NHC releases final report on Hurricane Florence

Report shows how unusual Florence's intensification was

JACKSONVILLE, Fla. – The National Hurricane Center (NHC released its final report on Hurricane Florence. Below is the summary, to read the entire report, click here.

Florence originated from a convectively active tropical wave, which was accompanied by
a broad low pressure system that moved off the west coast of Africa on 30 August. The lowl latitude disturbance fractured early on 31 August, with the northern portion of the wave moving
west-northwestward accompanied by a steady increase in convective organization. The southern
portion of the wave moved westward during the ensuing 2.5 weeks, eventually developing into
eastern North Pacific Tropical Depression 19-E on 19 September. After the fracture, the
convective organization of the pre-Florence disturbance gradually increased and the associated
low pressure system slowly became better defined over the next couple of days. It is estimated
that a tropical depression formed around 1800 UTC 31 August about 90 n mi southeast of
Santiago island in the southernmost Cabo Verde Islands. The depression strengthened into a
tropical storm 12 h later when it was passing about 110 n mi west-southwest of the southernmost
Cabo Verde archipelago. For the next several days, Florence maintained a steady westnorthwestward motion at about 15 kt while moving around the southern periphery of a massive
Bermuda-Azores ridge that extended from northwestern Africa and southern Europe westward to
the east coast of the United States. The “best track” chart of the tropical cyclone’s path is given
in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively. The best track
positions and intensities are listed in Table 11.

 Despite being embedded within a favorable vertical-wind-shear regime of about 5 kt, only
slow strengthening occurred during the next 48 h due to Florence moving over marginal sea surface temperatures (SST) around 26.5°C and entraining cooler and drier air from the north.
However, intermittent bursts of deep convection developed near the center during this time, which
caused the inner-core wind field to gradually contract. Florence became a 65-kt hurricane around
1200 UTC 4 September when the cyclone was located about 1200 n mi east-northeast of the
Lesser Antilles. Now possessing a 30-n-mi-diameter eye, Florence underwent a period of rapid
intensification (RI) over the next 30 h. Florence became a 115-kt, category 4 hurricane by 1800
UTC 5 September while centered over the central Atlantic about 1200 n mi east-southeast of
Bermuda. Florence’s RI phase was quite remarkable considering that it occurred within an
environmental regime characterized by deep-layer southwesterly vertical shear of 15–20 kt, SSTs
<27°C, and mid-level relative humidity values < 50% — conditions that are typically not
considered conducive for significant strengthening (Fig. 4). 

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Within 12 h after becoming a category 4 hurricane, Florence underwent a period of rapid
weakening (RW) due to strong southwesterly vertical wind shear of near 25 kt, and became a
tropical storm by 0000 UTC 7 September. The magnitude of Florence’s weakening rate during
the RW phase was nearly the same as during the RI phase despite SSTs being at least 1°C
warmer during the RW period. The increase in the vertical shear was due to a mid- to upper-level
shortwave trough passing just north of Florence. This trough also weakened the ridge, resulting
in the cyclone slowing down and making a jog toward the northwest. After the shortwave passed
Florence early on 7 September, the weakness in the ridge filled in, forcing the tropical storm on a
westward track over the central Atlantic.

Although passive microwave and conventional satellite imagery indicated that the innercore convection had been severely disrupted by the strong shear conditions, data from a NOAA
Hurricane Hunter aircraft on 8 September revealed that the inner-core wind field remained intact
and that Florence had retained its small 10–15 n mi RMW. Despite ongoing strong westerly to
southwesterly vertical wind shear and the presence of dry mid-level air characterized by humidity
values of around 45%, Florence slowly re-strengthened over the next 48 h. By early on 9
September, the vertical shear had decreased to 5–10 kt when the tropical storm moved into an
upper-level col region, and a ring of deep convection formed in the inner core – a pre-cursor to
the redevelopment of an eye. By 1200 UTC that day, Florence began moving toward the westnorthwest and had regained hurricane status. Pronounced outflow jets formed in the northwestern
and southeastern quadrants of the hurricane. These features enhanced the upper-level outflow
across the hurricane’s center, allowing to Florence to undergo a second RI period and strengthen
50 kt during the 24-h period ending at 1800 UTC 10 September (Figs. 3 and 4). Some minor innercore fluctuations occurred for the next 24 h, with Florence reaching its peak intensity of 130 kt
around 1800 UTC 11 September when the hurricane was located about 725 n mi east-southeast
of Cape Fear, North Carolina.

Although the SSTs were quite warm (≥ 29.5°C), the depth of the warm water along
Florence’s track became fairly shallow, which promoted cold upwelling and mixing. This is
indicated by the upper-ocean heat content reaching a maximum of about 56 kJ cm-2 when
Florence was at its peak intensity, followed by a sharp decrease to less than 25 kJ cm-2 just 24 h
later (Fig. 4). While maintaining a steady west-northwestward motion around the southwestern
periphery of the Bermuda-Azores high pressure ridge, Florence began to slowly weaken, likely
due to the effects of cold upwelling (Fig. 5) and the onset of an eyewall replacement cycle (ERC).
Data from Air Force Reserve and NOAA reconnaissance aircraft, along with passive microwave
satellite imagery (Fig. 6), indicated that the diameter of Florence’s eye had increased to 25–30
n mi and eyewall convection had started to erode in the southeastern semicircle by late on 12
September. In addition, the hurricane’s outer wind field expanded and Florence’s peak winds
dropped below major hurricane status by 1200 UTC 13 September when the cyclone was located
about 150 n mi east-southeast of Wilmington, North Carolina. As the weakening hurricane
approached the southeastern coast of North Carolina late on 13 September, steering currents
collapsed as another shortwave trough weakened the ridge over the southeastern United States.
This resulted in a very slow westward motion of around 5 kt, and Florence made landfall as an
80-kt hurricane near Wrightsville Beach, North Carolina, around 1115 UTC 14 September. 

After landfall, Florence made a slight jog toward the west-southwest while maintaining a
slow forward speed. This track allowed the center of the cyclone to remain close to the warm
waters of the Gulf Stream just offshore, and the tropical cyclone gradually weakened during this
period. Florence became a tropical storm by 0000 UTC 15 September when the cyclone was
located over eastern South Carolina just north of Myrtle Beach. The tropical storm turned
westward and moved slowly across central and northern South Carolina, weakening to a tropical
depression by 1800 UTC 16 September while centered about 35 n mi south of Florence, South
Carolina. The depression then accelerated northward around the western periphery of a narrow
high pressure system centered just east of the North Carolina Outer Banks on 17 September, and
passed over western North Carolina, eastern Tennessee, and western Virginia, before reaching
western West Virginia by 1200 UTC that day where it became extratropical. The low turned
northeastward ahead of an approaching frontal system and steadily weakened within an
environment of strong westerly vertical wind shear. The system eventually dissipated over
Massachusetts shortly after 1200 UTC 18 September.