Tropical Storm Florence Discussion Number  35
NWS National Hurricane Center Miami FL       AL062018
1100 PM AST Fri Sep 07 2018

Florence remains a sheared tropical cyclone.  Satellite images
indicate that the low-level center is partially exposed on the
southwesterly edge of a large convective mass, with the overall
circulation somewhat elongated from southwest to northeast.
A blend of the latest Dvorak wind speed estimates from TAFB/SAB and
the CIMSS SATCON gives a value of 50 kt for this advisory.

While the winds at 200 mb are already from an easterly direction
near the center of Florence, there is significant shear from
northwesterly winds from 300-500 mb, undercutting the outflow layer.
This shear is forecast to relax by the global models over the next
36 hours as an anticyclone builds to the north of the storm, which
should promote some strengthening by Sunday. After 48 hours, the
deep-layer flow becomes easterly near the cyclone, with very little
shear while the system is over very warm waters.  This pattern
favors significant intensification, and most of the guidance brings
Florence back to a category 4 hurricane in 4 or 5 days.  The
intensity forecast is very similar to the last one, and is raised
slightly at days 3 and 4 to come into better agreement with the
guidance.  It is interesting to note that even with a lower initial
intensity, the guidance is higher than the last cycle, which speaks
to the strength of the signal for intensification in the long range.

The initial motion estimate is 265 degrees at 6 kt.  Florence is
expected to continue moving slowly westward for the next 48 hours
under the influence of a weak mid-level ridge over the western
Atlantic.  By days 4 and 5, an exceptionally strong blocking ridge
is forecast to develop between Bermuda and the Northeast U.S. and
build westward, keeping Florence on a west-northwestward trajectory
with a notable increase in forward speed by the end of the forecast
period.  It feels like a broken record to mention that the overall
guidance envelope keeps shifting southwestward, and the official
forecast is moved in that direction.  Unfortunately with such a
large well-defined steering current from the ridge becoming likely,
the extended-range risk to the United States keeps rising, which is
confirmed by the majority of the latest ensemble guidance.

Key Messages:

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

2. The risk of other direct impacts associated with Florence along
the U.S. East Coast next week has increased.  However, there is
still very large uncertainty in model forecasts of Florence's track
beyond day 5, making it too soon to determine the exact location,
magnitude, and timing of these impacts.  Interests near and along
the U.S. East Coast should monitor the progress of Florence through
the weekend and ensure they have their hurricane plans in place.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 24.8N  53.2W   50 KT  60 MPH
12H  08/1200Z 24.7N  54.1W   50 KT  60 MPH
24H  09/0000Z 24.7N  55.2W   55 KT  65 MPH
36H  09/1200Z 24.7N  56.3W   60 KT  70 MPH
48H  10/0000Z 25.0N  57.8W   70 KT  80 MPH
72H  11/0000Z 25.8N  62.3W   95 KT 110 MPH
96H  12/0000Z 27.5N  68.5W  115 KT 130 MPH
120H  13/0000Z 30.5N  74.5W  115 KT 130 MPH

$$
Forecaster Blake