Tuesday, September 11, 2012

Hurricane Nadine (2012)

Storm Active: September 11-October 4 (21.75 days as a tropical cyclone)

On September 7, a low pressure system emerged off of the western coast of Africa. The system moved over the Cape Verde Islands the next day, bringing some periods of heavy rain and gusty winds to the islands. The low slowly gained organization, with convection becoming closer to the center over the next few days as it moved westward.

Dry air briefly hindered development somewhat on September 10, but the system became slightly better organized on September 11, meriting the designation Tropical Depression Fourteen late that morning. The system turned toward the northwest that afternoon, as a weakness developed in the ridge to its north.

A central overcast developed late that night, and the central pressure dropped, so Fourteen was upgraded to Tropical Storm Nadine. Steady strengthening occurred over the next day as convection deepened, bringing the system to near hurricane strength by September 13. However, strengthening leveled off that day, as Nadine came under the influence of some shear out of the southwest. On September 14, Nadine skated around the periphery of a subtropical ridge, turning to the north and northeast by that evening, still maintaining strong tropical storm intensity.

During that day, the direction of the wind shear affecting Nadine had shifted and upper-level winds began to blow out of the west, allowing the cyclone to become slightly better organized. Therefore, Nadine was upgraded to a hurricane overnight. Another ridge began building to the north of the cyclone the next day, and it was pushed into an eastward motion on September 15. Since it was moving away from the source of the shear, it was able to maintain Category 1 intensity, despite a slight elongation of the circulation, and achieved winds of 80 mph and a pressure of 983 mb.

Some changes occurred within the circulation of Nadine on September 16. The western side of the cyclone eroded considerably, and though the circulation became less tilted, satellite images indicated that the system had weakened slightly as cooler waters began to take their toll. On September 17, Nadine became embedded in the flow of a trough over the Azores, and it turned to the northeast once again, with a decrease in forward speed. The cyclone weakened to a tropical storm, as its inner core became all but void of convection that morning, but a flare up of deep thunderstorms to the north of the center allowed the system to maintain strong tropical storm intensity through the evening.

Nadine continued to persist as tropical on September 18, maintaining an eyewall on the north side of the center and fairly prominent rain bands. Additionally, surf increased in the Azores as the center as the cyclone approached. By this time, Nadine was, in many respects, a hybrid system. The cyclone exhibited some extratropical properties such as resilience to strong shear, but still maintained a tropical-like central structure. This unusual combination allowed Nadine to survive the marginal conditions of the northeast Atlantic.

On September 19, a blocking pattern formed over the northeast Atlantic, a very rare event. This caused Nadine to remain nearly stationary through the afternoon, and eventually to move slowly to the east-southeast, actually increasing in organization on September 20 as its central pressure dropped to 981 mb, a new low for the system.

Overnight, a cold front that had exited the east coast about six days earlier began to interact with the circulation of Nadine, pushing it to the south at a slightly faster speed. The front also caused it to lose some tropical characteristics, and its convection to decrease. However, on September 21, the cyclone developed a prominent banding feature, and though the windfield had expanded, it was not yet non-tropical. Therefore, during the afternoon of that day, Nadine was reclassified a subtropical cyclone. Such an event, a transition from tropical to subtropical, is somewhat rare, and Nadine was only the fifth Atlantic cyclone to do this since subtropical storms were introduced in 1968.

However, as Nadine continued to lose convection overnight, it no longer qualified as even a subtropical cyclone and was downgraded to a remnant low. On September 22, the system regained much of its lost convection as it drifted southward over warmer waters, though most was displaced to the north of the center. By the morning of September 23, satellite classifications indicated that Nadine had regenerated into a tropical cyclone, continuing the lifetime of this unusual storm.

After its regeneration, it lost nearly all the convection it had recovered, but retained just enough to remain tropical into September 24. A trough of low pressure to the northeast of Nadine continued to cause wind shear that day, but upper-level winds gradually lessened as the storm moved generally westward, steered by another ridge to its north.

On September 25, Nadine weakened slightly, but its outflow greatly improved in the favorable upper-atmospheric environment. Also, a prominent central void appeared on infrared imagery that morning, closely resembling an eye, despite the fact that the cyclone was only a weak tropical storm. That evening, the system became more organized as stronger bands developed and the eye contracted, and the system strengthened slightly.

On September 26, the cyclone was steered southward under the influence of the ridge, and convection increased further, causing gradual strengthening over the next day, and bringing Nadine to strong tropical storm intensity by the afternoon of September 27. Overnight, Nadine steered around the edge of the ridge, and began to accelerate to the northwest, and ultimately north-northwest. Meanwhile, Nadine began to interact with an upper-level low to it west, which actually helped to shield it from the worst of the shear. During the morning of September 28, an eye began to sporadically appear on visible imagery, and Nadine was upgraded to a hurricane.

