"I had a feeling we were going to be in the middle of this thing."

At 7 a.m. on Saturday, August 27, the National Weather Service issued Hurricane Katrina Intermediate Advisory No. 16A. The massive storm system was chugging along at a sluggish 7 miles per hour out in the Gulf of Mexico, the message said, 180 miles west of Key West, Fla., 430 miles southeast of the Mississippi River. The advisory concluded, tentatively, that Katrina would make a "gradual turn toward the west-northwest . . . during the next 24 hours." For seasoned veterans like the National Hurricane Center's Max Mayfield, this was a bit unusual. Sometimes, big storm systems like Katrina would claw their way up the Florida peninsula, wreaking havoc; sometimes they blew out over the Gulf and died. A lot depended on whether the storms were accompanied by "steering winds," and if so, how strong they were. The last description, at the moment, seemed to fit Katrina's profile best. Which meant that everyone from the Keys to the low-lying Gulf Coast needed to keep a close eye on Katrina.

That's just what Marc Levitan was doing. He and his colleagues at Louisiana State University's Hurricane Center had cranked up their massive storm-surge computer hours earlier. The SuperMike mainframe was one of the two or three fastest computers on a university campus anywhere. The devilishly complex computer simulation, called ADCIRC, looked at every possible element that could make or break a big storm. Climate and meteorological data were two of the most obvious factors. But ADCIRC also calculated the effects of the built environment along the coast where a big blow might hit, as well as the shape and contour of the seafloor. The latter was a critical factor in determining the amount of "run-up" a hurricane might cause. Run-up refers to how much wave heights might increase as sea depth decreases.

A little after 1 p.m., after grinding the data for nine hours, ADCIRC spat out its "model run" analysis. Among the data points the computer had taken into account was the water temperature of the Gulf. As it veered west away from Florida, Katrina passed over something climatologists call the "loop current." The strong flow of tropics-hot sea water surges up into the Gulf between Mexico's Yucatan Peninsula and Cuba. Later on in the hurricane season, big storms that move out into the Gulf often blow themselves out as they encounter cool water. In late August, however, the loop current is just about as warm as it gets, even in its depths. The warm water, in other words, would only feed Katrina's strength. Bill Lokey had left for Baton Rouge in the morning after being dispatched by Michael Brown, the director of the Federal Emergency Management Agency. FEMA's emergency watch center was already operating at Level 1, its highest alert status. Watching Katrina's progress across the Gulf, Lokey described the effect as "pouring gasoline on a fire."

Briefing emergency planners in New Orleans, Levitan put it in more clinical terms. ADCIRC had spat out a colorful map of the Louisiana coast. On it were some words in red ink. Levitan read them quietly: "Special Note: Levee overtopping predicted for west New Orleans (Kenner) but may also occur in New Orleans East and St. Bernard Parish, where waves will cause run-up."