With water around 28°C (83°F) and low wind shear, the cyclone continued to rapidly intensify on the morning of September 18. The intensity leveled off somewhat that afternoon as a strong Category 2 hurricane with winds of 110 mph (175 km/h). Late in the evening of September 18, Karl attained Category 3 status, becoming the sixth major hurricane of 2004. On September 19, Karl continued to strengthen, maintaining a well-defined eye, and strengthening late that day into a Category 4 hurricane with winds of 135 mph (215 km/h) and a minimum pressure of 948 mbar. Early on September 20, the storm weakened slightly as a result of an eyewall replacement cycle, weakening back to a strong Category 3 hurricane. At this time Karl began to turn sharply northward into a weakness in the subtropical ridge.