Thursday, September 11, 2014

Hurricane Edouard (2014)

Storm Active: September 11-19

On September 7, a tropical low emerged off of the coast of Africa, already showing signs of organization as it moved west. Though a broad circulation was evident in association with the system from the beginning, convection remained decentralized through the next few days. On September 8, the system passed to the well south of the Cape Verde Islands, with minimal impacts. At the same time, the low began a gradual turn toward the northwest, exploiting a weakness in the Bermuda High. Upper-level winds prevented development through September 10. Thereafter, shear abated, allowing the low to acquire organization. By the morning of September 11, the appearance of banding features and a better-defined circulation merited the classification of the system as Tropical Depression Six.

Overnight, denser convection developed near the center, and the cyclone was upgraded to Tropical Storm Edouard. Meanwhile, vertical shear kept the center near the southwestern edge of the convective canopy. The circulation of Edouard gained definition over the next day, leading to some modest intensification as the central pressure dropped and the outflow improved. Even though September 13, however, dry air continued to enter the system from the south, fighting the development of a central dense overcast. But upper-level winds continued to become more favorable and waters were anomalously warm, resulting in continued strengthening. During the morning of September 14, an eye made a brief appearance on visual imagery, and Edouard was upgraded to a category 1 hurricane.

During that day, only the entrainment of dry air, which disrupted the formation of a full eyewall, prevented the rapid intensification of the cyclone. Still, the central pressure dropped considerably that evening and overnight. By the morning of September 15, Edouard had become a category 2 hurricane. Later that day, the system began to navigate around the western edge of a subtropical ridge and assumed a more poleward motion. Edouard developed a larger and more symmetric eye during the afternoon while rain bands extended farther from the center, especially on the western side of the circulation. Overnight, the cyclone lingered just below major hurricane strength.

During the morning of September 16, Edouard strengthened into the first major hurricane of the 2014 Atlantic season, and in doing so reached its peak intensity of 115 mph winds and a central pressure of 955 mb. Meanwhile, the storm made its closest approach to Bermuda, passing just over 400 miles to the east as it began to curve towards the north and northeast that evening. As it encountered less favorable thermodynamic conditions, Edouard slowly decreased in convection and weakened. However, the circulation remained vigorous through September 17 as the system accelerated toward the northeast. That afternoon, Edouard's convective banding structure briefly became concentric, when an inner eyewall and a larger circular rain band beyond it. This resulted in a relatively gentle pressure gradient out from the center. Therefore, though the central pressure of the cyclone was quite low, its winds were only that of a category 1 hurricane.

As Edouard moved over progressively cooler water during the next day, however, the eyewall began to decay and gradual weakening continued. By late morning on September 18, the system had reached the north edge of a mid-level ridge, and was heading due east. It became a tropical storm that afternoon. Over the next day, wind shear increased significantly, stripping all convection from the circulation and displacing it to the southeast. By the afternoon of September 19, Edouard had become post-tropical.

The above image shows Hurricane Edouard on September 16, shorting after becoming the first major hurricane of the 2014 season.

During its time as a tropical cyclone, Edouard did not affect any landmasses.

Monday, September 1, 2014

Tropical Storm Dolly (2014)

Storm Active: September 1-3

A tropical wave formed over the Central Atlantic around August 23 and began to produced disorganized shower activity as it moved westward. Atmospheric conditions remained hostile for development along the path of the system for nearly a week as it moved generally westward into the Caribbean. Even as upper-level winds improved on August 30, the tropical wave began to interact with the Yucatan Peninsula, inhibiting further development. Convection increased the next day even as the center of the developing low pressure center moved over land. By September 1, the broad low pressure system had moved west-northwestward into the Bay of Campeche, where a tighter circulation appeared. Though the center had primarily left the deep convection behind over the Yucatan, the system was organized enough by that afternoon to be classified Tropical Depression Five.

Initially, the only significant rain band associated with Five was well south and east of the center of circulation, in part due to moderate wind shear out of the north-northwest. However, convection flared up near the center of circulation overnight. Meanwhile, the center itself underwent several reformations, shifting the position of the system significantly to the north. By the morning of September 2, organization had increased significantly, and the system strengthened into Tropical Storm Dolly. That afternoon, Dolly's instability continued as the center reformed once again, this time significantly south of its former position. This movement made the cyclone more symmetric, and the central pressure decreased to 1002 mb, a minimum for Dolly. Near midnight, the system made landfall in Mexico.

The main threat associated with Dolly was flooding rains, which portions of Mexico received in abundance, for even as Dolly rapidly weakened into a tropical depression during the morning of September 3, new rain bands continued to appear over water and move inland. 5-10 inches of rain were expected for much of the affected area, with up to 15 inches locally. The assembly line of moisture continued through the next few day even though Dolly dissipated within 12 hours of landfall, fueled in part by the nearby Hurricane Norbert in the East Pacific.

The above image shows Tropical Storm Dolly on September 2.

The above image shows the track of Dolly.