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Hurricane Helene Discussion Number  23
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Wed Sep 12 2018

Helene's surface circulation has become rather difficult to find
this evening in shortwave and enhanced infrared satellite imagery.
Based on an earlier SSMI/S microwave pass, the center was removed
well to the south of the convective mass.  A blend of the Dvorak
intensity estimates from TAFB and SAB, as well as a SATCON analysis
of 72 kt, yields an initial intensity of 70 kt for this advisory.

Global models and the ECMWF/GFS Decay SHIPS models show the
southwesterly shear increasing significantly in 24 hours, however,
the warm sea surface temperatures are expected to limit the
weakening trend some. Beyond the 48 hour period, Helene is expected
to traverse cooler waters while the harsh upper wind environment
persists.  Therefore, additional weakening is forecast at a
faster rate.  Toward the end of the period, the global models, as
well as the Florida State Cyclone Phase Evolution analysis and
forecast product, show Helene transitioning to a post-tropical
extratropical cyclone as it moves toward Ireland and the United
Kingdom.

Helene is estimated to be moving northward, or 350/11 kt, within the
mid-tropospheric southerly flow produced by an amplifying deep-layer
trough over the central Atlantic.  At the 48 hour period, the
cyclone should turn to the north-northeast followed by a
northeastward turn with an increase in forward motion toward the
end of the forecast period.  The large-scale models, and the
HWRF hurricane model, are clustered fairly close together and
indicate that Helene will pass near the Azores in about 72 hours.
Interests in those islands should closely monitor the progress of
Helene over the next several days.  The NHC forecast lies in the
middle of the guidance envelope and is nudged toward the TVCN
multi-model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  13/0300Z 22.4N  36.9W   70 KT  80 MPH
 12H  13/1200Z 24.4N  37.1W   60 KT  70 MPH
 24H  14/0000Z 27.3N  37.0W   55 KT  65 MPH
 36H  14/1200Z 30.7N  36.4W   55 KT  65 MPH
 48H  15/0000Z 34.3N  35.3W   55 KT  65 MPH
 72H  16/0000Z 40.1N  30.5W   50 KT  60 MPH
 96H  17/0000Z 43.4N  22.4W   50 KT  60 MPH
120H  18/0000Z 48.7N  12.9W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Roberts

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