Hurricane Isabel - Meteorological History

Meteorological History

A tropical wave moved off the coast of Africa on September 1. An area of low pressure associated with the wave moved slowly westward, and its convection initially appeared to become better organized. On September 3, as it passed to the south of the Cape Verde islands, organization within the system degraded, though convection increased the next day. The system gradually became better organized, and Dvorak classifications began early on September 5. Based on the development of a closed surface circulation, it is estimated the system developed into Tropical Depression Thirteen early on September 6. Hours later, it intensified into Tropical Storm Isabel, though operationally the National Hurricane Center did not begin issuing advisories until 13 hours after it first developed.

Located within an area of light wind shear and warm waters, Isabel gradually organized as curved bands developed around a circular area of deep convection near the center. It steadily strengthened as it moved to the west-northwest, and Isabel strengthened to a hurricane on September 7 subsequent to the development of a large, yet ragged eye located near the deepest convection. The eye, overall convective pattern, and outflow steadily improved in organization, and deep convection quickly surrounded the 40-mile (60 km)-wide eye. Isabel intensified on September 8 to reach major hurricane status while located 1,300 miles (2,100 km) east-northeast of Barbuda. On September 9, Isabel reached a primary peak intensity of 135 mph (215 km/h) for around 24 hours, a minimal Category 4 hurricane on the Saffir–Simpson Hurricane Scale.

Early on September 10, the eyewall became less defined, the convection near the eye became eroded, and northeasterly outflow became slightly restricted. As a result, Isabel weakened slightly to a Category 3 hurricane. The hurricane turned more to the west due to the influence of the Bermuda-Azores High. Later on September 10, Isabel restrengthened to a Category 4 hurricane after convection deepened near the increasingly organizing eyewall. The hurricane continued to intensify, and Isabel reached its peak intensity of 165 mph (270 km/h) on September 11, a Category 5 hurricane on the Saffir–Simpson Hurricane Scale. Due to an Eyewall replacement cycle, Isabel weakened slightly, though it retained Category 5 status for 24 hours. As Isabel underwent another eyewall replacement cycle, outflow degraded in appearance and convection around the eye weakened, and early on September 13 Isabel weakened to a strong Category 4 hurricane. A weakness in the ridge to its north allowed the hurricane to turn to the west-northwest. After completing the replacement cycle, the hurricane's large 40 mile (65 km) wide eye became better defined, and late on the 13th Isabel re-attained Category 5 status. A NOAA Hurricane Hunter Reconnaissance Aircraft flying into the hurricane launched a dropsonde which measured an instantaneous wind speed of 233 mph (375 km/h), the strongest instantaneous wind speed recorded in an Atlantic hurricane. Cloud tops warmed again shortly thereafter, and Isabel weakened to a strong Category 4 hurricane early on September 14. Later that day, it re-organized, and for the third time it attained Category 5 status while located 400 miles (650 km) north of San Juan, Puerto Rico.

Cloud tops around the center resulted in Isabel weakening to a Category 4 hurricane early on September 15. Later that day, the inner core of deep convection began to deteriorate while the eye decayed in appearance. As a ridge to its northwest build southeastward, it resulted in Isabel decelerating as it turned to the north-northwest. Increasing vertical wind shear contributed in weakening the hurricane further, and Isabel weakened to a Category 2 hurricane on September 16 while located 645 miles (1035 km) southeast of Cape Hatteras, North Carolina. Convection remained minimal, though outflow retained excellent organization, and Isabel remained a Category 2 hurricane for two days until making landfall between Cape Lookout and Ocracoke Island on September 18 with winds of 105 mph (165 km/h). It weakened after it made landfall, though due to its fast forward motion Isabel remained a hurricane until reaching western Virginia early on September 19. After passing through West Virginia as a tropical storm, Isabel became extratropical over Western Pennsylvania including Pittsburgh. Later that day, it crossed Lake Erie into Canada, and was absorbed by a larger extratropical storm over the Cochrane District of Ontario.