Meteorological history of Hurricane Florence

On August 28, 2018, the National Hurricane Center (NHC) began monitoring a tropical wave—an elongated area of low air pressure—over West Africa for possible tropical cyclogenesis over the subsequent five days as it progressed west.^[2] Development into a tropical cyclone became increasingly likely the following day;^[3] and a more defined low coalesced along the coast of Senegal on August 30.^[4] On this day, the system split in two, with the northern one ultimately developing into Florence and the southern portion later becoming Tropical Depression Nineteen-E in the Eastern Pacific.^[1] Favorable environmental conditions, including ample moisture and low wind shear,^[5] enabled further organization and development of broad shower and thunderstorm activity. Lacking a well-defined center but posing an immediate threat to Cape Verde, the NHC began issuing advisories on the system as Potential Tropical Cyclone Six later that day.^{[nb 1]} Easterly trade winds propelled the disturbance along a west to west-northwest trajectory.^[8] Through much of the day and into August 31, convection remained confined to the southwest of the disturbance within a monsoon trough and precluded its classification as a tropical cyclone.^[9] Toward the end of August 31, convective organization became sufficient for the NHC to mark the formation of Tropical Depression Six as the system passed south of Santiago in Cape Verde.^[10]

By September 1, the primary steering factor shifted to a strong, expansive subtropical ridge anchored well to the north. This ridge extended from Europe to the eastern United States and remained the dominant steering factor through much of the storm's history.^[1] Moderate wind shear temporarily stunted development and displaced convection to the eastern side of the depression.^[11] Pronounced banding features surrounded the circulation and the depression intensified to a tropical storm; accordingly the NHC assigned the system the name Florence.^[12] Satellite intensity estimates indicated Florence achieved maximum sustained winds of 60 mph (95 km/h) by 09:00 UTC on September 2.^[13] Thereafter, shear and entrainment of dry air displaced convection from the surface low, leaving it exposed.^[14] Considerable uncertainty in the forecast for Florence arose, as weather models began to depict various solutions.^[15] Fluctuations in organization and intensity continued through September 3.^[16][17]

Florence as a Category 4 hurricane at 18:00 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.

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.^[18][19] Broad-scale conditions—moderate wind shear, sea surface temperatures below 81 °F (27 °C), and low relative humidity values—did not favor further intensification.^[1] However, these conditions were averaged over a large area and did not accurately represent localized conditions in the vicinity of Florence.^[20] The compact system unexpectedly rapidly organized within a small area of low wind shear.^[21] The hurricane's core structure and outer banding improved markedly, catching forecasters off-guard and intensifying beyond model outputs.^[22] In stark contrast to model guidance,^[23] 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.^[1] Situated at 22°42′N 46°36′W / 22.7°N 46.6°W / 22.7; -46.6 (Hurricane Florence Category 4),^[24] Florence became the northernmost Category 4 hurricane east of 50°W.^[25]

The hurricane's unforeseen intensification caused it to track farther north, out of the localized low shear.^[26] 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.^[27][28] 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.^[1][26] Meteorologist Robbie Berg described the intensity forecasts for Florence as a "self-defeating prophecy" owing to the "nuances of the environmental shear".^[26][29] A building mid-level ridge halted Florence's northward movement, leading to a slow westward turn.^[26][29] Weather models became increasingly consistent on the storm's future track, leading to greater confidence in a major impact to the Southeastern United States.^[30] 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.^[31] This trajectory proved climatologically unusual, with United States hurricane impacts primarily originating farther south and west of Florence's position on that day.^[32]

Environmental conditions became increasingly conducive to reorganization on September 8 as NOAA Hurricane Hunters began reconnaissance of the cyclone.^[33] Convective banding blossomed around the storm and a formative eye appeared on satellite imagery.^[34] The storm's central dense overcast became more defined and a complete eyewall developed within its core. Florence reattained hurricane-status around 12:00 UTC on September 9, with the Hurricane Hunters observing 76 mph (122 km/h) sustained winds at the surface.^[1][35] Fueled by sea surface temperatures of 84.2 to 85.1 °F (29 to 29.5 °C), Florence rapidly intensified overnight. Convective bursts with frequent lightning surrounded the eyewall,^[36] giving rise to a well-defined 12 mi (19 km) wide eye. Expanding outflow ventilated the cyclone, enabling continued growth.^[37] The system rapidly achieved Category 4 intensity by 16:00 UTC, with reconnaissance aircraft recording surface winds near 130 mph (215 km/h) and a pressure of 946 mbar (hPa; 27.93 inHg).^[38] The hurricane's motion accelerated and shifted northwest by this time, a trajectory it would maintain for several days.^[37]

Hurricane Florence achieved its initial peak intensity late on September 10 with sustained winds of 140 mph (220 km/h) and a pressure of 940 mbar (hPa; 27.76 inHg).^{[1][nb 2]} The extent of hurricane-force winds doubled in size and well-defined mesovortices rotated along the inner eyewall.^[39] Slight weakening ensued thereafter as an eyewall replacement cycle started; convection surrounding the eyewall became ragged and the eye itself filled.^[40][41] This process completed the following day, with the newly formed eye spanning 35 mi (55 km) across. Extensive outflow became established over the cyclone, extending northwest and east, providing ample ventilation and deformation which enabled Florence to continue expanding.^[42] The system intensified again and reached its peak intensity with winds of 150 mph (240 km/h) and a pressure of 937 mbar (hPa; 27.76 inHg) at 18:00 UTC.^{[1][nb 3]} The future track of the hurricane became increasingly complex as it approached the Carolinas. A strengthening trough moving inland over the Pacific Northwest amplified ridging over the Northeastern United States and western Atlantic Ocean, steering Florence to the west-northwest. A collapse of steering currents was anticipated around the time of landfall on September 14, which would result in the hurricane meandering near the coast or just inland for a prolonged period of time.^[43]

Fluctuations in the organization of Florence continued through the remainder of September 11 into September 12. Environmental conditions remained highly favorable for intensification and the NHC forecast the system to strengthen just below Category 5 status by September 13;^[44] however, the system soon weakened and degraded to Category 3 status by 18:00 UTC.^[45] Florence steadily grew in size during this time, with tropical storm-force winds extending 195 mi (315 km) by the end of September 12.^[46] The hurricane's core soon unraveled on September 13 as mid-level wind shear increased and upwelling of colder water reduced available energy.^[1] The hurricane's eyewall eroded entirely along the south side and convection became disrupted. Reconnaissance data revealed an ongoing eyewall replacement cycle, with a 25–35 mi (35–55 km) wide inner-core and 60–70 mi (95–110 km) wide outer-core. Furthermore, sustained surface winds fell to an estimated 110 mph (175 km/h), marking the cessation of Florence's tenure as a major hurricane.^[47]