Katrina will be a storm for the record books. It first appeared a week ago — not off the coast of Africa like many hurricanes — but near the Bahamas, very close to the United States.
That, according to Max Mayfield, director of the National Hurricane Center, is not unusual.
"There's no rule that says tropical cyclones have to form in the far eastern Atlantic," he says. "I wish they did. That would give us a lot more time."
But Katrina got strong fast. After it crossed Florida, killing nine people, it got a lot of energy from the Gulf of Mexico.
Dr. Kevin E. Trenberth is head of the Climate Analysis Section at the National Center for Atmospheric Research.
"The sea temperatures are running 2 degrees Fahrenheit above normal. That's the fuel for the storms," he says.
The gulf is warm not just at the surface, but deep below, from a great stream of warm water called the loop current. All that energy along with the warm water vapor at the surface feeds into the hurricane system, pushing its towering layers of circulating clouds higher and making the winds stronger.
As Katrina approached the Gulf Coast, hurricane hunter planes determined it was the fourth most powerful storm ever to threaten the U.S. Only Gilbert in 1988, the Labor Day hurricane of 1935 and Allen in 1980 were more powerful.
But at the last moment Monday, there was one piece of good fortune. The eye wall of powerful hurricanes often split into two. That weakens the storm temporarily. Dr. Greg Holland, with the National Center for Atmospheric Research in Boulder, Colo., says with Katrina it happened right at landfall.
"It was just plain good luck for the Biloxi and New Orleans areas," says Holland. "It meant that Katrina definitely came ashore much weaker than many had feared and many of us were forecasting."
Even with its slight weakening Katrina was one of the biggest hurricanes ever — and many scientists say we can expect such storms more often as global warming increases sea temperatures.