PHILADELPHIA — In the early-morning hours of Friday, Katrina exited the Florida peninsula and entered the Gulf of Mexico as a regular, unleaded tropical storm.

By Friday evening, though, it had blossomed into a potent hurricane, on its way to becoming one of the deepest, most-powerful storms on record.

Meteorologists said Katrina was pushed around by large-scale weather systems, triple its size, over the United States and out in the Atlantic Ocean, and drew on some of its own chaotic energy.

But it clearly was recharged by a deep, extensive pool of warm water in the Gulf of Mexico, part of the so-called Loop Current associated with the Gulf Stream.

Katrina's track virtually followed the trail of the serpentine pool, where water temperatures were warm to a depth of more than 300 feet, according to an analysis by the University of Miami.

"It got into the higher octane fuel," said Lynn Shay, a professor of meteorology and physical oceanography at the university's Rosenstiel School.

As it approached land early yesterday, where the warm layer was shallower, Katrina weakened, albeit slightly, from a Category 5, with winds of 160 mph, to one with winds of 145 mph.

Surface temperatures across the Gulf are quite high, in the mid- to upper-80s. But the depth of the warmth makes all the difference, hurricane experts say.

Shay likened it to two boiling pots. One with shallow water will heat faster, but a pot with deeper water will generate far more water vapor and steam and a richer supply of heat energy.

Shay said the role that water temperatures play in steering storms is unclear, but he suspects that the atmosphere isn't the only player.

"They must be working together," he said.

Whatever was guiding Katrina, it was no friend of New Orleans or the city's Gulf Coast neighbors.

Katrina's landfall, about 40 miles east of New Orleans, spared the city a catastrophic storm surge, but not the blinding rains and punishing north winds that whipped across brimming Lake Pontchartrain.

And the track did not spare the Louisiana, Mississippi and Alabama coasts storm surges of up to 18 feet. Those surges actually are tides upon which storm waves are superimposed. Thus, a 10-foot wave atop a 10-foot surge reaches a height of 20 feet, said Asbury Sallenger, of the U.S. Geological Survey in St. Petersburg, Fla.

Sallenger said the agency would be flying reconnaissance missions to assess surge levels. He said the tides might have been as catastrophic as the surges along the Gulf Coast in Hurricane Opal, in 1995, and Ivan, last year, which led to one of the biggest coastal building collapses ever.

A hurricane's storm tides are worst to the east of a storm's center, because powerful winds from the south cause the waters to pile up. Winds move counterclockwise around the centers of hurricanes.

New Orleans was on the western flank of the storm because Katrina made a slight jog to the northeast in the final hours before landfall.

"It was a little wrinkle at the end," said Chris Sisko, a meteorologist at the National Hurricane Center, in Miami. Although captured in upper-air steering currents, Katrina appeared to "wobble" on its own, "perhaps trying to modify its environment."

That caused it to deviate slightly from a due-northerly course.

"The storms tend to wobble about a track," he said. "We're still not certain why."

Katrina's movement was tied to an area of low pressure, or trough, over the Great Lakes, that affected an area of higher pressure, or ridge, centered over the Southeast coast. Winds blow clockwise around the centers of a high, and winds to the west of the center are from the south, which explains in part why Katrina was moving south to north.

But Shay's analysis suggests the storm also was influenced by the warm water.

"It comes down to the complex interplay of these larger-scale systems," Sisko said.