Tropical Depression Fifteen (2007)

Tropical Depression Fifteen was the fifteenth and weakest tropical cyclone of the 2007 Atlantic hurricane season. The origins of the depression were in a large area of disturbed weather, possibly associated with the remnant low-pressure area of Hurricane Karen, on October 4. The disturbance gradually organized, and was declared Tropical Depression Fifteen on October 11 while located 740 mi (1,190 km) east-southeast of Bermuda. Strong wind shear prevented the depression from intensifying further as it drifted over the open waters of the central Atlantic basin. It degenerated into a remnant low the next afternoon, as all that remained of the storm was a swirl of clouds. After being declared a remnant low, the storm transitioned into an extratropical cyclone and intensified. The storm may have brought strong winds to the western Azores, although there are no official reports that verify that possibility. The extratropical storm was absorbed into a larger extratropical system the next day.

Meteorological history
The origins of Tropical Depression Fifteen can be traced back to a large and complex area of disturbed weather extending from the northwestern Caribbean Sea to the western Atlantic Ocean on October 4. The disturbance may have been related to the remnants of Hurricane Karen, which degenerated into a remnant low-pressure area on September 29 while moving towards the west-northwest. However, because there was insufficient evidence to prove this, the system was not re-designated as Karen. The system slowly organized over the next several days, and on October 8, a surface low pressure had developed while located 170 mi (275 km) northeast of the Turks and Caicos Islands. Convection associated with the storm steadily increased as the low moved towards the northwest. By October 11, the low had gained enough convection to be declared Tropical Depression Fifteen while located 740 mi (1,190 km) east-southeast of Bermuda. The low was declared a tropical depression on the basis that deep convection had persisted around the center of circulation for about 12 hours. The depression was located to the east of an upper-level low pressure area, causing strong southwesterly wind shear, which was forecast to shift to the north and inhibit development of the storm. At the same time, the upper-level low steered the depression, causing it to move towards the east-northeast at 15 mph (25 km/h).

By the morning of October 12, the steering currents for the depression rapidly changed, causing a sudden decrease in foreword motion. A low- to mid-level subtropical ridge became the major factor in the movement of the storm, causing it to track at a slow pace of 3 mph (5 km/h) to the east. Persistent wind shear began to deteriorate the structure of the storm, causing the center to separate from the deepest convection, and restricting the outflow to all areas around the storm, except for the eastern quadrant. Increasingly strong wind shear, estimated at values of between 45 and 60 mph (75 to 95 km/h), was forecast to affect the depression by the end of the day. This prompted forecasters to predict the storm would degenerate into a remnant low-pressure area within 36 hours. Later that morning, the center of the storm became exposed and the deep convection associated with the depression was limited to a few small cells north of the center. By the afternoon, the depression nearly stalled, limited to a slight drift to the north. All of the convection was sheared away from the depression, and it was classified a remnant low. As a result, the final advisory from the National Hurricane Center was issued while the low was located 910 mi (1,465 km) east of Bermuda. The remnant low persisted for the next several days while picking up speed and taking a gradual turn towards the northeast. The low transitioned into an extratropical cyclone on October 14 and began to intensify. By October 16, the extratropical storm was nearing the Azores and had intensified to a storm equivalent in strength to a tropical storm. The storm reached its peak intensity as an extratropical storm while located to the north of the Azores with winds sustained at 50 mph (85 km/h) and a minimum pressure of 996 mbar (hPa; 29.41 inHg). Less than a day later, the storm was absorbed into a larger extratropical storm.

Impact
Because Tropical Depression Fifteen never approached land as a tropical cyclone, no watches or warnings were issued for any land masses. However, as an extratropical cyclone, the storm may have produced tropical storm equivalent winds across the western Azores on October 16. Despite this, there were no reports of damage. Tropical Depression Fifteen was, in terms of barometric pressure, the weakest tropical cyclone of the 2007 Atlantic hurricane season.