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Hurricane FLORENCE


Hurricane Florence Discussion Number  58
NWS National Hurricane Center Miami FL       AL062018
500 PM EDT Thu Sep 13 2018

The satellite and radar presentations of Florence have changed
little this afternoon with a 20-25 nmi wide eye waxing and waning
as intrusions of dry air have occasionally eroded the southern and
eastern eyewall. Despite the occasional ragged appearance of
Florence's eye, reconnaissance aircraft data indicate that the
central pressure has remained steady at 955 mb. The last reports
from the Air Force Reserve hurricane hunters support an intensity
of 85 kt, and this is corroborated by average Doppler velocity
values of near 105 kt at 2500-3000 ft ASL. A report of a 10-minute
average wind of 59 kt and a gust to 74 kt was recently received from
the Cape Lookout C-MAN station (CLKN7). The 59-kt 10-minute wind
speed is roughly equivalent to a 65-kt 1-minute wind.

Florence has continued to slow down, and radar fixes over the past
couple hours suggest that Florence has possibly stalled due to a
re-organization of the eye/eyewall. Smoothing through the fixes
over the past 6 h yields an initial motion estimate of 295/04 kt.
There is no change to the previous forecast reasoning. The ridge to
the north and east of Florence remains intact over the Atlantic
Ocean, but water vapor imagery and special upper-air observations
indicate that a shortwave trough has weakened that portion of the
ridge along the U.S. mid-Atlantic coast, resulting in a weakening
of the steering currents. The ridge is forecast by all of the
latest model guidance to remain intact, albeit weak, for the next 72
hours, which will nudge Florence on a slow westward to west-
southwest track into central South Carolina. On days 4 and 5,
Florence is expected to become an extratropical low as it interacts
with a front while moving northward and northeastward along the
Appalachian Mountains. The official forecast track is similar to,
but slightly south of the previous advisory track through 72 hours,
with little change indicated on days 4 and 5. This scenario closely
follows the simple consensus model TVCN/TVCA, which is north of the
corrected-consensus models HCCA and FSSE, which are heavily weighing
the southernmost model, ECMWF, which keeps Florence over or near the
Atlantic through about 48 hours.

Radar data indicate that Florence may be developing an outer
eyewall. If this trend continues, then little change to the
intensity is likely until landfall occurs in about 24 hours due in
part to the low vertical wind shear conditions and the warm, deep
waters of the Gulfstream current. Florence is expected to weaken
after landfall, but the rate of weakening may be tempered somewhat
due to much of the hurricane's circulation remaining over the warm
waters of the Atlantic Ocean and Gulfstream. Once Florence moves
farther inland over central South Carolina, rapid weakening of
the inner-core wind field should occur due to land interaction and
the cyclone's slow forward speed of 5 kt or less. However, intense
rainbands are expected to continue developing over the Atlantic
waters and keep moving along the coast and inland, likely producing
strong wind gusts through Saturday night.

Aircraft and satellite wind data show that Florence remains a large
hurricane.  Life-threatening storm surge, heavy rainfall, and
damaging wind will cover a large area regardless of exactly where
the center of Florence moves.

Key Messages:

1. A life-threatening storm surge is highly likely along portions of
the coastlines of South Carolina and North Carolina, and a Storm
Surge Warning is in effect for a portion of this area. The greatest
storm surge inundation is expected between Cape Fear and Cape
Hatteras, including the Neuse and Pamlico Rivers and western Pamlico

2. Life-threatening, catastrophic flash flooding and prolonged
significant river flooding are likely over portions of the Carolinas
and the southern and central Appalachians through early next week,
as Florence is expected to slow down as it approaches the coast and
moves inland.

3. Damaging hurricane-force winds are likely along portions of the
coasts of South Carolina and North Carolina beginning this evening,
and a Hurricane Warning is in effect. Strong winds could also spread
inland into portions of the Carolinas.

4. Large swells affecting Bermuda, portions of the U.S. East Coast,
and the northwestern and central Bahamas will continue this week,
resulting in life-threatening surf and rip currents.


INIT  13/2100Z 33.7N  76.2W   85 KT 100 MPH
 12H  14/0600Z 34.1N  77.2W   85 KT 100 MPH
 24H  14/1800Z 34.2N  78.2W   75 KT  85 MPH...INLAND
 36H  15/0600Z 33.9N  79.0W   50 KT  60 MPH...INLAND
 48H  15/1800Z 33.8N  79.9W   35 KT  40 MPH...INLAND
 72H  16/1800Z 34.8N  82.3W   25 KT  30 MPH...INLAND
 96H  17/1800Z 37.9N  82.6W   20 KT  25 MPH...POST-TROP/INLAND
120H  18/1800Z 42.7N  76.3W   20 KT  25 MPH...POST-TROP/INLAND

Forecaster Stewart