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Hurricane Helene Discussion Number  19
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Tue Sep 11 2018

Satellite imagery indicates that Helene is gradually weakening,
likely due to cool SSTs of around 25.5 deg C beneath the cyclone.
The eye has become a little less distinct and cloud tops associated
with the inner-core convection have warmed. The initial intensity
has been lowered to 85 kt, based on a blend of Final-T and CI
numbers from TAFB. Continued weakening is likely for the next 36 to
48 h since Helene will remain over marginal SSTs, and the intensity
guidance is in good agreement. After that time, the cyclone will
reach somewhat warmer waters and will begin to interact with an
upper-level trough to the northwest. While this interaction will
cause an increase of shear over Helene, it could also provide some
baroclinic support to help maintain Helene's intensity before it
undergoes extratropical transition. Some of the dynamical models
suggest that reintensification is possible through this period,
while the statistical models quickly weaken the cyclone. For now the
official intensity forecast continues to show little change in
intensity from 72-120 h and remains close to the various intensity
consensus aids, but confidence in this portion of the forecast is
lower.

Virtually no change has been made to the official track forecast.
Helene is currently moving northwestward, and a steady turn
northward, and eventually northeastward is expected as the cyclone
is steered between a mid-level ridge to the east and a trough to
west. All of the global models are in good agreement, with the
forward speed of the cyclone being the only notable difference from
model to model. Since I have no reason to depart from the consensus
at this point, the NHC forecast remains very close to HCCA through
day 5.

FORECAST POSITIONS AND MAX WINDS

INIT  12/0300Z 18.0N  35.4W   85 KT 100 MPH
 12H  12/1200Z 19.3N  36.3W   75 KT  85 MPH
 24H  13/0000Z 21.2N  37.3W   65 KT  75 MPH
 36H  13/1200Z 23.6N  37.7W   60 KT  70 MPH
 48H  14/0000Z 26.6N  37.7W   55 KT  65 MPH
 72H  15/0000Z 33.7N  35.9W   50 KT  60 MPH
 96H  16/0000Z 40.5N  31.0W   50 KT  60 MPH
120H  17/0000Z 45.0N  23.0W   50 KT  60 MPH

$$
Forecaster Zelinsky

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