seven years ago, but the latest storm to batter Louisiana's Gulf Coast is punching above its weight class in more ways than one, scientists say.

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The 2005 Hurricane Katrina, which devastated Louisiana and parts of Mississippi and Alabama, was a Category 3 storm (sustained winds of 125 mph) moving at about 15 mph when it made landfall on the Gulf Coast. By comparison, Isaac was a weak Category 1 storm as measured on the Saffir-Simpson scale, with sustained winds of 74-95 mph. By Wednesday afternoon, Isaac had been downgraded to a tropical storm, although it still was close to the Gulf Coast and continued to dump torrential rain.

While Isaac is considerably less intense than Katrina, it is large and slow — a dangerous combination — and it's moving west of the Mississippi River, a track that intensifies storm surge, says Timothy Shott, a meteorologist at the National Weather Service's Tropical Cyclone Program.

"This storm has sustained tropical storm force winds currently extending out to about 175 miles from the center and the hurricane force about 45 miles from the center," Shott told NPR at about noon ET Wednesday.

Measuring Isaac on three criteria — storm surge, rainfall and wind — Shott would rate the storm "high impact" on all of them.

Even though the winds are Category 1, the slow movement of the storm increases their effect, he says. "There's also a connection between the size of the storm and the storm surge," Shott says. "We're seeing the storm surge inundation values coming in at 8, 9, 10 feet in those southeast Louisiana parishes."

When it comes to predicting storm surge, a lot of factors come into play, says Brian McNoldy, a senior research assistant at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.

Photo shows a Plaquemines Parish vehicle riding through rising floodwater behind the levee as Isaac approaches on Tuesday. Credit NPR.

Read more on Isaac at NPR.

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