While trekking across the central Caribbean Sea, Janet was only slightly larger than while it was moving over the Windward Islands, with gale-force winds extending out from the center by September 25. Remaining a Category 4 hurricane as it moved erratically westward across the Caribbean, a reconnaissance flight mission during the night of September 25–26 indicated strong rainbands with frequent lightning strikes and a well-defined eye, evidence that the storm was once again rapidly intensifying. As it neared the Yucatán Peninsula on September 26, Janet began accelerating in forward speed. After the reconnaissance flight "Snowcloud Five" was lost while making a penetration into the hurricane's eye, another flight early on September 27 reported a minimum pressure of , with winds in excess of "by a large and incalculable amount." The hurricane was estimated to have intensified to Category 5 hurricane intensity—the highest rating on the modern-day Saffir–Simpson hurricane scale—at 1200 UTC on September 27, while in the western Caribbean Sea. Janet continued to intensify afterwards, eventually reaching its peak intensity with maximum sustained winds of , with gale-force winds having expanded to at least away from the center of the hurricane. Janet maintained peak intensity as it passed over the Swan Islands before making landfall in extreme southern Quintana Roo, Mexico, just east of Chetumal, Mexico. In that city, a barometer indicated a minimum barometric pressure of in the eye of Janet. At the time, this was the second-lowest pressure ever recorded on land in a landfalling Atlantic hurricane, behind the 1935 Labor Day hurricane, though Janet was later surpassed by hurricanes Dean and Gilbert, which also made landfall on the Yucatán Peninsula as Category 5 hurricanes.