Unpredictable Art of Tracking a Hurricane

Billowing white clouds drift across the blue sky above the headquarters of the National Hurricane Center. Just another calm summer day in south Florida. But behind the 10-inch thick concrete walls of the bunker-like headquarters, staff members have no time to enjoy the weather. Since late last week, all eyes here have been on computer screen images of a swirling mass of wind, clouds, and rain that is lashing a huge expanse of ocean 500 miles to the east. The storm is roughly the size of the entire state of Florida. "This is a big hurricane. Its winds extend almost 400 miles in diameter," says Chris Landsea, a meteorologist at the center. The mere threat of a hurricane was enough to disrupt cruise-ship schedules and send many coastal residents scurrying to supermarkets and hardware stores for supplies. About 200,000 residents were told to evacuate North Carolina's Outer Banks Aug. 25.

Battening down In North Myrtle Beach, S.C., Jane Brown woke up, looked out at the roaring surf from her beachfront condo, and turned on the Weather Channel. If the storm heads her way, she has an evacuation plan ready. She's arranged to stay with friends who live across Highway 17, a mile from the shore. Her most precious valuables will go with her, and the rest will get put in the bathtub. But even as residents in the Carolinas nailed plywood to their windows and stockpiled bottled water, forecasters say it is not a scenario they will have to repeat often this year.

Hurricane watchers predicted a moderate hurricane season. Until last week, it had been downright quiet. Then came tropical storm Charley, which dumped as much as a foot of rain on portions of Texas.

Bonnie came next, stalling for two days off the Bahamas, and then proceeding northward up the Atlantic coast. As if Bonnie wasn't enough to watch, meteorologists are also tracking a new tropical storm, Danielle, which appears to be gaining strength while approaching the Windward Islands.

This year, forecasters watching the Atlantic and Caribbean predicted seven storms, five of which they said would become hurricanes. Of those, the prediction goes, two would develop into major hurricanes.

Bonnie is the first major Atlantic hurricane of 1998. And hurricane watchers believe that Danielle may soon become the second. Predicting the path of a hurricane is still as much an art as a science. And the primary purpose of the hurricane center is to provide an early-warning system. The last thing forecasters want is to advise folks that they are safe, only to have a hurricane make an unpredicted shift and catch an entire region unprepared.

One key to the formation of hurricanes, forecasters say, are the upper-level winds that in most years act like a giant fire hose, slowing down and disrupting the cyclical flow of tropical storm clouds as they move across the ocean and into the warm Caribbean. This year, the upper-level winds are relatively weak.

Tracing a storm's path As a storm begins to form, hurricane watchers rely on satellite images to track its movement and development. But as it moves closer to land, forecasters need reliable information about conditions inside and around the storm to be able to accurately predict where the hurricane will likely make landfall. They gather that information by dropping sensors from planes flying around and through the hurricane. Each $600 sensor weighs about 5 pounds, is attached to a parachute, and can take as long as 20 minutes to reach the ocean surface. They relay a new data reading to the hurricane center once every half second.

During the descent, the sensors record wind speed, wind direction, barometric pressure, temperature, and humidity. The resulting raw data are loaded into each of the 12 computer models the hurricane center is using to predict the strength and direction of hurricane Bonnie.

The data offer a real-time glimpse of actual weather conditions influencing the development and movement of the storm. "This is an incredibly data-rich environment," says Frank Lapore, a spokesman for the center. The more data obtained, the better the computer models will run, and forecasters will be better able not only to warn those in the path of the storm, but also to advise those who are no longer threatened that the danger is past.

* Staff writer Alexandra Marks contributed to this report from Myrtle Beach, S.C.