Monday, July 31, 2017

Tropical Storm Emily (2017)

Storm Active: July 31-August 1

During the last week of July, a cold front extending from Texas to the northeast U.S. pushed south and east, weakening as it did so. By July 30, the southern half of the cold front had moved over the Gulf of Mexico and stalled. A non-tropical low quickly formed along it just south of the Florida panhandle and moved slowly toward the east. Overnight, thunderstorm activity became clustered around the circulation center and the system became organized enough to merit tropical depression status early on July 31. Just two hours after formation, radar indicated that Tropical Depression Six's maximum winds had risen to 45 mph and it was upgraded to Tropical Storm Emily.

The storm moved just south of east through the morning and made landfall near Tampa, Florida before noon. Heavy rain fell even as the system weakened over land, with the heaviest south of the center. By the afternoon, the system had weakened to a depression. It accelerated and turned toward the northeast overnight and emerged over open Atlantic waters east of Florida early on August 1. Emily's grip on tropical cyclone status was quite tenuous by this time: only scattered bursts of disorganized convection remained. Late that night, it was downgraded to a remnant low. This low continued out to sea over the next several days before dissipation.

The above image shows Tropical Storm Emily making landfall in Florida within 12 hours of formation.

Emily was yet another short-lived tropical storm, the fifth of the 2017 season. In fact, the combined ACE (accumulated cyclone energy) of these storms was the lowest on record for the first five of an Atlantic hurricane season.

Monday, July 17, 2017

Tropical Storm Don (2017)

Storm Active: July 17-18

A mid-July tropical wave crossed westward from the coast of Africa to more than half-way across the tropical Atlantic without generating much thunderstorm activity. However, on July 16, the system began to organize, despite the proximity of dry air. A low pressure center formed shortly afterward, even as convection remained quite limited. During the afternoon of July 17, a curved band developed about the center and the circulation became better-defined. As a result, the low was upgraded to Tropical Storm Don about 500 miles east of the Windward Islands.

Over the next day, Don moved quickly toward the west. It strengthened briefly as a central dense overcast appeared, but increasing shear reversed this slight intensification as quickly as it had occurred. By midday on July 18, Don's disorganized thunderstorm activity was moving over the Windward Islands. That evening, before the system passed over the islands, Don lost its circulation center in the face of strong shear and dissipated. Scattered gale force winds and heavy rain did continue, however, as its remnants entered the Caribbean.

Tropical Storm Don was only a small cyclone for its brief existence, forming as it did on the edge of a dry air mass with limited moisture supply.

Don existed for less than two days before succumbing to high wind shear as it entered the Caribbean Sea.

Thursday, July 6, 2017

Tropical Depression Four (2017)

Storm Active: July 5-7

At the beginning of July, a tropical wave located southwest of the Cape Verde Islands began to organize. The system was moving rather slowly for its latitude over the next several days, allowing it to began circulating more easily than it otherwise would. Slowing development, however, was its interaction with the intertropical convergence zone (ITCZ). Even though this interaction generated a great deal of convection, the disturbance needed to separate from the ITCZ to initiate development. On July 4, the system began to veer toward the west-northwest and gain some latitude. The next day, it acquired a circular area of strong thunderstorms near its center and became Tropical Depression Four over the open tropical Atlantic.

Shortly afterward, however, the system began to feel the effects of a Saharan dry air encroaching from the north and east. On July 6, the depression continued to the west-northwest, but its thunderstorm activity slowly declined as it entrained dry air. In addition, the cyclone increased in forward speed, making it difficult for the circulation to persist. It did not persist long, in fact: the system lost a closed circulation and dissipated during the afternoon of July 7, far from any land.

The above image shows Tropical Depression Four over the open Atlantic.

The short-lived tropical depression fell victim to a large dry air mass quickly after formation.