October 20, 2005
Thursday

Even though they'd been expecting the Atlantic basin's 21st tropical storm of the season to rev up for several days, the pace and scope of Hurricane Wilma's intensification to the strongest storm on record stunned forecasters Wednesday.

Lixion Avila, the forecaster on duty at the National Hurricane Center while an Air Force Reserve Reconnaissance plane plunged into the heart of Wilma overnight, even warned colleagues that instruments should go through a calibration check before they accepted the hurricane's record low-pressure reading of 882 millibars in the eye.

Wilma's remarkable transformation from a newly formed, 60-mph tropical storm early Tuesday to a monster Category 5 hurricane with steady winds of 175 mph before dawn Wednesday was the fastest wind-speed increase recorded since hurricane hunter planes first started probing the storms in the 1950s, forecasters said.

Similar wind speeds were recorded for both Hurricanes Rita and Katrina well before they made landfall along the Gulf Coast this year, and like those storms, Wilma was not expected to be a Category 5 storm by the time it reaches southwestern Florida this weekend.

Forecasters tend to rely more on the central low pressure of a storm than wind speed measurements to rate the strength of hurricanes because wind gauges are often damaged or destroyed when gusts reach greater than 120 mph or so.

For instance, Hurricanes Camille in 1969 and Allen in 1980 were estimated to have top sustained winds of 190 mph, but there were no instruments left intact to actually record them.

A hurricane, or any cyclonic storm, achieves such high winds because the atmospheric pressure in the center - where warm, moist air is getting pumped skyward - is much less than the barometric pressure outside the eye walls. Air under higher pressure tries to rush into this vacuum, creating the extreme winds of a hurricane.

Usually, the lower the pressure inside the eye, the higher the winds. But this varies with the pressure of the surrounding air and the size of the eye, among other factors.

With Wilma stalled for several days over some of the warmest seawater in the Caribbean south of Cuba, "we knew there was a good possibility of rapid intensification,'' said May Mayfield, director of the hurricane center. "But the bottom really did drop out on this storm."

Wilma's pressure fell from 982 millibars to 882 in 24 hours, a rate of 4.2 millibars an hour. By contrast, the previous low-pressure record-holder in the Atlantic, Hurricane Gilbert, dropped 3 millibars an hour to a low of 888 in 1988. The lowest pressure ever recorded for a tropical cyclone was 870 mb inside Typhoon Tip in the northwest Pacific in 1979.

Wilma is expected to fluctuate in intensity for another day or so as the hurricane threads through the Yucatan Channel between Mexico and Cuba into the Gulf of Mexico.

By Friday, Mayfield said, the storm will probably have winds less than 155 mph, and with water temperatures in that part of the Gulf several degrees cooler, the storm isn't likely to get much stronger, but it will still pose a considerable threat to Florida and perhaps elsewhere on the Atlantic seaboard.

Although the exact path of Wilma isn't certain - Mayfield says he's going to be pushing for a "forecast cone" as wide as possible - forecasters anticipate it will hit someplace along the southwestern coast of Florida over the weekend as a Category 3 or 4 storm, still with winds over 110 mph and the potential for storm surges of 10 to 15 feet or greater, similar to water levels seen in Rita and Katrina.

Although forecasters still can't precisely predict the intensity of a storm as it makes landfall, Mayfield said it's likely that Wilma will "become larger in size" once prevailing winds start moving the system eastward across the Gulf.

On Wednesday, the intense eye of the hurricane was only about 4 miles in diameter, with hurricane force winds extending out about 50 miles from the center. The tradeoff for the reduced intensity by landfall Saturday or Sunday will be a bigger eye that generates a bigger swath of hurricane winds, and a greater potential for storm surge along more of the coastline, particularly in areas that lie south of the center of the storm, Mayfield said.

Moreover, Wilma is expected to cross Florida in less than 12 hours, exiting into the Atlantic still a formidable hurricane that several computer models suggest could be drawn by another low-pressure system back toward already sodden New England by early next week.

Wilma is the 12th Atlantic hurricane of the season, the same number reached in 1969, and the highest since record keeping began in 1851. It is also the 21st tropical storm, tying a record set in 1933.

Wilma is the last on the 21-name list for storms this year. If any other storms form, letters from the Greek alphabet would be used, starting with Alpha, for the first time since the naming system for Atlantic storms was put in place in 1953.

Scientists are divided on whether global warming is increasing the intensity of hurricanes. Some studies suggest that since warm ocean waters fuel hurricanes, warming temperatures may be increasing the size and strength of hurricanes. Other scientists say both hurricane activity is cyclical, and that since the mid-1990s we have been in a cycle of increased hurricane activity.

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http://www.nhc.gov

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