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For more than a year, El Niño has disturbed the weather around the world. Now, as El Niño is replaced by its alter ego La Niña, the 1998 Atlantic hurricane season has heated up considerably.
The powerful El Niño of 1997 caused abnormally strong westerly winds to blow at high altitudes across the tropical Atlantic, cutting off the tops of developing tropical storms. After an active start, the Atlantic hurricane season simply stopped around the end of July 1997 and only one storm, Erica, was able to form later. Now that El Niño has ended, the coast is clear for new tropical storms to blossom into hurricanes.
Up to mid August, the 1998 hurricane season was every bit as dull as the end of the 1997 season. One storm, Alex, managed to form off the African coast on July 28, only to fall apart 5 days later as it approached the West Indies. However, in late August, disturbances began popping up throughout the tropical Atlantic. One of these disturbances grew into Hurricane Bonnie, which pounded eastern North Carolina as it made its leisurely way across the state August 26-27.
Hurricanes can form anywhere between Africa and the coast of Texas, or off the southeast U.S. coast, but during the peak of the season, late August through September, the most powerful storms all start as tropical "waves" (clusters of thunderstorms) coming off the African coast near the Cape Verde Islands. These storms, called Cape Verde hurricanes, can persist for three weeks or more as they cross the ocean to threaten the U.S. or the nations of the Caribbean. All of North Carolina's greatest hurricanes--Hazel, Donna, Fran--have been Cape Verde "blows."
Hurricane Bonnie was no exception, but it was a late bloomer. Starting as a relatively weak wave off Africa, Bonnie didn't acquire its name until August 20, as it approached the Leeward Islands east of Puerto Rico. After a fast passage past the Antilles, Bonnie stalled for 48 hours (August 22-24) near San Salvador in the Bahamas. When it finally got moving again, it moved with single-minded determination to Cape Fear, only to stall again as it ran against the force of an approaching cold front.
Bonnie taught Tar Heels to fear the rain of a hurricane as much as the wind. Six to ten inches of rain poured down on much of the flat eastern part of the state, flooding roads, fields, and homes.
How many hurricanes will there be this year? The experts have disagreed sharply over the probable course of the 1998 hurricane season. William M. Gray and his forecast team at Colorado State University originally predicted 6 named storms, but as El Niño faded more rapidly than expected they have raised their forecast numbers. In early August, they predicted a total of 10 named storms, including 6 of hurricane strength and 2 of intense strength (category 3, 4, or 5 on the Saffir-Simpson scale).
For the first time, Gray also issued predicted probabilities of hurricane strikes on the U.S. coast. For the coast of the Carolinas (from the Savannah River through Cape Hatteras) his predictions are that the probability of a named storm striking within 50 kilometers of a given location is about 6%. The probability of a strike from an intense hurricane is a little less than 2%. (With those numbers, the probability of three intense hurricanes striking the Cape Fear area in three years (as Bertha, Fran, and now Bonnie did) is only about 1 chance in 5000---but that's what happened!)
What will happen next? We'll have to wait and see. Fortunately, few topics are as well represented on the Internet as hurricanes. There are dozens of sources of current hurricane information; listed below are some of the best.
Internet Sources
FEEDBACK: We'd be happy to have your comments and suggestions.
Copyright © 1998, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
Posted August 20, 1998. Revised August 27 to reflect Hurricane Bonnie.
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