page_title: Meteorological history of Hurricane Florence
section_title: Unexpected intensification
hierarchical_section_title: Meteorological history of Hurricane Florence / Unexpected intensification
caption_attribution_description: English: Hurricane Florence on September 5, 2018
context_page_description: The meteorological history of Hurricane Florence spanned 22 days from its inception on August 28, 2018, to its dissipation on September 18. Originating from a tropical wave over West Africa, Florence quickly organized upon its emergence over the Atlantic Ocean. Favorable atmospheric conditions enabled it to develop into a tropical depression on August 31 just south of the Cape Verde islands. Intensifying to a tropical storm the following day, Florence embarked on a west-northwest to northwest trajectory over open ocean. Initially being inhibited by increased wind shear and dry air, the small cyclone took advantage of a small area of low shear and warm waters. After achieving hurricane strength early on September 4, Florence underwent an unexpected period of rapid deepening through September 5, culminating with it becoming a Category 4 hurricane on the Saffir-Simpson scale. Thereafter, conditions again became unfavorable and the hurricane quickly diminished to a tropical storm on September 7.
  From September 7–9, Florence's forward momentum slowed as it turned west. Favorable conditions again fostered intensification on September 9 and the system regained hurricane status.
context_section_description: Development of a small central dense overcast and a mid-level eye feature signified that Florence achieved hurricane strength early on September 4, roughly 1,240 miles (2,000 km) west-northwest of the Cape Verde islands. Broad-scale conditions—moderate wind shear, sea surface temperatures below 81 °F (27 °C), and low relative humidity values—did not favor further intensification. However, these conditions were averaged over a large area and did not accurately represent localized conditions in the vicinity of Florence. The compact system unexpectedly rapidly organized within a small area of low wind shear. The hurricane's core structure and outer banding improved markedly, catching forecasters off-guard and intensifying beyond model outputs. In stark contrast to model guidance, Florence continued to intensify and attained major hurricane status around 12:00 UTC on September 5. Sustained winds rose to 130 mph (215 km/h) and its pressure fell to 950 mbar (hPa; 28.05 inHg); this ranked it as a Category 4 hurricane on the Saffir-Simpson scale. Situated at 22.7°N 46.6°W, Florence became the northernmost Category 4 hurricane east of 50°W.
  The hurricane's unforeseen intensification caused it to track farther north, out of the localized low shear. Persistent shear finally took its toll on Florence on September 6 through September 7, causing convection to become asymmetrical and tilting the storm's core southwest to northeast. By the early hours of September 7, rapid degradation of Florence's structure occurred. Its low-level circulation became exposed as convection became displaced to the northeast and the previously well-defined eye dissipated. Scatterometer data revealed the system weakened to tropical storm intensity by 00:00 UTC. Meteorologist Robbie Berg described the intensity forecasts for Florence as a "self-defeating prophecy" owing to the "nuances of the environmental shear". A building mid-level ridge halted Florence's northward movement, leading to a slow westward turn. Weather models became increasingly consistent on the storm's future track, leading to greater confidence in a major impact to the Southeastern United States. This threat was enhanced by the downstream effects of Typhoon Jebi's extratropical remnant influencing a ridge off the eastern United States which, in turn, forced a trough away from Florence faster than initially anticipated. This trajectory proved climatologically unusual, with United States hurricane impacts primarily originating farther south and west of Florence's position on that day.
caption: Florence as a Category 3 hurricane at 16:30 UTC on September 5 (left) and as a rapidly weakening hurricane at 14:05 UTC the following day (right). Strong wind shear displaced all convection to the north and east of the circulation, rendering its surface circulation exposed.
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