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Tropical Storm Florence Discussion Number  40
NWS National Hurricane Center Miami FL       AL062018
500 AM AST Sun Sep 09 2018

Florence's cloud pattern has continued to gradually become better
organized overnight, with an increase in convection near the center
and a developing central dense overcast feature.  However, the
cloud tops are not particularly cold and the outer banding features
remain fragmented likely due to some nearby dry mid-level air.
Dvorak intensity estimates from TAFB and SAB range from 55 to 65
kt, so the initial intensity remains 60 kt, just shy of hurricane
strength.  The NOAA Hurricane Hunters are scheduled to investigate
the storm later today, and that data should provide a better
assessment of Florence's intensity.

The upper-level outflow is becoming well established over the
cyclone, and the global model guidance indicates that Florence
will remain in a very favorable upper-level environment while the
cyclone moves over the warm waters over the southwestern
Atlantic.  These conditions favor strengthening with the only
apparent negative factor being nearby dry air, which will likely
remain away from the inner core due to the low shear conditions.
The NHC intensity forecast again calls for a period of rapid
strengthening within the next 12-36 hours, and Florence is forecast
to become a major hurricane on Monday with additional strengthening
early in the week.  This means that Florence is likely to be a very
powerful hurricane as it moves over the western Atlantic toward the
southeastern United States.  The new NHC intensity forecast is near
the various intensity consensus aids and is very similar to the
previous official forecast.

Florence is currently located between a couple of mid-level ridges
and a slow westward motion is expected today.  Another strong
high pressure ridge is forecast to build to the north of Florence
on Monday, which should cause the storm to begin moving
west-northwestward to northwestward at an increasingly faster
forward speed.  The dynamical model guidance is tightly clustered
for the first 2-3 days with increasing spread thereafter.  The GFS
remains along the right side of the guidance envelope with the HWRF
and ECMWF bracketing the left edge.  It should be noted that both
the GFS and ECMWF ensemble means are a little to the left or west
of their operational runs.  As a result, the NHC track forecast
lies to the left of the TVCA multi-model consensus, but is not as
far to the west as the FSSE and HCCA corrected consensus models at
day 5.  The models are in agreement that Florence is likely to slow
down near the end of the forecast period as a blocking high
pressure ridge builds to the north of the hurricane.

Key Messages:

1. Florence is forecast to be a dangerous major hurricane near the
southeast U.S. coast by late this 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 affecting Bermuda and portions of the U.S. East
Coast will continue this week.  These swells will result in
life-threatening surf and rip currents.

FORECAST POSITIONS AND MAX WINDS

INIT  09/0900Z 24.5N  55.8W   60 KT  70 MPH
 12H  09/1800Z 24.6N  56.7W   70 KT  80 MPH
 24H  10/0600Z 24.8N  58.3W   85 KT 100 MPH
 36H  10/1800Z 25.3N  60.4W  100 KT 115 MPH
 48H  11/0600Z 26.1N  63.1W  110 KT 125 MPH
 72H  12/0600Z 28.2N  69.1W  125 KT 145 MPH
 96H  13/0600Z 31.3N  74.8W  120 KT 140 MPH
120H  14/0600Z 34.4N  77.9W  105 KT 120 MPH...INLAND

$$
Forecaster Brown

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