Hurricane Iris - Meteorological History

Meteorological History

Toward the end of September 2001, a poorly defined tropical wave moved westward across the tropical Atlantic Ocean. It moved through an area of hostile wind shear, caused by a large upper-level low associated with a trough to the northeast of the Lesser Antilles. A few days later, the upper-level low detached from the trough and moved southwestward over the Caribbean Sea. This allowed for the formation of an upper-level ridge, or high pressure area, over the tropical wave. The ridge provided a favorable environment for organization, and subsequently an area of convection increased along the wave axis. As the wave approached the Lesser Antilles, a mid-level circulation formed within the deep convection, and gradually a low-level circulation became more pronounced on satellite imagery; though its low-level circulation was very small and poorly defined, the system organized enough to be classified as Tropical Depression Eleven at 1200 UTC on October 4, while located about 100 mi (160 km) southeast of Barbados. Nine hours after developing, Hurricane Hunters confirmed the formation of the depression.

In the first discussion on the depression, around which time the depression was passing between St. Vincent and St. Lucia, the NHC noted the potential for it to degenerate into a tropical wave if the cyclone maintained its fast forward motion. However, the official forecast was for the depression to gradually intensify, with its movement to the west-northwest under the influence of a strong ridge to its north. Compared to its appearance 24 hours prior, the depression had improved outflow and more distinct convection, although the circulation initially remained very poorly organized. In the hours after its formation, flights by the Hurricane Hunters failed to report a closed circulation, despite its well-organized appearance on satellite imagery. Late on October 5, the Hurricane Hunters reported a circulation with flight-level winds of 74 mph (119 km/h), which was adjusted to a surface intensity of 60 mph (95 km/h); based on the data, the depression was upgraded to Tropical Storm Iris about 155 mi (250 km) south of the southern coast of Puerto Rico. In post-season analysis, the NHC estimated Iris attained tropical storm status about nine hours earlier.

Despite its intensification and well-organized satellite appearance, the circulation remained very poorly defined. One forecaster noted the center as fragile, and that the cyclone could dissipate quickly if it encountered stronger wind shear to its south; however, the official forecast was for Iris to attain hurricane status while passing south of Hispaniola and Jamaica. Its overall appearance did not change significantly, although on October 6 the Hurricane Hunters reported a closed eye, 23 mi (37 km) in diameter, with a stadium effect. Late that day, Iris attained hurricane status just to the southwest of the southern tip of the Dominican Republic. Upon becoming a hurricane, Iris maintained well-established outflow, and the NHC noted land interaction with the Greater Antilles as the only factor impeding further development. After reaching winds of 85 mph (140 km/h) early on October 7, its intensity remained steady for about 24 hours. During that time, its satellite appearance became slightly ragged as its outflow became restricted. By late on October 7, the area of hurricane force winds associated with Iris extended only 25 mi (35 km) from its 16 mi (22 km) wide eye.

Early on October 8 after turning west-southwestward away from the Greater Antilles, Iris began strengthening again. With warm waters and an absence of significant wind shear, it was forecast to intensify further. The NHC predicted peak winds of about 105 mph (165 km/h) before hitting Belize. It rapidly intensified with the favorable conditions, intensifying from 95 mph (150 km/h) to 140 mph (225 km/h) in a 12 hour period on October 8, making Iris a Category 4 hurricane on the Saffir-Simpson scale; in the same 12 hour period, the minimum central pressure dropped 38 mbar (1.12 inHg). While intensifying, the hurricane developed concentric eyewalls, with an innermost eye having a diameter of 7 mi (11 km). For comparison, the smallest known eye diameter on record for an Atlantic hurricane was about 3 mi (5 km) during Hurricane Wilma in 2005. With Iris having such a small eye, a Hurricane Hunters flight could not deploy a dropsonde into the center of the eye, and shortly after the flight, the innermost eye collapsed as the core paralleled the Honduras coastline just offshore. This resulted in a temporary and slight weakening during an eyewall replacement cycle, and within a few hours Iris re-intensified to attain peak winds of 145 mph (230 km/h) just offshore Belize. At 0200 UTC on October 9, Iris made landfall at peak intensity in Monkey River Town in the southern portion of Belize.

Initially, Hurricane Iris was forecast to remain a tropical cyclone while crossing Central America and for it to re-intensify in the eastern Pacific Ocean; had it done so, it would have retained the name Iris. However, officials noted the potential for the storm to dissipate, due to its small circulation. The hurricane rapidly weakened after moving ashore into the mountainous terrain of Guatemala, and within six hours of landfall the hurricane weakened to a tropical storm. Late on October 9, within sixteen hours of landfall, the tropical cyclone dissipated over extreme southeastern Mexico. As the remnants approached the Pacific Ocean, a new area of convection developed south of the original circulation of Iris. It gradually organized while continuing westward, developing into Tropical Storm Manuel and ultimately lasting until October 18 before succumbing to cooler waters and wind shear.