Immediately after the storm entered the Gulf of Mexico, the low wind shear, good upper-level outflow, and the warm sea surface temperatures of the Gulf Loop Current caused Katrina to intensify rapidly. On August 27, the storm was upgraded to Category 3 intensity, becoming the third major hurricane of the season. An eyewall replacement cycle disrupted the intensification of maximum winds for about 18 hours, but almost doubled the radius of the storm. A second period of rapid intensification started by 7:00 p.m. CDT on August 27, and by 12:40 a.m. CDT on August 28, Katrina was upgraded to a Category 4 hurricane with maximum sustained winds of . It became a Category 5 (the first in the Gulf of Mexico since Hurricane Allen 25 years prior) storm by 7:00 a.m. CDT, twelve hours after the beginning of the second round of rapid intensification, and reached its peak intensity at 1:00 p.m. CDT with maximum sustained winds of , gusts of and a central pressure of 902 millibars. The minimum pressure made Katrina, at the time, the fourth-most intense Atlantic hurricane on record (Hurricanes Rita and Wilma would later surpass Katrina that same year). As the hurricane approached New Orleans, the Weather Forecast Office in Slidell, Louisiana issued two strongly worded warnings of the storm's danger.