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Tropical Storm Helene Discussion Number  26
NWS National Hurricane Center Miami FL       AL082018
500 PM AST Thu Sep 13 2018

Helene has maintained deep convection this afternoon though only
in the northern semicircle.  Dvorak estimates from TAFB and SAB are
continuing to drop, and a blend of these with the CIMSS SATCON
supports lowering the intensity to 55 kt.

Strong tropospheric vertical shear along with only lukewarm waters
are likely contributing toward Helene's weakening.  The shear
should further increase during the next two days, though the sea
surface temperatures will rebound up to 27C along with an increase
in low-level moisture.  Helene is expected to be either slowly
weakening or steady state during this time.  Beginning in about
three days, Helene will commence baroclinic transition, and it is
expected to become an extratropical cyclone by 96 hours.  The
baroclinic forcing should preclude any additional weakening through
the remainder of the forecast period, and the official intensity
forecast is nearly the same as the last advisory.  This prediction
is based on a blend of the stronger HWRF/COAMPS dynamical models and
the weaker SHIPS/LGEM statistical schemes for the next couple of
days, and is based on a blend of the GFS/ECMWF/UKMET global models
at the extended lead times.

Helene is moving northward at about 12 kt between the subtropical
ridge to its east and a deep-layer trough to its west.  The system
should accelerate and turn toward the northeast over the next few
days as it rounds the subtropical ridge and starts getting picked
up by the mid-latitude westerlies.  The official track forecast is
based upon the tightly clustered variable consensus technique (TVCN)
and is nearly the same as the previous forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  13/2100Z 26.3N  37.0W   55 KT  65 MPH
 12H  14/0600Z 28.8N  36.7W   50 KT  60 MPH
 24H  14/1800Z 32.3N  35.8W   50 KT  60 MPH
 36H  15/0600Z 35.5N  34.2W   50 KT  60 MPH
 48H  15/1800Z 37.9N  31.7W   50 KT  60 MPH
 72H  16/1800Z 40.6N  24.4W   50 KT  60 MPH
 96H  17/1800Z 44.0N  17.5W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  18/1800Z 51.0N   7.5W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Landsea

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