Prolonged Period Away From Land Helped Make Record Storm : Warm Waters and ‘Steering Winds’ Fueled Hurricane

A combination of subtle factors has turned Hurricane Gilbert into one of the greatest storms in history.

Scientists said Thursday that such things as slightly warmer than normal ocean temperatures and high-level “steering winds” that kept the storm over water for a prolonged period gave Gilbert the intensity it needed to rewrite the record books.

And, as the great storm churned toward land, where eventually it will run out of “fuel” and meet its demise, experts were awed by its strength.

‘Most Intense ... We’ve Seen’

“It’s certainly the most intense storm we’ve ever seen,” said David Jorgensen, a research scientist with the National Oceanic and Atmospheric Administration in Boulder, Colo., who spent 10 years flying through hurricanes for the National Hurricane Center in Miami.

But how, the people who are waiting in its path must wonder, did Gilbert get so monstrous?

Jerome Namias, a research meteorologist with Scripps Institution of Oceanography and an expert on the role the oceans play in global weather patterns, said Gilbert got so big because it came along at just the right time.

Hurricanes are formed over tropical oceans, where rising air currents create a warm core of air surrounded by cooler air. The core draws water vapors from the warm sea, and, as the vapors rise, they condense into thunderstorms around the core--which becomes the eye of the storm.

The thunderstorms release warm rain that heats the atmosphere, creating a low-pressure system that in turn creates winds rushing toward the eye.

“That is a feedback mechanism,” the NOAA’s Jorgensen said.

The atmospheric pressure in the low-pressure system is the best measurement of the strength of the storm because it determines how much “fuel” the hurricane will be able to pull into itself in the form of water vapor and warmth, Jorgensen said.

Pressure Dropped

Hurricane Gilbert surpassed all previous storms in the Western Hemisphere Tuesday when the pressure in the hurricane’s eye dropped to 885 millibars. The standard atmospheric pressure at sea level is about 1,013 millibars.

Low pressure in a hurricane causes the sea under it to bulge up for miles, creating a wall of water called the storm surge. When this reaches shore, it can have devastating effects.

When Gilbert struck the Yucatan, it had a storm surge 20 feet high near the eye of the storm, lifting boats from their moorings and depositing them inland and causing widespread flooding.

Warmth Is Fuel

As long as a hurricane remains over its supply of warm water, it will have the fuel to sustain itself, Jorgensen said.

Scripps’ Namias said Hurricane Gilbert formed in an area where ocean temperatures have been “just a little warmer than average,” getting the storm off to a strong start.

He said the water did not have to be much warmer than normal--maybe a degree or so--because other factors also played into Gilbert’s hands.

“If you go over its track, you will see that its path is practically all over water,” Namias said. “That water is warm, and it may be slightly above normal, but not extremely so.”

Upper-level winds that pushed the storm toward the Gulf of Mexico, where it is now, kept the hurricane moving very slowly across the warm waters for a long time while steering it away from land, which would have eroded its strength.

Ideal Conditions

“These are ideal conditions for its intensification,” Namias said.

Fortunately for California, similar conditions do not exist here.

Strong hurricanes form in the Pacific off the west coast of Mexico, where winds push them westward toward the Hawaiian Islands.

“As they move, they go over progressively colder water, and, of course, the life and intensity of the hurricane depends on the warmth of the water in that the water provides the fuel through warmth and moisture,” Namias said. “If you bring it over cold water, it dies.”

Sometimes a storm will survive long enough to strike Hawaii, where prevailing upper-level winds usually will turn it back toward California. But, as the storm approaches the West Coast, it encounters cool ocean currents of about 60 to 70 degrees Fahrenheit--more than 10 degrees too cool to sustain the hurricane--and it fizzles out.

Unexpected Showers

“Once in a while we get the remnants of a hurricane and we get some moisture which produces some unexpected showers,” he said.

But the hurricane itself dies.

Although the mechanics of what causes a hurricane to form are fairly well understood, it is not always clear just how they are going to behave.

“Hurricanes can be quite erratic,” Jorgensen said, especially when the high-level “steering winds” are weak. “It’s kind of puzzling.”

Jorgensen said one reason Gilbert is moving so slowly in a fairly straight path may be its immense size.

“Because the storm is so large, it has a lot of inertia,” he said. “Smaller storms are more easily influenced by subtle changes in steering winds.”