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Tropical Storm Florence Discussion Number  36
NWS National Hurricane Center Miami FL       AL062018
500 AM AST Sat Sep 08 2018

Although Florence remains a sheared tropical cyclone, satellite
imagery during the past 6 h also indicates that the shear has
started to abate somewhat, which has allowed the dense cirrus
canopy to build back over the previously exposed low-level
circulation center. Furthermore, deep convection with overshooting
cloud tops near -80C and an abundance of lightning activity have
developed very close to the center. Based on these data along with
Dvorak intensity estimates of T3.5/55 kt from TAFB and SAB, the
initial intensity has been raised to 55 kt.

The initial motion estimate is 265/8 kt. The mid-latitude flow
across CONUS and the northern Atlantic is forecast to flatten out
and become more zonal over the next 48 h or so, resulting in the
development of a narrow east-west oriented ridge along 35/36N
latitude.  This large-scale feature is expected to steer Florence
in a general westward direction during that time. By days 3-5,
however, the flow across the central and western U.S. is forecast
to buckle and become more meridional as a  deep mid-/upper-level
trough over the northeast Pacific pushes inland over the western
U.S., causing downstream ridging over the northeastern U.S. and
northwestern Atlantic. The global models agree on this general
change in the synoptic-scale flow pattern, but they differ
noticeably on where a downstream mid-/upper-level high pressure cell
takes up residence over the Atlantic either to the northwest or
northeast of Bermuda. The farther west/east the high develops will
determine how far west/east Florence will eventually move and
possibly affect the U.S. east coast beyond the 5-day forecast
period. The new official forecast track is close to the previous
advisory track through 48 h, and then was nudged a little to the
left or west of the previous track, which is close to the consensus
model TVCN and is north of the corrected-consensus models FSSE and
HCCA since the bulk of the NHC model guidance lies north of those
latter two models.

The upper-level environment is expected to improve to significantly
during the next 12 h and beyond with the current 20 kt of
southwesterly shear forecast to give way to shear of less than 10
kt. By 72 h and beyond, light shear from the southeast and east
along with the development of strong upper-level outflow jets to
the north of Florence is expected to create an environment that
favors significant and possibly even rapid strengthening. The new
NHC intensity forecast has been increased over the previous advisory
in anticipation of these very favorable dynamical conditions
developing, and now shows Florence becoming a hurricane by Sunday
and a major hurricane in 3 days, followed by additional
strengthening over the very warm Atlantic waters of at least 29 deg
C that are about 2 deg C above normal right now. The consensus
models IVCN and HCCA were closely followed, which are a little
below the FSSE model.

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  08/0900Z 24.5N  54.2W   55 KT  65 MPH
 12H  08/1800Z 24.6N  55.0W   60 KT  70 MPH
 24H  09/0600Z 24.6N  56.0W   65 KT  75 MPH
 36H  09/1800Z 24.6N  57.2W   75 KT  85 MPH
 48H  10/0600Z 24.9N  59.0W   90 KT 105 MPH
 72H  11/0600Z 26.0N  64.0W  105 KT 120 MPH
 96H  12/0600Z 28.0N  70.2W  120 KT 140 MPH
120H  13/0600Z 30.9N  75.8W  125 KT 145 MPH

$$
Forecaster Stewart

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