Hurricane Florence Discussion Number  23
NWS National Hurricane Center Miami FL       AL062018
1100 PM AST Tue Sep 04 2018

GOES-16 imagery shows that the eye of Florence has become better
defined during the last several hours, with even some mesovortices
in the eye present on the shortwave infrared channel.  Satellite
intensity estimates continue to rise, and the initial wind speed is
set to 85 kt, just below the latest TAFB estimate of 90 kt.

Florence remains on a west-northwestward course at 300/10 kt.  This
general course is expected to continue through 36 or 48 hours while
the hurricane remains near the southwestern edge of the subtropical
ridge.  After that time, the forecast becomes more uncertain,
depending on the presence of a narrow mid-level ridge over the
north-central Atlantic Ocean.  A look at the ensemble guidance
shows a bifurcation developing by day 5, with the ECMWF favoring
a more northward turn, and the UKMET ensembles showing a stronger
ridge and a continuation of a west-northwest track.  The new NHC
forecast is adjusted westward at long range, in line with the
corrected-consensus aids, but don't be surprised if this forecast
undergoes some large changes in the next few cycles, given the
split in the guidance.

This intensity forecast is also difficult.   Florence certainly has
exceeded expectations during the last day or so, with the hurricane
on the verge of rapidly intensifying during the last 24 hours
despite a marginal environment.  Some more strengthening is
called for in the short term to reflect the current trend.
However, the global models continue to insist that southwesterly
shear will increase over the next couple of days which, in
combination with considerable dry air aloft, should cause some
weakening.  Later tomorrow, a slow weakening trend should begin and
continue through 48 hours, although not weakening as much as shown
in the past advisory.  This can't be considered a high-confidence
prediction in light of what Florence has done so far.  On Friday, an
upper-level low could cut off to the south of the cyclone, which
would lessen the shear near Florence, and the hurricane should be
moving over steadily increasing SSTs. Restrengthening is forecast at
long range, and it wouldn't be surprising if the new NHC prediction
turns out to be too low.  It is best to be conservative, however,
since the track uncertainty is increasing by the end of the
forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  05/0300Z 20.7N  43.9W   85 KT 100 MPH
12H  05/1200Z 21.5N  45.5W   90 KT 105 MPH
24H  06/0000Z 22.6N  47.6W   80 KT  90 MPH
36H  06/1200Z 23.7N  49.4W   70 KT  80 MPH
48H  07/0000Z 24.7N  50.9W   65 KT  75 MPH
72H  08/0000Z 26.0N  53.5W   65 KT  75 MPH
96H  09/0000Z 27.3N  55.8W   70 KT  80 MPH
120H  10/0000Z 29.0N  57.5W   80 KT  90 MPH

$$
Forecaster Blake