A well-organized tropical wave moved off the coast of Africa on September 15. Showers and gusty winds were observed as the disturbance moved over the Cape Verde Islands on September 17.[50] Over the following days convective activity increased over the system while strong ridging over the eastern Atlantic forced the system westward.[4] Based on satellite imagery, the system was designated a tropical depression on September 19 and Tropical Storm Helene on September 20.
When it reached the mid-Atlantic, Helene's track turned northwest under the influence of a major trough.[50] Favorable conditions allowed the storm to deepen at a moderate rate. On September 21 Helene became a hurricane over the mid-tropical Atlantic. The storm deepened further, reaching major hurricane status late on September 22 and Category 4 intensity on the following day.[51]
On September 23 Helene turned northward under a secondary trough and continued moving in that direction for the next week. The hurricane gradually weakened to a minimal hurricane over the next several days. It approached the southern boundary of a strong jetstream on the September 28, which enhanced the storm's outflow and briefly restrengthened it.[52] Helene's forward speed increased to 60 mph (97 km/h) on September 30 as the hurricane became extratropical over the north Atlantic. Observed as a hurricane for nine days via satellite, Helene was the longest-lived hurricane of the 1988 Atlantic season.[4]
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