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Hurricane Gert Discussion Number  18
NWS National Hurricane Center Miami FL       AL082017
500 AM AST Thu Aug 17 2017

The expected weakening trend of Gert appears to have started.  The
eye of the hurricane is no longer evident in satellite images, and
the convective pattern has become less symmetric during the past
several hours.  The initial wind speed is lowered a little to 85 kt,
in agreement with a blend of the Dvorak final T- and CI-numbers
from TAFB and SAB.  The hurricane is crossing the north wall of the
Gulf Stream current, and it will soon be moving over substantially
colder water.  These colder waters combined with a significant
increase in wind shear and drier air should cause rapid weakening
during the next day or so.  Gert is now forecast to lose its
tropical characteristics in about 24 hours, when it will be over
very cool sea surface temperatures of about 16 deg C.  The NHC
intensity forecast is an update of the previous one, and it
generally follows the GFS model.

Gert is embedded in fast mid-latitude flow and it is racing
east-northeastward over the central Atlantic at 34 kt.  A decrease
in forward speed and a slight turn to the northeast is expected
during the next couple of days while Gert interacts with a large
extratropical low to the east of Atlantic Canada.  The global
models show this extratropical system absorbing, or merging with,
Gert in about 3 days.  The NHC track forecast is adjusted a little
to the right of the previous one to come into better agreement with
the latest consensus models.

The track, intensity, and wind radii forecasts have incorporated
guidance from NOAA's Ocean Prediction Center.

Swells from Gert should continue to affect the coast of the
northeastern U.S. and Atlantic Canada through tonight.  These swells
are likely to produce dangerous surf and rip current conditions.
Please consult products from your local weather forecast office for
more information.

FORECAST POSITIONS AND MAX WINDS

INIT  17/0900Z 41.7N  54.0W   85 KT 100 MPH
 12H  17/1800Z 44.2N  47.4W   70 KT  80 MPH
 24H  18/0600Z 48.0N  40.2W   55 KT  65 MPH...POST-TROP/EXTRATROP
 36H  18/1800Z 50.7N  36.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 48H  19/0600Z 52.0N  33.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
 72H  20/0600Z...DISSIPATED

$$
Forecaster Cangialosi

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