Wednesday, August 30, 2017

Hurricane Irma (2017)

Storm Active: August 30-September 12

On August 28, another vigorous tropical wave emerged off of the coast of Africa. Saharan dry air did not hamper it as it had its predecessors, and the wave maintained thunderstorm activity as it passed near the Cape Verde Islands on August 29, bringing locally heavy rain and gusty winds. Before long, its circulation became better-defined and it developed gale-force winds. As a result, advisories were initiated on Tropical Storm Irma during the morning of August 30. Already, the storm had 50 mph sustained winds, and strengthening continued steadily as Irma's inner core became better defined. The system turned a bit toward the west-northwest on August 31, and an eye feature suddenly appeared on satellite imagery. Irma was undergoing extremely rapid intensification, and was upgraded directly from a tropical storm to a category 2 hurricane and then a major hurricane by late afternoon.

The circulation was quite compact, with hurricane-force winds very close to the center. As is often true with small cyclones, Irma was subject to short-term fluctuations in intensity. A few eyewall replacement cycles, in which the eye clouded over temporarily and then reappeared, occurred between August 31 and September 2. This caused Irma to alternate between category 2 and category 3, an impressive intensity as it traversed only marginally warm waters. Meanwhile, the subtropical ridge to its north began to build southward, and the cyclone turned west and even slightly south of west by September 2. Over time, this brought the system toward warmer waters and moister air. By September 3, though Irma's intensity remained in the low-end category 3 range, the cyclone was beginning to grow larger, displaying outer banding features. That evening, Irma's central pressure began to decrease. This trend continued into September 4, when the maximum winds began to strengthen as well.

Meanwhile, Irma began to round the bottom of the subtropical ridge and assume a due westward path near 17° N. This brought it over areas of higher oceanic heat content and even higher atmospheric humidity levels. As a result, intensification once again increased in speed: Irma became a category 4 that afternoon. During the evening, its eye cleared and became quite large in the wake of another eyewall replacement. During the morning of September 5, yet another burst of intensification brought it to category 5 strength, with winds of 175 mph. This was the farthest east such high wind speeds had ever been recorded in the Atlantic. The outer part of Irma's circulation began to affect the northeast Caribbean islands that day. The trend of remarkable deepening continued as the system turned west-northwest, bringing the hurricane to a peak intensity of 185 mph winds and a minimum pressure of 914 mb. These winds were tied for the second-highest ever recorded in an Atlantic hurricane, and the pressure was the lowest known in the Atlantic outside of the Caribbean and Gulf of Mexico. That night, the cyclone passed directly over the northern Leeward Islands.

Despite the continuing evolution of the inner core, Irma did not undergo an eyewall replacement on September 6, and maintained its incredible category 5 intensity, though with some pressure fluctuations. The center passed through the U.S. Virgin Islands and then just north of Puerto Rico that evening. Overnight, the circulation was a bit disrupted from land interaction, and the winds began to slowly decrease as Irma moved west-northwest just north of Hispaniola. That evening, the center of circulation passed among the Turks and Caicos Islands. During the night, the outer eyewall of Irma finally got the better of the inner, weakening the maximum winds down to category 4 strength, but expanding the overall windfield. The system moved west-northwest between Cuba and the Bahamas the morning of the 8th as a strong category 4 hurricane. Late in the afternoon, Irma turned back toward the west and approached the northern coast of central Cuba. Just before making a direct hit on the northern archipelago of Cuba, the system briefly regained category 5 strength, reaching winds of 160 mph and a minimum pressure of 924 mb.

After passing over the northern islands, the center of circulation did not pass inland, but rather turned just north of west and paralleled the coast overnight, slowing down as it did so. Land interaction took a significant toll on the storm for the first time early on September 9, dropping Irma down to high-end category 3 strength. The eye wobbled a great deal along its path during the day, but the overall motion was a turn toward the north-northwest by the evening. Meanwhile, as conditions improved in Cuba, they deteriorated in Florida, as intense outer bands swept across much of the peninsula. These include tropical storm conditions, flooding rains, and several reports of tornadoes. During the night, Irma regained category 4 intensity, reaching a final peak of 130 mph winds and a pressure of 928 mb before passing over the Florida Keys early on September 10. Increasing wind shear and land interaction steadily weakened Irma from then on. It made landfall as a category 3 hurricane in southwestern Florida that afternoon. As with many sheared landfalling systems, Irma became lopsided, with almost all rainfall occurring north of the center, and the northern eyewall much more intense than the southern. The size of the system was such that, located over western Florida, it brought tropical storm force winds from the Florida panhandle to parts of Georgia and even extending well into South Carolina.

On September 11, Irma weakened to a tropical storm while centered over the northwestern Florida Peninsula, having caused more than 6 inches of rain throughout much of the state. By the afternoon, the center had pushed into Georgia and rains spread northwestward as they ended in Florida. Overnight, the system weakened to a tropical depression and then became post-tropical. In addition to those already mentioned, Irma set many other records. Its 3.25 days spent as a category 5 hurricane were tied for the most ever recorded in the Atlantic with the 1932 Cuba hurricane. Its category 5 landfalls in the Leeward Islands and Cuba were among the strongest ever recorded. Finally, Irma maintained 185 mph winds for a period of 37 hours, which was the most ever recorded in the entire world.

This image shows Hurricane Irma at peak intensity on September 6 as it passed directly over the northern Leeward Islands, causing catastrophic damage.

Irma's long track as a powerful hurricane brought devastating impacts to the northern Leeward Islands, the U.S. Virgin Islands, the Turks and Caicos Islands, Cuba, and Florida.

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