Tropical Storm Helene (2000) - Meteorological History

Meteorological History

A tropical wave moved off the African coast on September 10. Shortly thereafter, it lost most of its atmospheric convection and initially showed few signs of development as it moved westward. On September 14, convection reformed near the center of the system. It continued to organize, and the next day the National Hurricane Center (NHC) classified it as Tropical Depression Twelve. At this point, it was located 470 mi (765 km) east of the Leeward Islands. At this point, the depression had a weak circulation and ragged convection. An anticyclone to its north caused the depression to move generally to the west, and upper-level conditions were forecast to be favorable for intensification. However, the circulation moved away from the main area of convection before a Hurricane Hunters flight indicated that the depression degenerated into a tropical wave on September 16. Although there was not a closed circulation, the flight observed flight-level winds of 65 mph (105 km/h) to the north and east of the system.

The remnants of the depression continued westward, moving through the Lesser Antilles on September 17. Despite favorable conditions for redevelopment including low wind shear and warm ocean temperatures, the system remained a tropical wave as it moved across the Caribbean Sea. Late on September 19, another reconnaissance plane discovered a closed circulation to the northwest of Grand Cayman. Although there was minimal convection near the center, it organized enough to be re-classified as a tropical depression. Upon redeveloping, the depression was moved west-northwestward around a large anticyclone to the east of Florida. Its circulation was broad, though the NHC anticipated further strengthening. Conditions remained favorable for intensification, but instead the depression weakened as it approached Cuba. Around 1200 UTC on September 20, it moved across the western tip of Cuba into the Gulf of Mexico with minimal thunderstorms near the center. A few hours later, the NHC noted that "the cyclone on the verge of breaking open into an east-west oriented trough". The agency did not discontinue advisories due to an increase in convection near the center, although no intensification was expected. However, the system's circulation and convection became better organized, and early on September 21 the NHC upgraded the depression to Tropical Storm Helene.

Upon becoming a tropical storm, Helene gradually turned to the north while rounding an anticyclone to its east. It rapidly intensified after its upgrade, and reached its peak intensity of 70 mph (110 km/h) late on September 21. The storm was small with an asymmetric wind field, and wind shear displaced the circulation from the deep convection. Increasing wind shear prevented Helene from attaining hurricane status, and the storm began weakening on September 22 as it approached the northern Gulf Coast. By 0900 UTC that day, strong wind shear moved the convection away from the center and toward the north and northeast. This prompted NHC forecaster Lixion Avila to remark, "If I did not have a reconnaissance plane in the area, I would not know there was a tropical cyclone by just observing IR satellite imagery." The storm made landfall near Fort Walton Beach, Florida at 1200 UTC on September 22 after weakening from its peak intensity to winds of 40 mph (64 km/h).

After moving ashore, an area of convection redeveloped over Helene's center and the circulation became well-defined on radar imagery. The storm accelerated to the northeast into the Westerlies, and about six hours following its landfall, Helene weakened to tropical depression status after crossing into southeastern Alabama. A few hours later, the NHC issued the last advisory on the system and transferred warnings to the Hydrometeorological Prediction Center. Hurricane forecast models anticipated restrengthening, and due to the forecast track north of the Gulf Stream, the NHC predicted intensification as an extratropical cyclone. The agency noted that Tropical Storm Danny re-intensified in the same region as a tropical cyclone. Despite strong wind shear, convection increased over Helene's center as the storm moved through North Carolina. Stations along the Outer Banks reported sustained winds up to 59 mph (95 km/h). Satellite imagery and buoy data indicated that Helene re-intensified into a tropical storm inland over North Carolina. The storm emerged from the Virginia coast into an area of less wind shear, where conditions were thus more favorable for strengthening. The storm became compact over the northern Atlantic Ocean, with a diameter of 140 mi (230 km). Strong convection developed over the center on September 24, and the following day Helene re-attained its peak intensity of 70 mph (110 km/h) while southeast of Nova Scotia. The wind estimate was based on observations from the Neptune Olivine, a nearby ship that recorded 64 mph (103 km/h) winds and a barometric pressure of 988 mbar (29.2 inHg); because the ship was located to the south of the center, the storm's minimum pressure was estimated at 986 mbar (29.1 inHg). Helene continued moving rapidly to the east-northeast, and late on September 25 dissipated as it was absorbed by a cold front.