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Tropical Storm Florence Discussion Number  17
NWS National Hurricane Center Miami FL       AL062018
1100 AM AST Mon Sep 03 2018

While Florence's structure improved overnight, the cloud tops have
warmed and the deep convection has thinned during the past several
hours. The 12Z satellite intensity estimates range from 55 to 65 kt,
but given recent trends the initial intensity is set at the low end
of that range at 55 kt, although this is quite uncertain given the
recent fluctuations in the cloud pattern.

UW-CIMSS satellite diagnostics indicate that around 20 kt of
southwesterly shear is affecting Florence, while the SHIPS analysis
based on the GFS fields shows only about 10 kt. SSTs warm from this
point forward along the forecast track, but shear is expected to
be steady or strengthen, and the mid-level relative humidity values
decrease to around 50 percent during the next 48 to 72 hours. Given
these mixed factors, the NHC intensity forecast shows some
possibility for strengthening in the next 12 hours, followed by a
slow decay through 72 hours. Some restrengthening is forecast late
in the period as SSTs warm above 28C and the atmospheric moisture
increases. The NHC forecast is close to or a bit above the latest
IVCN consensus aid and about 5 kt above the previous NHC forecast
through 96 hours.

The initial motion estimate is 285/14. Florence will be steered
generally west-northwestward for the next 72 hours by the Atlantic
subtropical ridge, followed by a northwestward turn at days 4 and 5.
While there is large spread in the guidance between the HWRF on the
right and the UKMET on the left, the GFS, ECMWF, and their ensemble
means are more tightly clustered near the middle of the guidance
envelope. Since the overall track forecast reasoning has not
changed, the new NHC forecast remains near the middle of the
guidance. This forecast is a bit north of the previous NHC track
given the initial position and lies a little south of the consensus
aids to reflect less influence of the outlier HWRF model to the
north.

FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 18.3N  38.7W   55 KT  65 MPH
 12H  04/0000Z 18.6N  40.7W   60 KT  70 MPH
 24H  04/1200Z 19.1N  43.0W   60 KT  70 MPH
 36H  05/0000Z 19.9N  45.3W   55 KT  65 MPH
 48H  05/1200Z 20.8N  47.6W   50 KT  60 MPH
 72H  06/1200Z 23.0N  52.2W   45 KT  50 MPH
 96H  07/1200Z 25.0N  55.0W   50 KT  60 MPH
120H  08/1200Z 27.0N  58.0W   55 KT  65 MPH

$$
Forecaster Brennan

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