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TROPICAL STORM GASTON DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
1100 AM AST THU AUG 25 2016

The satellite presentation of Gaston has degraded some since the
previous advisory, with deep convection bursting northeast of the
center due to about 25 kt of southwesterly shear as analyzed by
UW-CIMSS.  An ASCAT-B pass from 1212 UTC was helpful in locating the
center and showed peak winds of around 55 kt, which support an
intensity of 60 kt for this advisory, a little above the latest
Dvorak estimates of T3.5/55 kt.  The strong shear is forecast to
continue for the next 24 hours until Gaston moves poleward of an
upper-level low currently located to its west.  Given the shear and
SSTs around 27-28C, little change in intensity, or perhaps some
slight weakening, is expected in the first 24 hours.  After that
time, the shear decreases and Gaston moves over SSTs of 29-30C,
which should allow for strengthening through the rest of the
forecast period.  The NHC intensity forecast is a little higher than
the previous one later in the period and is close to the SHIPS model
and the IVCN intensity consensus.

The initial motion estimate is 315/15.  Gaston should continue
moving quickly northwestward for the next 24 to 36 hours around a
mid-level ridge centered to its northeast.  Ridging then builds
westward to the north of the tropical cyclone, leading to a decrease
in forward speed and a turn toward the west-northwest in 48 hours.
Late in the period, Gaston will turn poleward into a weakness along
60W and then begin to recurve into the mid-latitude westerlies.  The
track model guidance agrees on this general scenario, however, there
are some differences in the timing and sharpness of recurvature.
The GFS, GFS ensemble mean, and UKMET are on the left side of the
guidance envelope with a broader turn, while the ECMWF, HWRF, and
GFDL show a sharper turn on the right side of the guidance.  Given
the uncertainty, the NHC forecast remains near the previous one in
the middle of the guidance envelope, and is close to the TVCN
multi-model consensus.

The initial and forecast 34-kt and 50-kt wind radii were adjusted
based on data from the ASCAT pass.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 20.4N  44.4W   60 KT  70 MPH
 12H  26/0000Z 22.1N  45.9W   55 KT  65 MPH
 24H  26/1200Z 24.4N  48.2W   55 KT  65 MPH
 36H  27/0000Z 26.0N  50.9W   60 KT  70 MPH
 48H  27/1200Z 27.1N  53.1W   65 KT  75 MPH
 72H  28/1200Z 29.4N  56.6W   80 KT  90 MPH
 96H  29/1200Z 31.0N  58.0W   90 KT 105 MPH
120H  30/1200Z 33.0N  57.0W   90 KT 105 MPH

$$
Forecaster Brennan

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