Saturday, October 27, 2018

Hurricane Oscar (2018)

Storm Active: October 26-31

On October 23, an area of low pressure a few hundred miles east-northeast of the Lesser Antilles began to produce some shower and thunderstorm activity. Over the next few days, the disturbance moved slowly northward and atmospheric conditions for development improved. As westerly shear diminished, convection persisted closer to the center of the developing low. Late on October 26, the low-level center had become well-defined. However, due to its interaction with a nearby upper-level low, the system was classified Subtropical Storm Oscar. This was the seventh named storm in 2018 to be subtropical during some part of its lifetime, making 2018 the first known season for such an occurrence.

Interaction with the upper-level low caused Oscar to turn sharply westward on the 27th. Meanwhile, significant deepening took place, indicating that Oscar's maximum winds had increased. Oscar's structure evolved throughout that day until the cyclone possessed a small core of deep convection and maximum winds close to the center. As a result, it was reclassified as a tropical storm. A ridge pushed the system south of west on the 28th and favorable conditions allowed an eye feature to begin forming. Around the same time, Oscar strengthened into a hurricane. The trend of gradual intensification persisted on October 29 and the system became a category 2 that evening, reaching its peak intensity of 105 mph winds and a pressure of 970 mb. Meanwhile, a mid-latitude frontal system approaching from the west began to sheer the cyclone toward the north.

By the 30th, Oscar was picking up speed toward the north and north-northeast and began to encounter cooler waters as it passed east of Bermuda. As a result, deep convection near the center waned, the maximum winds dropped, and the system began to take on extratropical characteristics. Nevertheless, it remained a potent cyclone and brought rough surf to Bermuda that day. Around midday on October 31, Oscar transitioned to a hurricane-strength extratropical storm as it sped north-northeastward over the open Atlantic. The post-tropical low deepened over the north Atlantic during the following days, reaching a minimum pressure of 950 mb on November 2. It dissipated a few days later near Iceland.

This image shows the small hurricane Oscar at peak intensity as a category 2 hurricane.

Oscar did not directly affect any land areas during its time as a tropical or subtropical cyclone.

Tuesday, October 9, 2018

Tropical Storm Nadine (2018)

Storm Active: October 9-12

A late-season tropical wave entered the Atlantic ocean on October 6 and began to show signs of development. Waters in the eastern Atlantic were still fairly warm and shear was low, so organization proceeded fairly quickly. By October 8, convection had wrapped nearly around the disturbance, but it still lacked a closed circulation. The next day, it formed into Tropical Depression Fifteen. Within a few hours, satellite intensity estimates supported its upgrade into Tropical Storm Nadine. Nadine formed unusually late in the season for a system so far east in the tropical Atlantic.

The cyclone was fairly small, and hence prone to rapid changes in intensity. Over the next day, it took advantage of a favorable environment and strengthened quickly to its peak intensity as a strong tropical storm with 65 mph winds and a pressure of 997 mb on October 10. However, wind shear sharply increased that night and displaced all of Nadine's convection to the east of the center by October 11. As a result, the storm decayed rapidly as it moved northwest. The next day, Nadine dissipated over the central Atlantic.

Nadine was a small cyclone that quickly succumbed to unfavorable atmospheric conditions a few hours after formation.

The short-lived Nadine did not affect any land areas, but was an unusually late-season storm to form in the central tropical Atlantic.

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.