Sunday, October 7, 2018

Hurricane Michael (2018)

Storm Active: October 7-12

During the first few days of October, a broad area of low pressure developed in the southwestern Caribbean, with associated showers and thunderstorms extending from central America all the way to Jamaica and Haiti. Such systems are common in this region in the autumn, and are known as Central American Gyres (CAGs). CAGs tend to bring heavy rainfall to a wide area of central America, which held true in this case. In addition, they can sometimes spawn tropical cyclones. Nevertheless, the large circulation of a CAG takes time to consolidate, and the system only slowly organized as it moved northwestward. By October 6, the center of the low was just north of Honduras and the region of strong upper-level winds that had been affecting the system retreated to the north. This allowed further organization, and a flare up of organized thunderstorm activity led to the classification of Tropical Depression Fourteen early on October 7.

Even immediately after formation, the storm had an impressive satellite signature, with very cold cloud tops to the east of the center of circulation. Soon after, satellite and aircraft data indicated an immense radius of gale force winds, extending over 200 miles from the center in some quadrants, and the depression was upgraded to Tropical Storm Michael. Under the influence of some westerly shear, the center of Michael underwent some reformations toward the east that day, but the large cyclone strengthened steadily into the evening as it moved slowly northward. Already, the outer rainbands were hitting the southern tip of Florida, even though the center was still just east of the Yucatan Peninsula. By the morning of October 8, Michael had strengthened into a hurricane.

During that day, the inner core gradually became more organized and the cyclone steadily intensified as it passed near the western tip of Cuba. A large eyewall struggled to surround the center throughout the day, but coverage increased during the evening. The storm became a category 2 early on October 9. The system gained some forward speed toward the north that day and the warm waters of the Gulf of Mexico supported extremely intense convection. Shear also lessened, and the eyewall became complete early that evening, bring Michael to category 3 strength. The outer bands of the storm were now crossing the Gulf coastline, but proximity to land did nothing to slow the system's intensification. A symmetric eye cleared out on satellite imagery overnight and Michael rocketed to category 4 status, deepening even when the center was within 50 miles of landfall. The powerful cyclone reached a peak intensity as a category 5 hurricane with 160 mph winds* and a pressure of 919 mb when it slammed into the Florida Panhandle around 1:00pm local time on October 10. In terms of pressure, Michael became the third strongest hurricane ever on record to make landfall in the United States, behind only Hurricane Camille of 1969 and the Labor Day Hurricane of 1935. At the time, it also broke the top 10 overall list for strongest landfall recorded for an Atlantic hurricane. Furthermore, it was the only category 5 ever known to hit the Florida panhandle.

The storm surge that Michael brought to the coastline was unprecedented and record-braking in some areas and the wind damage was catastrophic, though the worst of it was confined to quite a small area where the eyewall made landfall. However, the rainfall was not especially severe, as the system accelerated northeastward as it moved inland and did not linger. In fact, the system entered southwestern Georgia before losing major hurricane status, thus becoming the first major hurricane to impact the state since 1898. Nevertheless, the core did rapidly deteriorate once inland, and Michael weakened to a tropical storm early on October 11 over central Georgia. Later that day, it moved through the Carolinas, bringing heavy rain and wind to regions inundated by Florence's rains the previous month. Fortunately, the storm was moving so fast that the flooding impacts were not as severe as they otherwise would have been. The cyclone emerged over the Atlantic near the border of North Carolina and Virginia and became post-tropical early on October 12. The system crossed the Atlantic and eventually brought some rain and wind to western Europe on October 15.

The above image shows Hurricane Michael making landfall in the Florida panhandle at category 5 intensity. This was among the strongest landfalls ever recorded for an Atlantic hurricane.

Michael's speedy development amid only marginally favorable conditions and rapid strengthening prior to landfall were very unexpected and demonstrate how far there is to go in modeling intensity changes in tropical cyclones.

*Note: Hurricane Michael was operationally identified as a top-end category 4 hurricane with 155 mph winds at landfall, but this was changed to 160 mph (category 5 strength) in post-season analysis.

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