Friday, August 1, 2014

Hurricane Bertha (2014)

Storm Active: July 31-August 6

On July 24, a tropical wave entered the east Atlantic. By July 27, a large area of disorganized shower activity had developed in the vicinity of the system. Convection continued to increase over the next two days, and a circulation became evident in the southeastern portion of the tropical wave early on July 29. Later that day, the low began a turn toward the west-northwest, and entered a drier air mass. Thunderstorm activity decreased markedly that night and into July 30. However, the circulation itself continued to become better defined and the low deepened into July 31. By this time, gale force winds were occurring in portions of the circulation, and only a lack of convection prevented the classification of the system as a tropical storm, since shower activity remained confined to the south and east of the center. Hurricane hunter aircraft which investigated the system noted winds up to 45 mph that afternoon near the center of circulation, even though these areas were devoid of thunderstorms. Finally, that evening, an area of convection appeared and persisted near the system's center, and advisories were initiated on Tropical Storm Bertha.

Even as Bertha entered a more moist and unstable atmosphere, promoting convective development, higher wind shear began to affect the system, restricting thunderstorm activity to the eastern half of the circulation. For a period on August 1, the center became completely exposed due to incoming shear, as Bertha's quick motion towards the west-northwest made it even more difficult for the cyclone to maintain convective coverage. Amidst these fluctuations, however, the circulation remained intact, and the storm entered the Caribbean Sea that afternoon. On August 2, heavy rain bands just north of Bertha affected Puerto Rico and the neighboring islands as the center passed to the south.

As interaction with land began that day, Bertha became even less organized, and a closed circulation was nearly undetectable, even on Doppler radar imagery. During the late afternoon, the system passed over the eastern Dominican Republic, bringing localized heavy rain and gusty winds. By this time, the cyclone had navigated around the southern edge of a ridge to its north, and therefore turned towards the northwest.

On August 3, Bertha brushed the easternmost Bahamas before moving away from land, and in addition away from hostile wind shear. For the first time since its formation, the tropical storm was able to develop healthy outflow and a central dense overcast near the center. When this occurred late that night, Bertha intensified rapidly: its central pressure dropped from 1012 to 999 mb in 12 hours, and winds approached hurricane strength. During the morning of August 4, through Bertha appeared quite disorganized on satellite imagery, a more intense eyewall and slight hints of an eye feature appeared at the center of circulation, even though there was no convection north of the eyewall! However poor the visual presentation, aerial data indicated that the cyclone now had hurricane force winds. Bertha thus reached its peak intensity as a category 1 hurricane with winds of 80 mph and a central pressure of 998 mb.

The system began to accelerate northward and north-northeastward that evening, and began to weaken as an oncoming frontal boundary greatly increased wind shear. By the morning of August 5, the center of circulation was intermittently exposed as thunderstorm activity was repeatedly displaced, and Bertha quickly lost strength, again becoming a tropical storm. Over the following day, the system weakened further and accelerated to the northeast away from the east coast of the United States. The system became extratropical on August 6.

Hurricane Bertha reached category 1 hurricane status (shown above) without much convective organization.

Bertha curved around the edge of a subtropical ridge and therefore missed landfall on the U.S. East Coast.

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