Friday, September 7, 2018

Hurricane Helene (2018)

Storm Active: September 7-16

On September 6, an extremely large tropical wave entered the Atlantic basin from west Africa, to which it had brought torrential rains over the past few days. By the time it hit water, the system already possessed a clear spin, and only a lack of central convection lay between it and tropical cyclone designation. This was immediately remedied on September 7 as more concentrated thunderstorms developed near the center. That afternoon, it was designated Tropical Depression Eight, only a few hundred miles west of Senegal. The massive system consolidated fairly quickly for its size and strengthened into Tropical Storm Helene shortly after.

There was some Saharan dry air to the north of Helene, but convection still managed to wrap around the center on September 8 and the storm continued to intensify. That evening, the center passed well south of the Cabo Verde islands, bringing some rain due to its large size, but sparing them from worse impacts. The next day, Helene was upgraded to a hurricane as it moved west away from the islands and it closed off a very large eye that evening. The cyclone turned toward the west-northwest by September 10. The eye also became better defined and Helene intensified into a category 2 hurricane. The next day, Helene reached its peak intensity of 110 mph sustained winds and a minimum central pressure of 966 mb.

By this time, a weakness in the subtropical ridge to the north of Helene (caused by a low over the northeast Atlantic) allowed the system to turn northwest and then toward the north. As it gained latitude, the cyclone encountered lower water temperatures and a drier atmosphere. It weakened to a category 1 on September 12, and to a tropical storm the next day.

Helene began to appear less tropical as it accelerated northward since it became increasingly asymmetric. Nevertheless, it maintained its status as a strong tropical storm into September 14. Soon after, rain bands in its eastern semicircle began to sweep across the Azores Islands. The center of circulation passed just west of the islands on September 15 and veered northeastward. Helene picked up more forward speed and transitioned into an extratropical cyclone over the cold northeastern Atlantic on September 16. The system eventually brought some stormy conditions to Ireland and the UK a few days later.

Helene was a large system, and developed an unusually large eye when it intensified into a hurricane.

Though it did not affect any large landmasses, Helene did have impacts in both the Cabo Verde and Azores Islands.

No comments: