Storm strength not easy to gauge

Hurricane forecasters watching Tropical Storm Fay make its way to Florida had a much better idea where it would go than how strong it would be when it arrived.

They knew the factors that could make Fay a big threat: a long stay over warm water, low wind shear, good structure. But understanding exactly how these could come together to make a dangerous storm or just a windy, rainy day is still beyond their reach.

There are a huge number of factors that control intensity, "and you have to get them all right," said Hugh Willoughby, professor of meteorology at Florida International University's International Hurricane Center. "And even if you get them all right, you might not get the forecast right."

Since 1990, forecasters have halved their errors in predicting where a storm will hit, from about 250 to 300 miles on either side of a line down to roughly 125 to 150 miles. They have been unable to improve much on storm intensity. On average, forecasters are 16 mph off in their intensity predictions, a rate unchanged in the past two decades, said Chris Landsea, science and operations officer at the National Hurricane Center in Miami. Some forecasts are off by 30 mph.

"That's one of the reasons why when emergency managers are getting ready at the county and city level for a strike by a storm, they plan for one category higher, given our rather limited skill in intensity prediction," Landsea said.

The biggest danger for coastal residents is when a hurricane undergoes what forecasters call rapid deepening, ramping up a category or more just before making landfall.

Most hurricanes are weakening as they make landfall, but a few go through rapid deepening.

It happened with Hurricane Charley, which went from a Category 1 to a strong Category 4 in six hours just before striking Punta Gorda on Aug. 13, 2004. It happened with Hurricane Andrew, which powered up over the deep warm Gulf Stream to a Category 5 storm just before plowing into Homestead in August 1992. The Gulf Stream is a current of warm water that runs north along the east coast of Florida, similar to the warm deep Loop Current off the west coast of Florida.

Forecasters and researchers are trying to improve intensity forecasts, but the task is difficult. They do not fully understand the physics of what makes hurricanes grow stronger or weaker. They have an incomplete, though improving, picture of the internal workings of a storm.

Another problem is that computer models need massive amounts of computer brain power to analyze millions of bits of data about a storm. The lack of abundant computer power makes looking at a storm a little bit like looking at a poor quality digital snapshot: The closer you look, the less clear the picture.

The biggest improvements in getting a good fix on what is going on inside a hurricane -- and so better information to feed computer models -- has come from aircraft that circle storms and even venture inside. They use Doppler radar to see a storm's internal winds and drop instruments into it to gauge pressure, temperature and humidity inside.

"Everybody wants to know what the track is because they hope it's going somewhere else," FIU's Willoughby said. "The people in the bull's-eye want to know the intensity."