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Hurricane Florence Discussion Number  30
NWS National Hurricane Center Miami FL       AL062018
500 PM AST Thu Sep 06 2018

Vertical shear has taken its toll on Florence today as evidenced by
a continued degradation of the overall cloud pattern.  The
circulation appears tilted, with the low-level center partially
exposed to the southwest of the deep convection.  Subjective and
objective Dvorak Current Intensity (CI) numbers have fallen, and a
blend of the various estimates supports an initial intensity of 70
kt.

The intensity forecast is relatively straightforward in the
short-term as shear is expected to remain strong, which should
continue Florence's weakening trend, potentially taking the system
below hurricane strength.  By 24 hours, vertical shear is forecast
to decrease, and the SSTs gradually warm along the forecast track.
Assuming the overall circulation remains intact, Florence shouldn't
have any problems restrengthening beginning in a day or so. In
fact, guidance suggests that Florence could once again become a
major hurricane in 4 or 5 days.  The new NHC intensity forecast is
essentially an update of the previous one, and is only adjusted to
account for recent trends.  Largely, the official forecast is close
to the various consensus aids.

Owing to the degraded structure and tilted nature of the system,
Florence has wobbled a bit to the west, but the longer-term motion
estimate is 305/09.  Low- to mid-level ridging should cause
Florence to turn toward the west-northwest and west between 12-48
hours while the cyclone recovers from the strong shear.  Thereafter,
Florence is anticipated to become a deep cyclone again, but an even
stronger ridge should maintain the west-northwestward motion, at a
faster speed, through day 5.  The ridge is forecast to be
sufficiently strong such that some track models show a motion just
south of due west during the next 12-36 hours.  Beyond day 3, the
track forecast becomes increasingly uncertain due to differing
evolutions of the steering pattern over the western Atlantic.  While
all of the global models show a progressive trough eroding the
ridge, they differ in the strength of the trough and the ridge to
the north of Florida.  These differences result in a great deal of
bifurcation in the track guidance, especially among global model
ensemble members, at the end of the forecast period. In such
situations, prudence suggests a reliance on continuity, and the the
new official NHC track forecast is shifted only slightly south
towards the TVCN multi-model consensus and HCCA.  It is important to
note that deterministic track models in these types of situations
often display considerable run-to-run changes, and the uncertainty
in this forecast remains larger than normal.

Key Messages:

1. Regardless of Florence's eventual track, large swells will begin
to affect Bermuda on Friday and portions of the U.S. East Coast this
weekend, resulting in life-threatening surf and rip currents.

2. There is still very large uncertainty in Florence's track
beyond day 5, and it is too soon to determine what, if any, other
impacts Florence could have on the U.S. East Coast next week.

3. Since we are near the peak of hurricane season, this is a good
time for everyone who lives in a hurricane-prone area to ensure they
have their hurricane plan in place.

FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 25.0N  49.6W   70 KT  80 MPH
 12H  07/0600Z 25.4N  50.7W   60 KT  70 MPH
 24H  07/1800Z 25.5N  52.1W   65 KT  75 MPH
 36H  08/0600Z 25.5N  53.4W   70 KT  80 MPH
 48H  08/1800Z 25.6N  54.6W   75 KT  85 MPH
 72H  09/1800Z 26.4N  57.1W   90 KT 105 MPH
 96H  10/1800Z 28.0N  61.0W  100 KT 115 MPH
120H  11/1800Z 30.0N  67.0W  105 KT 120 MPH

$$
Forecaster Berg/Rhome

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