The meteorological history of Hurricane Wilma, the strongest tropical cyclone known in the Western Hemisphere, began in the second week of October 2005. A big weather system formed across much of the Caribbean Sea and slowly organized to the southeast of Jamaica. By late on October 15, the system was became strong for the National Hurricane Center to name it Tropical Depression Twenty-Four.
The depression slowly moved southwestward, and in conditions that were good for strengthening, it strengthened into Tropical Storm Wilma on October 17. In the beginning, development was slow because of its large size, although thunderstorms slowly organized. From October 18, and over the next day, Wilma underwent explosive deepening over the open waters of the Caribbean; in a 30-hour period, the system's central atmospheric pressure dropped from 982 mbar (29.00 inHg) to the record-low value of 882 mbar (26.05 inHg), while the winds increased to 185 mph (300 km/h). At its strongest, the pinhole eye of Wilma was about 3 miles (5 km) in diameter, the smallest known eye in an Atlantic hurricane. After the inner eye died off because of an eyewall replacement cycle, Wilma weakened to Category 4 status, and on October 21, it made landfall on Cozumel and on the Mexican mainland with winds of about 150 mph (240 km/h).
Wilma weakened over the Yucatán Peninsula, and reached the southern Gulf of Mexico before speeding up northeastward. Despite increasing amounts of wind shear, the hurricane re-strengthened to hit Cape Romano, Florida as a major hurricane. Wilma weakened as it quickly crossed the state, and entered the Atlantic Ocean near Jupiter, Florida. The hurricane again re-intensified before cold air and wind shear penetrated the inner core of convection. On October 26, it turned into an extratropical cyclone, and the next day, the remnants of Wilma were absorbed by another extratropical storm over Atlantic Canada.
Formation
The origin of Hurricane Wilma is complicated. During the second week of October, an unusually large, monsoon-like lower-level circulation and a large area of disturbed weather developed over much of the Caribbean Sea. The system was enhanced by diffluence—the rate at which a fluid moves—from an upper-level low across the southwestern Atlantic. By October 13, a broad area of low pressure developed and persisted about 150 miles (240 km) southeast of Jamaica, possibly aided by the passage of tropical waves through the area at the time. Convection increased and became slightly better organized, though upper-level wind shear originally stopped development. The system moved westward, and early on October 14 the convection became more concentrated and a little better organized as upper-level wind shear lessened slightly.
Later on October 14, the system became much better organized, with increasingly organized shower and thunderstorm activity, as conditions in the upper levels of the atmosphere became significantly more favorable. It was then that the National Hurricane Center first said that it was possible for a tropical depression to develop in the area. Dvorak classifications were initiated on October 15. The system continued to organize, with the National Hurricane Center remarking the system could ultimately become a hurricane. By late on October 15, the surface circulation became defined well-enough, with enough organized deep convection, for the National Hurricane Center to designate the system as Tropical Depression Twenty-Four while it was about 220 miles (345 km) east-southeast of Grand Cayman.
The depression tracked slowly westward, a motion due to weak steering currents caused by a high pressure system to its north across the Gulf of Mexico. Initially, the center of circulation was broad without a defined inner core; forecaster Lixion Avila remarked, "The area of minimum pressure could [have been] anywhere within 60 miles (95 km) of its [initial advisory position]." Originally, the tropical depression was forecast to drift west-southwestward before turning to the north; within 120 hours of the forecast's issuance, the system was predicted to be about 80 miles (130 km) south of the Isle of Youth as a 105 mph (170 km/h) hurricane. However, the National Hurricane Center noted in the first advisory on the depression that there were "all indications that there could a dangerous hurricane in the northwestern Caribbean Sea in 3 to 5 days." This was due to the depression being in an environment very ideal for development, specifically low amounts of wind shear and very warm water temperatures.