Tropical Storm Florence Discussion Number  37
NWS National Hurricane Center Miami FL       AL062018
1100 AM AST Sat Sep 08 2018

Florence has undergone a dramatic structural evolution just within
the past 6 hours or so.  Vertical shear has decreased just enough
for the storm to take on a more symmetric shape, with convection
developing in a ring around the low-level center, and an elongated
band wrapping around to the northeastern part of the circulation.
The convection has waned a little bit in intensity, however, due to
the presence of dry air, and Dvorak estimates support maintaining
an initial intensity of 55 kt for now.  A NOAA P-3 aircraft is
approaching Florence now on a research mission and should provide
some useful data to better assess the storm's intensity.

Recent WindSat microwave data revealed that Florence has a
well-defined low-level ring in the 37-GHz channel, which tends to
be a harbinger of strengthening when environmental conditions are
favorable.  Since vertical shear is decreasing and should be 10 kt
or less by later today, and Florence is heading toward a deeper
pool of warm water over the southwestern Atlantic, a significant
phase of intensification is likely to begin by tonight, continuing
through Tuesday or Wednesday.  In fact, the official intensity
forecast explicitly shows rapid intensification occurring between
24 and 48 hours from now, and Florence is expected to become a
major hurricane by Monday.  The HFIP Corrected Consensus (HCCA) and
the Florida State Superensemble (FSSE), both of which tend to do
well in these scenarios, are both near the upper end of the guidance
suite, especially through day 3.  Even by days 4 and 5, the HWRF,
HMON, and ICON intensity consensus are near the top end of the
guidance, close to HCCA and FSSE.  Given the signals in the
environment, and the solutions provided by these models, the NHC
intensity forecast shows Florence reaching category 4 intensity by
day 3 and maintains that through the end of the forecast period.

Florence's longer-term motion is 265/6 kt.  The cyclone appears to
be slowing down as was expected, and this type of motion is likely
to continue for the next 24-36 hours.  In fact, the track guidance
has slowed down during this period, and the updated NHC track
forecast is a little slower than the previous one.  After 36 hours,
the most notable change in the models was a northeastward shift in
the 06Z GFS.  However, that run appears to be an outlier from the
rest of the dynamical models, and its trend opposes the slight
westward shift noted in the HCCA and FSSE aids.  The updated NHC
track forecast is therefore very close to or slightly west of the
previous forecast on days 4 and 5.  The exact path of Florence as it
approaches the southeastern U.S. coastline will depend heavily on
the position and strength of the blocking high pressure that is
expected to develop north of Bermuda and extend westward over the
eastern U.S., and so far there has not been much more clarity on
those important details.

Key Messages:

1. Florence is forecast to be a dangerous major hurricane near the
southeast U.S. coast by late next week, and the risk of direct
impacts continues to increase. However, given the uncertainty in
track and intensity forecasts at those time ranges, it's too soon to
determine the exact timing, location, and magnitude of those
impacts.

2. Interests along the U.S. East Coast, particularly from north
Florida through North Carolina, should closely monitor the progress
of Florence, ensure they have their hurricane plan in place, and
follow any advice given by local officials.

3. 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.

FORECAST POSITIONS AND MAX WINDS

INIT  08/1500Z 24.5N  54.3W   55 KT  65 MPH
12H  09/0000Z 24.6N  55.0W   60 KT  70 MPH
24H  09/1200Z 24.6N  56.1W   70 KT  80 MPH
36H  10/0000Z 24.8N  57.5W   85 KT 100 MPH
48H  10/1200Z 25.1N  59.3W  100 KT 115 MPH
72H  11/1200Z 26.3N  64.9W  115 KT 130 MPH
96H  12/1200Z 28.5N  71.5W  125 KT 145 MPH
120H  13/1200Z 31.5N  77.0W  120 KT 140 MPH

$$
Forecaster Berg