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Hurricane Helene Discussion Number  15
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Mon Sep 10 2018

Helene remains a well organized hurricane with a fairly large 25 n
mi diameter eye and ring of cold cloud tops surrounding that
feature.  There are a few dry slots beyond the inner core, however,
which have prevented some of the Dvorak estimates from increasing
this cycle.  A blend of the latest subjective and objective
satellite intensity estimates supports raising the initial intensity
slightly to 95 kt.

The hurricane has a limited amount of time to strengthen as the
current environment of low wind shear and relatively warm SSTs will
be changing soon.  Helene is expected to cross the 26 deg C isotherm
within 24 hours and it will enter an environment of increasing
south-southwesterly shear just beyond that time period.  These
conditions should promote a steady weakening trend beginning late
Tuesday.  By the end of the forecast period, however, SSTs increase
along the forecast track, and there could be some baroclinic
enhancements to end the weakening trend and perhaps even cause some
strengthening.  This scenario is supported by the HWRF/HMON regional
models and the GFS/ECMWF global models.  The NHC intensity forecast
is largely an update of the previous one, except it is slightly
higher at the end of the period.

The hurricane is gradually turning to the right, with the latest
initial motion estimate being 295/13 kt.  A large mid- to
upper-level trough over the north Atlantic is expected to dig
southward causing a significant weakness in the subtropical ridge.
In response to this change in the steering pattern, Helene is
forecast to gradually turn northward and then north-northeastward
during the next several days.  The track models are in fairly good
agreement, and the NHC track forecast lies near the middle of the
guidance envelope.  This forecast is only a little to the right of
the previous track.

FORECAST POSITIONS AND MAX WINDS

INIT  11/0300Z 15.5N  32.4W   95 KT 110 MPH
 12H  11/1200Z 16.2N  34.0W   95 KT 110 MPH
 24H  12/0000Z 17.2N  35.7W   85 KT 100 MPH
 36H  12/1200Z 18.6N  37.2W   75 KT  85 MPH
 48H  13/0000Z 20.1N  38.0W   65 KT  75 MPH
 72H  14/0000Z 24.6N  39.0W   60 KT  70 MPH
 96H  15/0000Z 30.0N  38.0W   50 KT  60 MPH
120H  16/0000Z 36.2N  34.0W   55 KT  65 MPH

$$
Forecaster Cangialosi

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