Meteorological History of Hurricane Dean - Caribbean Sea and First Landfall

Caribbean Sea and First Landfall

At 0930 UTC on August 17, the center of Hurricane Dean passed into the Caribbean Sea through the Saint Lucia Channel between the islands of Martinique and St. Lucia. The northern eyewall passed over Martinique where a weather station in the island's capital of Fort-de-France reported 13 in (33 cm) of rainfall. By this time the eyewall had closed, forming a distinct eye, and in an environment of low wind shear and increasing ocean temperature the hurricane began to intensify rapidly. Hurricane Dean strengthened to a Category 3 hurricane by the evening of August 17. Satellite imagery showed that a well defined eye and numerous cyclonically curved convective bands remained over the Lesser Antilles. That evening, another reconnaissance aircraft reached the hurricane and discovered that it had strengthened into a Category 4 hurricane, and by 0600 UTC on August 18, Dean reached Category 5 intensity for the first time with 165 mph (270 km/h) winds. The storm's wind radii increased in all quadrants as the storm grew in both intensity and size. At 0800 UTC August 18, Hurricane Dean passed directly over NOAA sea buoy 42059 which reported a significant wave height (average size of the largest 33% of waves) of 33 ft (10 m). On August 18, Hurricane Dean developed a double eyewall, indicating that an eyewall replacement cycle was taking place and causing short term fluctuations in intensity as Dean weakened back to a Category 4 hurricane. That afternoon the hurricane continued to improve its outflow, and its numerous spiral bands gave it a well defined satellite presentation. Hurricane Dean finished the eyewall replacement cycle early on August 19 with some trochiodal wobbles.

On the morning of August 19, the storm remained slightly weakened from its peak strength. As a Category 4 hurricane with wind speeds between 140 mph (220 km/h) and 145 mph (230 km/h), the center of Hurricane Dean passed 90 mi (150 km) south of Haiti, and that evening passed 25 mi (40 km) south of Jamaica. Two weather stations on the island of Jamaica, one at Ingleside and the other at Morant Bay, both reported in excess of 13 in (33 cm) of rainfall. In contrast, the weather station at Les Cayes, Haiti recorded only 1.18 in (3 cm) of rainfall.

Hurrican Dean intensified through the night of August 19 and reinforced its completed eyewall replacement cycle by forming a tight single-walled eye. At 0100 UTC August 20, the storm passed 120 mi (190 km) to the south of Sea Buoy 42056, which recorded a significant wave height of 36 ft (11 m). A concentric eyewall was briefly observed again on the morning of August 20, but it did not last long. In conditions of low wind shear, Hurricane Dean moved westward over waters with increasingly high heat content, and the storm exhibited a classic upper-tropospheric outflow pattern. The high pressure system over the southeastern United States continued to steer the storm west towards the Yucatán Peninsula. The eyewall became even better defined throughout the day. The cloud tops cooled, the minimum central pressure fell, and its winds increased to 160 mph (260 km/h), making Hurricane Dean a Category 5 hurricane once again. This time, it was less than 210 mi (335 km) from its first landfall.

Although many of the convective bands were already located over the Yucatán Peninsula, Hurricane Dean continued to intensify until the eye made landfall. As the eye moved over Mexico near the town of Majahual in the Costa Maya area, the NHC estimated surface level winds of 175 mph (280 km/h), making Dean the first storm to make landfall as a Category 5 hurricane in the Atlantic basin since Hurricane Andrew in 1992. At the same time, a dropsonde reading from the hurricane's eye estimated a central pressure of 905 mbar, making Dean the third most intense landfalling Atlantic storm in history (after the Labor Day Hurricane of 1935 and Hurricane Gilbert of 1988) and tying Dean with Camille and Mitch as the seventh most intense hurricane ever recorded in the Atlantic basin. The landfall itself occurred in a sparsely populated area of the Costa Maya region of the Mexican state of Quintana Roo near 18.7 N 87.8 W at 0900 UTC August 21 and brought with it a storm surge of 12–18 ft (3.7–5.5 m). A weather station at Chetumal (the capital of Quintana Roo, Mexico) reported 6.65 in (17 cm) of rainfall during Hurricane Dean's landfall. As expected, the landfall caused significant weakening of the storm; the eye filled and the cold cloud-tops warmed. The land severely disrupted the storm's organization, and by the time Dean crossed the Yucatán Peninsula it had weakened to a Category 1 hurricane.