In the fall of 1995, tropical storm Opal spun into the Gulf of Mexico, grew into a hurricane, and suddenly did something that atmospheric scientist John Molinari calls a "nightmare scenario."
It intensified overnight at a rate never before seen in an Atlantic hurricane - no prediction technique even hinted at the change. Few people would be awake to hear any new hurricane warnings, and those who were would have no time to evacuate.
Then, just as suddenly, the storm weakened, with most of the Florida panhandle experiencing hurricane-force winds much weaker than anticipated.
It was a worrying reminder that, while forecasters have made important leaps in charting hurricanes, they remain largely uncertain about how to estimate the storms' power. "We have no skill in intensity forecasts, and we don't know why," says Robert Gall, a scientist at the National Center for Atmospheric Research in Boulder, Colo., and head of the US Weather Research Program.
Now, new studies of data gathered during hurricane Opal are beginning to unlock the secrets behind such rapid changes. They may point to a need to pepper the ocean, as well as the atmosphere, with sensors. The goal, scientists say, is to help forecasters improve the lead time on warnings and ensure that warnings cover only the amount of coastline necessary.
Perhaps most significant, new research on storm intensity is focusing attention on the combined effects of atmospheric conditions and hard-to-spot, deep pools of warm water.
Forecasters anticipate an active 1999 hurricane season. The National Oceanic and Atmospheric Administration (NOAA), as well as a team from Colorado State, predict at least three intense hurricanes between this week and Nov. 30.
Both efforts cite La Nia - cooler than normal sea-surface temperatures in the central and eastern tropical Pacific - as one key influence on this year's outlook. And the Colorado State team, led by William Gray, suggests that changing patterns in ocean circulation will mean more activity in the next few years - and more hurricanes that travel farther up the East Coast.
Forecasters' ability to predict the track of such storms has improved by as much as 31 percent compared with forecasts based on older sampling techniques, according to NOAA figures. An array of expendable sensors and a new Gulf Stream jet - which can fly higher and farther than older hurricane trackers - have helped forecasters get a more detailed look at hurricane conditions.
But "evacuation is based on intensity, not just on the storm's track," notes Lynn Shay, associate professor of meteorology and physical oceanography at the University of Miami in Coral Gables, Fla. And intensity studies, he says, generally took a back seat to research to improve storm-track forecasts - until Opal.
Clues to Opal's explosive intensification come from air as well as from sea.
Dr. Molinari, a professor of earth and atmospheric science at the State University of New York at Albany, has looked at the atmospheric conditions that existed at the time Opal blossomed. He found that the jet stream, a river of air that snakes its way across North America from west to east, dipped into the subtropics and spun off an eddy that merged with Opal, adding to its strength. He says this merger was the major reason for Opal's rise.
But Dr. Shay and colleagues at the university and at NOAA's Hurricane Research Division say there was much more to it. They point to a 130-mile-wide pool of warm water some 700 feet deep. Known as warm-core rings, these features spin off a deep current of warm water that loops northward and back between Mexico's Yucatan Peninsula and Cuba. The loop spawns warm-core rings every 11 to 14 months.
This is important, because hurricanes typically feed off warm water on the surface. But as the hurricane passes over it, wind and wave action mix the surface water with deeper, cooler water, and the storm loses strength.
Just at the time the jet stream's eddy was giving Opal a boost, the storm passed over the warm-core ring. The wind and waves churned up more warm water, feeding the storm.
Forecasters didn't see this, Shay says, because most weather satellites measure only surface temperatures. But using ocean-altitude data from a special NASA satellite, Shay and his colleagues were able to spot indicators that showed the larger mass of warm water beneath.
This summer, the group plans to study this coupling of ocean and atmosphere more closely, using ocean-temperature and current sensors.
In the meantime, his team is making specially processed NASA data available. "We've been so focused on atmospheric conditions," Shay says, "that we've lost sight" of the conditions at the boundary between ocean and atmosphere that fuel hurricanes.