Isaac is less than 24 hours from its center's landfall on the Louisiana or Mississippi coast. The main computer-forecast models are finally in reasonable agreement on the location, although the small differences would change exactly who gets the worst storm surge.
The center has jogged a bit to the right tonight, likely due to the asymmetry of the system. If that trend continues, the surge will be higher in Mississippi, and the surge on the south-facing coast of Louisiana would be much less. This far out, however, more slight variations are possible.
The somewhat good news is that dry air continues to invade the core of the system, which is inhibiting or at least tempering the intensification process. Large-diameter storms like this generally intensify more slowly, as well. On the other hand, the pattern aloft looks very favorable for strengthening tomorrow, the heat content of the water will go up some, and the friction of the circulation's initial interaction with the coast can turn the winds in toward the storm's center kicking up the wind speed. Given the short window of time left before landfall, it looks most likely that Isaac will come ashore as a Category 1 storm, though Category 2 can't be ruled out.
The bad news is that the models are in lockstep on the LONG duration that the storm will batter areas near the coast, including the city of New Orleans. Tropical-storm force winds may continue over southeast Louisiana and Mississippi for 24 hours or more, with hurricane force winds for 8 hours or more. The storm is expected to move very slowly once it makes landfall, with tropical-storm force winds, at least in gusts, continuing into Thursday as the storm center moves away to the north.
If the storm tracks over or just west of New Orleans as is forecast, the heart of the city will experience stronger winds than it did in Katrina. And the duration of the strong winds will be far far longer.
The dry air in the circulation may temper the rain totals a bit, though there's no way to know it won't mix out before landfall. in any case, a foot or more of rain will fall along the coast and inland.
The levee and pumping system in New Orleans is NOT designed to keep the city dry in all eventualities. It's designed to keep the city safe. If a foot or more of rain falls on the area inside the levee, there will be flooding, just like there would be just about anywhere else. The difference is that the water is PUMPED out of New Orleans - one inch in the first hour and one-half inch each hour after. So people still need to use common-sense precautions in low-lying areas.
Elsewhere, the combination of heavy rain and storm surge is going to create an enhanced flood threat. The storm surge will push Gulf water up rivers, creeks, and inlets where the wind is onshore. The rainwater will want to drain out those same waterways. The storm surge will win and the rain water won't have anyplace to go.
Life-threatening storm surge will affect the coast for hundreds of miles east of where the center comes ashore. And, the slow movement is expected to hold the Gulf water over the coast and into inlets and bays for at least two high-tide cycles. The amounts at any one location will vary with the exact final track, but be ready for water higher than 6 feet above the land from Louisiana to Alabama, and dangerous surge in the Florida Panhandle as well.
The best we can hope for is that Isaac doesn't intensify, although at least some intensification is expected.
There is very little time to take precautions that will make a tremendous difference in how uncomfortable the period during and after the storm will be. Follow emergency instructions, use common sense, and stay safe.