Lasting from late August to mid-September 2017, the storm was the strongest open-Atlantic tropical cyclone on record and the first Category 5 hurricane to strike the Leeward Islands.

[2] The NHC operationally began advisories at 15:00 UTC that day, with cyclone already at tropical storm status, based on scatterometer data and satellite estimates.

Strengthening was anticipated due to warm sea surface temperatures and low wind shear, and only slightly cooler waters and drier air were foreseen as a hindrance to intensification.

[5] Moving westward due to a mid-level ridge over the eastern Atlantic,[2] the nascent storm began developing upper-level poleward outflow, as an anticyclone became established over the system, with banding features becoming increasingly evident in satellite images.

[6] Early on August 31, a central dense overcast (CDO) and an eye feature developed, indicating that Irma intensified into a Category 1 hurricane around 06:00 UTC, only 30 hours after tropical cyclogenesis.

[2] The first aircraft reconnaissance flight into the hurricane occurred early on September 4; during the mission, an eye 29 mi (47 km) in diameter and surface winds of 115 mph (185 km/h) were observed.

[2] 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.

[14] As a consequence of this internal structure change, Irma weakened to a Category 4 hurricane as it passed south of the Turks and Caicos Islands early on September 8.

[2] At 05:00 UTC, early on September 8, the eye of Irma made landfall on the island of Little Inagua in the Bahamas, with winds of 155 mph (249 km/h) and a central pressure of 924 mbar (27.3 inHg).

Once the eyewall replacement cycle was completed, Irma began to re-intensify, and it re-attained Category 5 intensity at 18:00 UTC that day east of Cuba, as deep convection became more pronounced and organized.

[2] As the eye of Irma moved along the northern coast of Cuba, gradual weakening ensued due to land interaction,[15] with the eye becoming cloud-filled and the intensity falling to a high-end Category 2 later on September 9 – operationally, it had been assessed as never falling below 120 mph (190 km/h),[16] though it was lowered in post-analysis, possibly due to shoaling interfering with reconnaissance data.

[2] By later that night, the forward speed of Irma had begun to slow down, which allowed it to finally begin moving northwestward towards Florida in response to a complex steering pattern, which involved Irma moving around the southwestern edge of the subtropical high to its northeast and a low-pressure system that was located over the continental United States.

[17] Irma made landfall in Cudjoe Key, Florida at 13:00 UTC on September 10 at Category 4 intensity, with winds of 130 mph (210 km/h) and a central pressure of 931 mbar (27.5 inHg).

[2] Irma finally weakened to a tropical storm around 12:00 UTC that day as it entered southern Georgia and began its transition to a post-tropical cyclone.