 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 . 
