But a dramatic intensification suddenly and suprisingly kicked Opal's winds from 98 to 150 mph.
No one knew what happened. Until perhaps now, CBS News Correspondent John Roberts reports.
Research scientists Nick Shay of the University of Miami and Peter Black of the National Oceanic and Atmospheric Administration think deep warm water eddies in the Gulf of Mexico jump-started Opal.
Black said, "They seem to act like a fuel injector on an engine that gives it a sudden shot of energy which may last for 12 to 24 hours."
The swirling warm water eddies develop every year or so when the top of a warm water current flowing into the Gulf of Mexico breaks off. The eddies are wide - about 140 miles across - roughly the diameter of a hurricane and warm layers much deeper than the surrounding water. And because a hurricane's fury is driven by warm water, the eddies provide a reservoir from which a storm can find fuel.
"You can think of it as the difference between gas tanks - here's a shallow gas tank, and here's a very deep gas tank, which one has more fuel?" asked Shay.
Later this summer the researchers will drop tiny sensors into the warmer water during a storm. The findings they hope will help predict hurricane strength and landfall. That could assist local authorities determining where and when to evacuate... a process that can cost a million dollars a mile.
"You would definitely want to plan well in advance to get these people off barrier island, off the Keys, anywhere that people would be in harm's way." Shay explained.
So the good news is about an advance in hurricane research. The bad news is that a huge warm water eddy is forming today - just in time for hurricane season. And any storm that hits the eddy is almost sure to hit the coastline, since nowhere in the Gulf of Mexico is more than 500 miles from land.