Sunday, August 24, 2014

Hurricane Cristobal (2014)

Storm Active: August 23-29

On August 16, some scattered thunderstorms developed in association with a broad low pressure system situated over the central tropical Atlantic, about halfway between the western coast of Africa and the Caribbean. No further organization occurred until around August 19, when convection became a little more concentrated in association with the system. As the low approached the Leeward Islands on August 21, it remained poorly defined, but began to generate gale force winds. Land interaction limited development over the next day while the low continued west-northwest just to the north of Puerto Rico and Hispaniola. However, on August 23, the system slowed in forward speed and turned northwest towards the Bahamas. That afternoon, though the circulation remained somewhat broad, banding on the north and east sides of the circulation improved to the point that advisories were initiated on Tropical Depression Four.

Some wind shear was affecting the system, but ocean waters near the Bahamas were very warm, and deep convection gradually increased near the center of circulation. By the morning of August 24, the depression had strengthened into Tropical Storm Cristobal. Over the next day, the system moved slowly northward, bringing sustained heavy rain to portions of the Bahamas. Though shear from the north exposed the circulation of Cristobal during the early morning of August 25, the center reformed southeast of its former position, bringing it closer to the convective canopy, and allowing modest strengthening through that morning.

During that day, the system meandered slowly to the north and east, moving away from the Bahamas. Meanwhile, though thunderstorm activity developed closer to the center, Cristobal's overall satellite presentation remind disorganized. The tropical storm was interacting with a frontal boundary, giving the associated convection a more linear than circular pattern. Despite this, winds increased that night and Cristobal became a category 1 hurricane. The system acquired a faster and more definite northward motion on August 26. Atmospheric conditions became more favorable over the next day, allowing Cristobal to develop a more organized, albeit asymmetric, structure. By the morning of August 27, the cyclone had developed a partial eyewall in the northwest semicircle. Though shower activity to the south and east was still scarce, small intensification accompanied this improvement in structure.

Around noontime, Cristobal passed well to the west of Bermuda, bringing scattered showers, gusty winds, and dangerous ocean conditions to the island. Influenced by mid-latitude winds, the system then began its turn towards the northeast, accelerating rapidly. At the same time, the cyclone assumed a more symmetrical appearance. As the hurricane raced to the north and east over the next day, it acquired an eyewall and even an intermittent eye feature during the afternoon and evening of August 28. Cristobal reached its peak intensity that night above 40°N, with 85 mph winds and a minimum pressure of 970 mb.

That evening, however, the system moved over much cooler water, and began extratropical transition as it moved northeast past Nova Scotia with a forward speed of nearly 50 mph. By the morning of August 29, all remaining convection was displaced well to the north of the center. Further, the wind field had broadened, indicating that the cyclone was no longer tropical. The low that had been Cristobal maintained powerful winds for several days, and eventually affected Iceland.

Hurricane Cristobal reached its peak intensity less than a day before extratropical transition, briefly developing a well-defined eye.

Cristobal recurved off of the east coast of the United States, and did not have a significant impact on any landmasses as a tropical cyclone.

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.