Irma re-intensified into a Category 3 hurricane at 12:00 UTC on September 3 as it moved southwestwards under the influence of the ridge. Beginning early the following day, the eye began increasing in size and improving in appearance, while deep convection surrounding the eye became more symmetrical. Additionally, the hurricane headed into warmer sea surface temperatures as it approached the Leeward Islands. As a result, Irma underwent another period of rapid intensification, reaching Category 4 intensity at 18:00 UTC on September 4 and Category 5 status at 12:00 UTC on September 5. The extremely powerful hurricane grew in size and became increasingly symmetric, acquiring annular characteristics, with mesovortices being observed within the well-defined eye, signaling the continued intensification of the storm. Sustained winds peaked at 180 mph (285 km/h) near 18:00 UTC on September 5 – although it was operationally assessed at 185 mph (295 km/h). Early on September 6, the storm reached its minimum barometric pressure of just before 06:00 UTC, an estimate based on a dropsonde surface pressure observation and measurements from Barbuda and Saint Barthélemy. This was the lowest barometric pressure in the Atlantic since Dean in 2007.