Nadine fluctuated in intensity over the next day, as its interaction with the low altered the contour of the circulation. The system weakened to a tropical storm, and then regained hurricane strength for a third time, as it moved north-northwestward across its former path through the Atlantic and completed a loop, bringing it back to a position it occupied 12 days earlier.

Early on September 30, Nadine increased further in organization, as the eye became better defined and more circular, bringing Nadine to its peak intensity of 90 mph winds and a pressure of 978 mb that afternoon. At the same time, the forward motion of Nadine decreased, and it began a turn to the west as an oncoming trough blocked its progress. The same trough began to erode the northern hemisphere of the circulation that evening, and induced some steady weakening, as Nadine also moved over cooler waters.

The cyclone began a counterclockwise loop overnight, turning to the southwest, and soon weakening to a tropical storm. On October 1, Nadine recovered some of its convection, and maintained strong tropical storm intensity over the next day as it gradually made a turn to the east.

However, shear increased significantly over the next day as Nadine became entrenched in the flow of a trough and began to accelerate eastward. On October 3, the center became separated from the deep convection as the latter was displaced to the southeast, and Nadine weakened to a low-end tropical storm. Overnight, the system accelerated further in the northeast direction, and affected the Azores with thunderstorms and gusty winds as it passed near the islands for the second time. By the morning of October 4, the circulation of Nadine was indistinguishable from the oncoming trough. The long-lived cyclone had finally dissipated.

Nadine's lifetime spanned 23 days, and the cyclone spent 21.75 days as a tropical cyclone, placing it fifth on the list of all-time longest lasting Altantic hurricanes. Its longevity was a product of weak steering over the northeast Atlantic and a resilient structure that allowed the circulation to survive repeated transitions from tropical to extratropical, and even subtropical.

Nadine near peak intensity as a Category 1 hurricane over the north Atlantic. A trough to the north is also visible. This trough would block Nadine and cause it to perform a final loop before dissipation.

Track of Nadine.

Tuesday, September 4, 2012

Hurricane Michael (2012)

Storm Active: September 3-11

On September 1, a trough of low pressure formed in the eastern Atlantic. Over the next day, it began to interact with an upper-level low, the resulting union producing a wide area of isolated showers activity. Later on September 2, a surface low formed at the southern edge of the trough, accompanied by a small area of dense overcast.

Though upper-level winds were initially unfavorable, the circulation organized fairly quickly, and by September 3, the convection had increased enough for the low to be designated Tropical Depression Thirteen.

Up to that point, the system had been moving slowly west to west-southwest as a dip in the jet stream prevented it from accelerating significantly. After formation, Thirteen began to drift northwestward, and eventually northward in the wake of a trough to its north.

Some shear was evident on the west side of the circulation through September 4, but the compact system maintained its integrity and increased slightly in deep convection that day, keeping the center of circulation close to the most intense thunderstorm activity. As a result, the cyclone was upgraded to Tropical Storm Michael.

Overnight, due to the approach of an anticyclone from the west, the tropical storm turned to the northeast. However, its forward speed was still quite slow for a cyclone of its latitude, as it was also hemmed in by the Bermuda high to its east. As the low to Michael's north moved away, shear diminished, and allowed Michael to steadily strengthen through September 5. By the afternoon of that day, an eye feature had made a brief appearance, but it was quickly replaced by a central overcast. These factors supported an intensity near hurricane strength that evening.

During the evening, Michael's center contracted, and the cyclone underwent rapid intensification, bringing the cyclone to major hurricane strength by the morning of September 6, the first of the season. That afternoon, the system had reached its peak intensity of 115 mph winds and a pressure of 964 mb. Michael had expanded somewhat by this time, and had an extremely well-defined eye. However, as the cyclone continued to move northeast, sea surface temperatures began to decline, and Michael began to gradually weaken.

On September 7, Michael gradually made a turn to the northwest, but remained in an area of weak steering, and moved slowly, still maintaining Category 2 intensity for the next day. On September 9, the ridge to Michael's north strengthened considerably, and the cyclone turned toward the west, and even west-southwest that evening. The eye was still prominent, but convection was degrading by this time, especially in the northwest quadrant. Therefore, Michael weakened to a category 1 hurricane.

On September 10, Michael navigated around the western edge of the ridge to its north, and made a sharp turn to the north over the following day. Around the same time, shear increased sharply in the vicinity of Michael, partly due to the outflow of Tropical Storm Leslie. The eye disappeared for the last time, and Michael weakened to a tropical storm. By September 11, all convection had been ripped away from the system by upper-level winds, and the system was declared extratropical that afternoon, and absorbed the next day.

Michael at peak intensity as a low-end Category 3 hurricane.

Track of Michael.