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Hurricane Helene Discussion Number  11
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Sun Sep 09 2018

Satellite images indicate that Helene continues to strengthen.
During the past several hours, a large band of deep convection has
completely wrapped around the center resulting in the formation of
a large ragged eye.  Although there are well-defined outer bands in
the southern portion of the circulation, there are a few dry slots
just beyond the inner core.  The latest Dvorak classifications have
increased to 4.5/77 kt from both TAFB and SAB, and therefore, the
initial intensity is adjusted upward to 75 kt.

Helene is currently in a low wind shear environment and over fairly
warm 27-28 deg C SSTs.  These conditions are expected to prevail
for another 24-36 hours, so additional strengthening seems likely
during that period.  Beyond that time, however, the environment is
expected to gradually become less favorable for Helene with
south-southwesterly shear increasing, mid-level humidities falling,
and SSTs lowering along the future path.  All of these conditions
suggest a steady weakening trend beginning in a couple of days, and
the NHC intensity forecast follows the trend in the latest model
guidance.

The hurricane continues to pull away from the Cabo Verde Islands,
and the initial motion estimate is 285/11 kt.  A continued
west-northwest motion at about the same forward speed is expected
for about two days while a mid-level ridge remains the primary
steering feature.  After that time, a significant weakness in the
ridge, caused by a cut off deep-layer low pressure system, should
promote a northward turn with a gradual increase in forward speed.
The models are in relatively good agreement on this scenario, but
they differ on when and where Helene makes the turn.  The NHC track
forecast is adjusted a little to the east at the latter forecast
times, but it still favors the left side of the guidance envelope
to be near the HCCA model.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0300Z 13.9N  27.2W   75 KT  85 MPH
 12H  10/1200Z 14.4N  29.2W   85 KT 100 MPH
 24H  11/0000Z 15.1N  31.8W   90 KT 105 MPH
 36H  11/1200Z 15.7N  34.2W   90 KT 105 MPH
 48H  12/0000Z 16.4N  36.1W   85 KT 100 MPH
 72H  13/0000Z 19.0N  38.7W   70 KT  80 MPH
 96H  14/0000Z 23.5N  40.5W   60 KT  70 MPH
120H  15/0000Z 29.6N  41.2W   55 KT  65 MPH

$$
Forecaster Cangialosi

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