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Hurricane Florence Discussion Number  21
NWS National Hurricane Center Miami FL       AL062018
1100 AM AST Tue Sep 04 2018

Florence's structure has gradually increased in organization, with
SSMIS passes from a few hours ago revealing the development of a
mid-level microwave eye.  Dvorak estimates have responded in
kind--TAFB is up to T4.0, SAB is at T4.5, and the objective ADT is
in between at T4.4.  Since there still appears to be moderate
southwesterly shear inducing some tilt to the cyclone and
disrupting the infrared satellite pattern, the initial intensity is
raised conservatively to 65 kt, making Florence a hurricane.

The current motion remains west-northwestward, or 295/10 kt, with
Florence positioned near the southwestern edge of the subtropical
ridge.  The steering pattern ahead of Florence is rather complicated
and will be evolving over the next few days.  A large and complex
mid-/upper-level trough located northeast of the Leeward Islands is
expected to give way to the development of two upper-level highs
centered near the Greater Antilles and southwest of the Azores, with
Florence slowing down and turning northwestward between these new
features.  Despite this complex pattern, the spread in the track
models is less than normal, which increases the confidence in the
NHC track forecast for the next 5 days.  There is some spread which
begins to develop around day 5, with the ECMWF model moving Florence
a little faster toward the north while the GFS maintains a slower
speed and keeps the system to the south.  The updated NHC track
forecast has been nudged slightly to the east of the previous
forecast on days 4 and 5, close to the TVCX consensus but not as far
as the HCCA and ECMWF models.  There is still too much model spread
after day 5 to speculate what Florence might do beyond the official
forecast period.

Despite Florence becoming a hurricane, the southwesterly shear
affecting the cyclone is expected to increase over the next day or
two, which should prevent further intensification.  In fact, the
increasing shear, as well as mid-level relative humidities below 50
percent, should cause weakening between 24 and 72 hours.  After 72
hours, decreasing shear and warmer sea surface temperatures should
foster some re-intensification, with Florence expected to reattain
hurricane intensity by day 5.  The intensity guidance is in good
agreement, and the NHC forecast is very close to a blend of HCCA,
the Florida State Superensemble, and the ICON consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  04/1500Z 19.7N  42.5W   65 KT  75 MPH
 12H  05/0000Z 20.3N  44.0W   65 KT  75 MPH
 24H  05/1200Z 21.3N  46.2W   65 KT  75 MPH
 36H  06/0000Z 22.4N  48.2W   60 KT  70 MPH
 48H  06/1200Z 23.6N  50.0W   55 KT  65 MPH
 72H  07/1200Z 25.6N  52.7W   50 KT  60 MPH
 96H  08/1200Z 27.5N  55.0W   55 KT  65 MPH
120H  09/1200Z 29.5N  56.5W   65 KT  75 MPH

$$
Forecaster Berg

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