After intensification stopped on September 20 due to the eyewall cycle and slightly increased wind shear, Karl quickly restrengthened that evening over very warm water, and early on September 21 reached its peak intensity as a powerful Category 4 hurricane with 145 mph (230 km/h) winds and a minimum central pressure of 938 mbar. That morning, as Karl was moving northward, another eyewall replacement cycle began to take place and vertical shear increased, again weakening the storm temporarily. The trend continued into the evening, and the storm weakened to a Category 2 hurricane by early on September 22. The weakening trend slowed and eventually leveled off that afternoon with Karl remaining a Category 2 hurricane as the storm turned towards the north-northeast. Late that evening, Karl began to restrengthen once again as wind shear diminished. The intensification continued into the morning of September 23, as Karl became a major hurricane a second time, reaching a final peak of 125 mph (205 km/h) winds.