Right 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 strength, becoming the third major hurricane of the season. An eyewall replacement cycle inhibited strengthening of its maximum winds for about 18 hours, but nearly doubled the size of the storm as a result. The storm then began a second period of rapid intensification starting at 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 145 mph (233 km/h). It became a Category 5 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 175 mph (280 km/h), gusts of 215 mph (344 km/h) and a central pressure of 902 mbar (26.64 inHg). The minimum pressure made Katrina, at the time, the fourth most intense Atlantic hurricane on record (Hurricanes Rita and Wilma would later beat Katrina's record that same year). As the hurricane came closer New Orleans, the Weather Forecast Office in Slidell, Louisiana gave out two strongly-worded warnings of the storm's danger.