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Tropical Storm Florence Discussion Number  38
NWS National Hurricane Center Miami FL       AL062018
500 PM AST Sat Sep 08 2018

A NOAA P3 aircraft conducted a research mission several hours ago
in Florence and measured SFMR surface winds of around 60 kt and
winds up to 65 kt at a flight level of 8000 feet.  Velocities of
65-70 kt at 500 meters were also measured by the Doppler radar on
the plane.  Based on these data, the initial wind speed is raised to
60 kt.  Dropsonde data also showed that the central pressure was
down to 989 mb.

Florence is slowly recovering from the dry air its circulation
ingested while it was under the influence of strong shear.
Convection in the outer bands is relatively thin but is deeper and
more persistent near the center.  Now that the shear has decreased
and the warm waters ahead of Florence reach deeper into the ocean,
continued strengthening is anticipated.  In fact, the official
forecast continues to show a period of rapid intensification, now
beginning 12-24 hours, with Florence reaching major hurricane
intensity between 36-48 hours.  One fly in the ointment is that the
SHIPS diagnostics are keeping mid-level relative humidities around
the cyclone around 50 percent, which isn't particularly moist, but
I'm going to assume that Florence will be able to scour out the dry
air within its circulation in the coming days.  The HCCA model and
the ICON intensity consensus support maintaining a forecast peak
intensity of 125 kt in 4 days or so, so no significant changes were
made from the previous forecast.  Regardless of the specifics of
the other models--some of which are higher and some of which are
lower--Florence is expected to be a powerful major hurricane on
days 3 through 5 as it moves across the southwestern Atlantic Ocean.

Florence is creeping westward (270 degrees) at 4 kt, trapped
between high pressure to its northeast and southwest.  A different
blocking ridge is expected to develop north and northeast of
Bermuda over the next few days, causing Florence to accelerate
toward the west-northwest and northwest between days 3-5.  There
have been some notable shifts in the model guidance on this cycle,
with the ECMWF model swinging to the northeast closer to the GFS,
and the HWRF model swinging farther south along the southern edge
of the guidance envelope.  Despite this change in the deterministic
ECMWF run, its individual ensemble members are still showing a
significant spread of solutions from just north of the Bahamas to
offshore the coast of North Carolina by day 5.  Because of this
spread, the updated NHC track forecast largely maintains
continuity and remains close to the TVCN multi-model consensus.
And despite the ECMWF's shift, this track prediction remains north
of the HCCA and FSSE solutions.

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 affecting Bermuda and portions of the U.S. East
Coast will continue into next week.  These swells will result in
life-threatening surf and rip currents.


FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 24.6N  54.7W   60 KT  70 MPH
 12H  09/0600Z 24.6N  55.4W   70 KT  80 MPH
 24H  09/1800Z 24.7N  56.6W   80 KT  90 MPH
 36H  10/0600Z 25.0N  58.3W   95 KT 110 MPH
 48H  10/1800Z 25.4N  60.5W  110 KT 125 MPH
 72H  11/1800Z 26.9N  66.4W  120 KT 140 MPH
 96H  12/1800Z 29.5N  72.5W  125 KT 145 MPH
120H  13/1800Z 32.5N  77.0W  120 KT 140 MPH

$$
Forecaster Berg

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