Gustav analysis: Some optimism for New Orleans

The Chronicle’s SciGuy, Eric Berger, has spent the last several days monitoring Gustav, posting and updating, and even holding live chats.
Now that it’s clear that Houston’s not going to be in the line of fire, though, he’s turned his formidable knowledge toward New Orleans, and what Gustav’s impact, under current circumstances (less than a day out) might be there.
In short, it’s better news than I expected, and his conclusion gave me some hope:

Assuming it will take a 15-foot surge to overtop or undermine the city’s post-Katrina levees, what conditions are necessary for catastrophic flooding in New Orleans?

• Gustav must strengthen back into a Category 4 hurricane like Betsy did in the open Gulf of Mexico. (Probably 50-50)
• Gustav must make landfall east of Grand Isle, La. (Probably less than 50-50)

By my admittedly very rough analysis, then, I’d say the chances of catastrophic flooding in New Orleans are roughly one-in-four at this time.

Those odds are soooo much better than they were this morning. Fingers crossed!