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Tropical Storm Florence Discussion Number  32
NWS National Hurricane Center Miami FL       AL062018
500 AM AST Fri Sep 07 2018

Florence's structure continues to be negatively affected by strong
southwesterly shear.  Cloud tops have generally warmed over the
past 6 hours, and recent microwave data show that the low-level
circulation center of Florence is displaced nearly 20 nmi to the
southwest of the mid-level center.  Satellite intensity estimates
have decreased since last night, and now support an initial
intensity of 55 kt.

Based on GFS and ECMWF SHIPS diagnostic output, the southwesterly
shear is near its peak now, and should gradually decrease over the
next 24 to 36 h.  All of the intensity guidance shows little change
in intensity through that time. From 48 h onward, a low shear/warm
SST environment should allow the tropical storm to re-strengthen.
However, the extent and timing of the strengthening varies greatly
from model to model, with the dynamical models generally showing
more intensification, and sooner, than the statistical models.  As
has been the case for most of Florence's existence thus far,
confidence in the intensity forecast, especially beyond 36 h, is
low.  The new official forecast is a little lower than the previous
advisory for the first 48 h, but close to it after that, and lies
between the more aggressive HCCA and less aggressive IVCN consensus
aids.

Nighttime Proxy-Vis and earlier microwave imagery indicate that
Florence has turned westward, with an estimated initial motion of
275/6 kt. Most of the track guidance has shifted slightly toward the
southwest, so the NHC track forecast has also been nudged in that
direction.  Over the next 3 days of the forecast, Florence should be
steered generally westward, and then west-northwestward, by a
mid-level ridge to its north.  By days 4 and 5, a developing
mid-latitude trough could create a weakness in this ridge and allow
Florence to move more toward the northwest, but there is still
considerable uncertainty in the global models and their ensembles
regarding the strength of the ridge and if the aforementioned trough
will have any notable impact on the track of Florence.  The NHC
forecast follows HCCA very closely, and is also near the middle of
the large combined envelope of the GFS, UKMET, and ECMWF ensembles.

Key Messages:

1. Regardless of Florence's eventual track, large swells will begin
to affect Bermuda later today and portions of the U.S. East Coast
this weekend, resulting in life-threatening surf and rip currents.

2. There is still very large uncertainty in Florence's track
beyond day 5, and it is too soon to determine what, if any, other
impacts Florence could have on the U.S. East Coast next week.

3. Since we are near the peak of hurricane season, this is a good
time for everyone who lives in a hurricane-prone area to ensure they
have their hurricane plan in place.

FORECAST POSITIONS AND MAX WINDS

INIT  07/0900Z 25.1N  50.7W   55 KT  65 MPH
 12H  07/1800Z 25.1N  51.8W   55 KT  65 MPH
 24H  08/0600Z 25.0N  53.2W   55 KT  65 MPH
 36H  08/1800Z 25.0N  54.3W   60 KT  70 MPH
 48H  09/0600Z 25.1N  55.4W   70 KT  80 MPH
 72H  10/0600Z 26.2N  58.3W   95 KT 110 MPH
 96H  11/0600Z 27.5N  63.5W  110 KT 125 MPH
120H  12/0600Z 30.0N  70.0W  110 KT 125 MPH

$$
Forecaster Zelinsky

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