By the time Keith became a tropical storm, it was beginning to undergo rapid deepening, fueled by warm waters, low wind shear, and an organizing cloud pattern. There initially was uncertainty in the storm's future movement, due to a ridge in the Gulf of Mexico. Tropical cyclone prediction models differed on their assessments; three models anticipated a turn to the northeast toward Florida due to a trough, while others predicted a continued slow motion to the west-northwest, eventually reaching the Bay of Campeche. After continued strengthening, Keith became a hurricane on September 30, just 18 hours after becoming a tropical storm, and making it the seventh such storm of that intensity of the season. That day, an eye began developing as the structure became much better organized. In a 13‑hour period beginning at 1808 UTC on September 30, Keith underwent explosive deepening, as its barometric pressure dropped at a rate of nearly per hour. The NHC noted that Keith experienced conditions "ideal for strengthening", with the exception of the approaching land interaction with the Yucatán peninsula. The hurricane slowed until stalling offshore eastern Belize, caused by the ridge to the north and the precursor to Tropical Storm Leslie forming over Cuba. By early on October 1, Keith had developed a well-defined eye about in diameter, with what the NHC described as a "spectacular appearance." Later that day, Hurricane Hunters estimated a minimum pressure of and deployed a dropsonde that observed peak winds of . Based on the data, the NHC estimated that Keith attained peak winds of 140 mph (220 km/h) at 0700 UTC on October 1, while just offshore eastern Belize. This made it a Category 4 on the Saffir-Simpson Hurricane Scale.