The difference between a monster and a wimp for Gulf of Mexico hurricanes often comes down to a small patch of warm deep water that's easy to miss. It is called the Loop Current, and hurricane trackers say Gustav is headed right for it, reminiscent of Katrina.
Gustav is likely to reach this current late Saturday, experts say. What happens next will be crucial, maybe deadly.
If Gustav hits the Loop Current and lingers in that hot spot, watch out. If the storm misses it or zips through the current, then Gustav probably will not be much of a name to remember.
The meandering Loop Current, located in the southeastern gulf, provides loads of hurricane fuel. It was a key stopover for nearly all the Gulf Coast killers of the past, including Katrina and Camille, said Florida International University professor Hugh Willoughby, former director of the government's hurricane research division.
Lynn "Nick" Shay, University of Miami meteorology and oceanography professor, flew over the gulf Thursday in a federal hurricane research plane to measure the Loop Current. He saw Gustav's forecast track going "right down the throat" of it.
"That's kind of the scary part here," Shay said. "You look at this and say, 'Boy I hope this thing doesn't really explode,' but it probably will."
It happened in 2005. "Katrina went over the Loop Current and intensified rapidly," said Mark DeMaria, a Colorado-based expert on hurricane strength with the National Oceanic and Atmospheric Administration.
Then less than a month later a weak tropical storm named Rita followed Katrina into the Loop Current. Thirty hours later it was a Category 5 monster.
Both Katrina and Rita later weakened - which often happens to Category 3 storms by landfall.
In the last several years, meteorologists have focused more attention on the Loop Current, which is only a couple of hundred miles long and not even 100 miles wide. The evidence linking it to the worst storms is beyond circumstantial, Shay said.
What is crucial is the depth of warm water in the current - several hundred feet - because it provides continuous high-octane fuel for a storm. Hurricanes use the heat from the water to grow stronger and in the process they churn up cooler water from below, which then slows or stops the feeding process. But in the Loop Current, the deeper water is also warm and it further feeds the storm.
The Loop Current constantly shifts, growing and shrinking and sending out smaller eddies. It is now starting to contract, but not soon enough.
On Friday, the National Hurricane Center warned: "Gustav is expected to be a large powerful hurricane as it approaches the northern gulf coast."
The one hopeful sign is that on his hurricane flight Thursday, Shay saw a pool of extra cool water north and west of the Loop Current. That could help counteract what he fears will be rapid strengthening.
GustavFriday evening as New Orleans officials considered issuing evacuations orders in an effort to avoid repeating Katrina's toll on the city. Gustav has already killed 78 people in the Caribbean.