This is a rough sketch of the actual path of the eye of hurricane Katrina compared to the current projected path of the eye of hurricane Gustav.
Now, this is NOT Gustav’s actual path. We don’t know what the path is going to be. But this graphic allows for a few key points to be made.
First, realize that in the counter-clockwise spinning hurricane, the right hand side, and in particular, the ‘front right” quadrant, of the storm is where the winds are strongest … because we need to add together both the forward motion of the storm, maybe between 10 and 15 mph at landfall in this case, with the spinning rate of the storm. Also, this part of the storm is virgin … it is coming off the ocean, and it is probably carrying more rain. Most importantly, perhaps, is that the front right quadrant carries the highest storm surge, which in the case of Gustav, could be 15 feet plus waves.
Now, you’ll notice that Lake Ponchetrain and New Orleans itself was not in the right front quadrant of Katrina but it could be for Gustav. Gustav could also hit in a very different locaiton, putting N.O. in the left quadrant, or simply being far enough away that it does not matter too much.
But what is possibly more important, and actually predictable at this stage (probably) is the angle with which Gustav will strike the coast. Since it will be an oblique angle (if present predictions hold) this means more contact time with low coastal areas, and probably a prolonged period of weakening (meaning the storm stays strong longer then if it just plowed over land).
On addition, look at the shape of the bodies of water and delta formations here. A storm surge moving in a northwesterly direction could drive more water father inland, into steadily restricting channels, where the surge can be significantly magnified. If the eye is exactly where it is shown on this map, then the open water leading towards New Orleans could be 50 miles or so away from the eye, and an appreciable but diminished storm surge may be expected. But if the eye is half as far or less from New Orleans, and Gustav remains at, say, Category Three strength, Breton Sound and/or Mississippi Sound could become traps. This water would have to go somewhere, and this often means a fundamental reshaping of the landscape.
Right now, New Orleans is on Lake Pontchartrain. When this is over, Lake Pontchartrain could be on New Orleans.