Monday, August 29, 2011

Hurricane Katia (2011)

Storm Active: August 29-September 10

A low formed just off of the African coastline on August 27, associated with a tropical wave, and was already showing signs of organization. The broad area of showers and thunderstorms quickly became concentrated over the next day, as the system passed well south of the Cape Verde Islands. Rapid development continued, and the system became Tropical Depression Twelve early on August 29. The depression also formed at 9.4ยบ N, fairly far south for a tropical cyclone. Some shear out of the east affected the system from the beginning, and, as a result, the center remained on the eastern tip of the cloud cover through the day, A ore circular area of convection developed early on August 30, and although shear continued, the system was organized enough to be named Tropical Storm Katia.

Katia adopted a west-northwest motion that morning, and also accelerated somewhat in forward speed. Through the day, shear lessened significantly, and Katia began to rapidly intensify. Deep convection had enveloped the center by that afternoon, and Katia quickly became a strong tropical storm that evening. The intrusion of dry air on the circulation delayed intensification overnight, but Katia resumed a slow strengthening trend by August 31. Meanwhile, the cyclone moved even faster to the west-northwest, under the influence of a ridge to its northeast, its forward speed exceeding 20 mph. Katia continued to slowly organize, and the development of a well-defined eyewall merited the upgrade of the system to a hurricane late that night.

However, some dry air entered the circulation from the south, temporarily weakening the eyewall early on September 1, and causing the cyclone to stabilize in intensity. Katia also returned to a westerly motion that morning. However, shear increased during the day from an upper-level low to Katia's north, due to the close proximity of the low, and Katia again became disorganized weakening back to a tropical storm. A deeper burst of convection appeared with the system early on September 2, but the center remained on the periphery of this cloud cover during the early morning hours. Katia decelerated significantly, and turned once again to the west-northwest. Despite somewhat hostile conditions, Katia regained hurricane status later that morning, as a gradual turn to the northwest began.

Katia continued to struggle against wind shear throughout the day, and even developed an eye for a time that evening! However, the eye remained too close to the edge of convection to survive, and clouded over early on September 3. The cyclone's central pressure dropped, and Katia maintained minimal hurricane intensity. Thunderstorm activity decreased in the eastern half of the circulation that afternoon, and the cyclone became slightly lopsided. As a result, Katia again weakened to a tropical storm. Later in the evening, the fluctuations in intensity continued, as convection once again increased, and another eye formed. Due to this, Katia was once again upgraded to a hurricane during the morning of September 4. Through the night, Katia had still maintained a general northwest motion.

Later that morning, a well-defined eye developed, and Katia underwent rapid strengthening, becoming a Category 2 hurricane. That afternoon, Katia reached an intensity of 105 mph winds and a pressure of 965 mb. However, dry air invaded the system once again, this time from the northeast quadrant, and Katia weakened slightly during the early morning of September 5. Rather than disrupting the circulation in the long term, however, the dry air was incorporated into a large eye that appeared later in the morning. Katia once again strengthened rapidly, as outflow also improved that afternoon. Following these structural changes, the cyclone was subsequently upgraded to a major hurricane. Further intensification ensued late that evening, and Katia quickly reached its peak intensity as a Category 4 hurricane, with 135 mph winds and a pressure of 946 mb.

However, as the storm continued northwest, it began to encounter less favorable conditions, including lower ocean temperatures, and increased wind shear. A general weakening trend began, and Katia soon lost Category 4 status. The hurricane force wind field remained quite broad, though, and even became larger during the morning of September 6. These winds extended up to 55 miles from the center that afternoon. Rip currents and high surf were already beginning to affect Bermuda, and the threat increased on September 7. In the face of dry air and wind shear from the west, the circulation became more lopsided, with any remaining symmetry in the eyewall disappearing by that morning.

Bermuda also received gusty winds and scattered showers, being on the periphery of Katia's powerful east side. By that afternoon, the cyclone's winds had decreased to 85 mph, a Category 1 intensity. A turn to the north followed during the early evening hours, and Katia's forward motion increased. Despite moving into cooler waters, the upper atmospheric conditions near Katia improved that night and it strengthened slightly as it recovered the western half of the eyewall somewhat. The system made its closest approach to Bermuda the following morning, passing well to the west. Once again, Katia's convection actually increased over cool waters during the day of September 8. A turn to the northeast commenced that evening, and Katia's motion rapidly increased. By September 9, the cyclone was speeding away from the New England coast. Extratropical transition began later that day, and Katia finally became extratropical during the morning of September 10, after reaching a forward speed of over 50 mph.

At the time of the last advisory, Katia still packed winds of 80 mph, and became a powerful extratropical cyclone, impacting north portions of the British Isles on September 11 with high winds and rain as it passed to the north. Katia indirectly affected the Lesser Antilles, Bermuda, the east coast of the United States, and the United Kingdom. 2 fatalities, one direct and one indirect, were the result of Katia.

Katia near peak intensity as a Category 4 hurricane.

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