Friday, September 18, 2015

Tropical Storm Ida (2015)

Storm Active: September 18-27

Yet another tropical wave entered the eastern Atlantic on September 13, bearing generally westward south of the Cape Verde Islands. A low pressure center appeared in association with the system soon after. By the 15th, the disturbance assumed a more west-northwestward heading as the ridge to its north remained somewhat weak. Convection remained vigorous throughout the next few days, but the low was unable to take advantage of mainly favorable conditions. However, thunderstorm activity became much more concentrated near the center early on September 18, and the formation of Tropical Depression Ten followed later that morning. The depression strengthened into Tropical Storm Ida that night.

By the morning of September 19, the newly named tropical storm began to experience wind shear out of the northwest. Though banding features and deep convection remained healthy on the south and east quadrants, the center of circulation became exposed to the northwest. Over the next day, Ida continued to lose organization. During the afternoon of September 20, the cyclone's forward speed suddenly increased, but this trend was to reverse almost immediately as the storm approached an area of weak steering currents by that evening. Meanwhile, shear relaxed and Ida's convection came roaring back: a huge area of intense convection appeared and covered the center. As a result, the system began to strengthen.

However, this new trend was stopped by a new feature in the environment - a pronounced increase in mid-level shear. Throughout the day of September 21, the shower and thunderstorm activity slowly separated from the center. In addition, the lower- and upper-level centers themselves became separated, making the system difficult to pinpoint. By September 22, the influence of a large trough to the north of Ida caused it to cease progress towards the west-northwest and instead drift in the entirely opposite direction, toward the east-southeast. The havoc caused by the wind shear caused the system's circulation to fragment somewhat, with multiple low-level vortices evident as cloud swirls on satellite imagery west of the convective canopy. Overnight, as it hovered around minimal tropical storm strength, Ida even took an unusual southward turn. On September 23, however, the though began to carry Ida on a more steady path toward the east.

Though the trough began to move away from the system, the destructive influence of dry air and wind shear did not abate, causing Ida to weaken to a tropical depression on September 24. Shortly afterward, a ridge to the system's north forced it to switch directions again, this time to the north-northwest. The cyclone continued to persist in the face of unfavorable conditions, pulsating periodically with new convection. Therefore, it maintained tropical depression status into September 26. Ida's motion turned more toward the west that day and began to degrade further as bursts of shower activity became less frequent. By the afternoon of September 27, Ida had been devoid of convection so long that it no longer met the criteria for a tropical cyclone and degenerated into a remnant low. The low continued generally northwest for several days, even showing some signs of organization near the end of September. However, no redevelopment occurred and the system eventually merged with a frontal boundary.

The above image shows the disorganized tropical storm Ida over the central Atlantic.

Ida did not approach any landmass throughout its lifetime as a tropical cyclone.

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