Hurricane Gustav strengthened very rapidly today into a Category 4 storm, and forecasters believe it may reach Category 5 for a time Sunday, then weaken before it makes landfall, probably on the central Louisiana coast. The current forecast track has changed little over the past three days. Relatively speaking, this is good news for New Orleans. There will be miles of wetlands, suburbs, and the river (and its levees) interposed between the city and the center of the storm. Areas to the south and west of the city would bear the brunt of the storm surge. Most of this is sparsely-populate bayou country, but there is one substantial urban area, Houma-Thibodaux, population 32,000.
The Saffir-Simpson scale is somewhat misleading, especially where Louisiana is concerned, because it’s based on wind speed. What really matters in Louisiana is storm surge. A big storm pushes an incredible amount of water over the coastline’s flat, marshy topography. Channels and decaying marshes offer avenues for water to get further inland, where it can easily engulf levees and inhabited areas.
Gustav looks especially ominous for several reasons. The longer the hurricane remains at Category 4 and 5 before making landfall, the bigger its storm surge will be. This is what made Katrina so devastating – it was a large Category 4 and 5 storm for nearly two days. Wind speeds fell as it came ashore, but the huge wave it generated didn’t dissipate with them. Gustav is also coming in diagonally, which will increase the time it is pushing water inland.
My friend and colleague Mark Schleifstein has some reporting up on this today:
Hurricane Gustav will be at Category 4 strength with winds of 145 mph only 12 hours before it hits the central Louisiana coastline Monday afternoon, according to a 2 p.m. National Hurricane Center forecast.
On that track, a Louisiana State University coastal scientist says, storm surge could reach the top or overtop levees on the West Bank, could raise water to 8 feet — plus waves — along levees on the Mississippi River-Gulf Outlet, and combined with a water-swollen Tchefuncte River, could push surge into Madisonville on the Northshore.
Water also could rise as high as 8 feet in the Industrial Canal, he said.
In a nutshell: areas around New Orleans will get swamped. The city itself will see some high water, but not high enough to top the levees (of course, this didn’t help last time) but may avoid the worst.
Update: Mayor Nagin has ordered a mandatory evacuation of New Orleans starting Sunday morning. The 11 p.m. forecast has shifted the track slightly eastward, increasing the risk to New Orleans.
Speaking of history, here’s a variable that could cause a significant disruption to the media’s focus on next week’s Republican Convention. Hurricane Gustav is now over the Caribbean and is 
