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TROPICAL STORM GASTON DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 PM AST THU AUG 25 2016

Strong southwesterly shear continues to take a toll on Gaston.  The
cloud pattern has become more assymetric with all of the deep
core convection located north and east of the center.  This was
confirmed by a recent SSMIS microwave overpass that showed
significant southwest to northeast tilt between the low- and
mid-level centers.  Objective Dvorak T-numbers have decreased and
subjective intensity estimates from TAFB and SAB support an initial
wind speed of 55 kt.  A large upper-level low near 23N 54W that is
moving southwestward is expected to continue to produce an
unfavorable upper-level environment over Gaston for another 12 to
18 hours.  Some additional weakening is possible tonight, but the
official NHC forecast shows little change in strength through
Friday.  After that time, Gaston should be moving around the
northern portion of the upper-low and into a more favarable
upper-level wind pattern.  This combined with warm SSTs and a moist
atmosphere should allow Gaston to re-strengthen over the weekend
and once again become a hurricane.  The updated NHC intensity
forecast is slightly above the previous advisory after 36 hours, and
is in good agreement with the latest SHIPS guidance.

The initial motion remains northwestward at 15 kt.  Gaston should
move northwestward during the next day or so around a mid-level
ridge over the east-central Atlantic.  In 36 to 48 hours, a ridge is
forecast to build to the north of Gaston, which should cause the
cyclone to turn west-northwestward.  Early next week, the ridge
is forecast to weaken and Gaston is expected to turn northward,
then recurve into the mid-latitude westerlies in 4 to 5 days.  The
track guidance is in good agreement during the first 48 hours, but
there is some spread as to what longitude recurvature begins.  The
ECMWF shows a slower motion near the end of the forecast period and
a track along the eastern side of the guidance envelope, while the
GFS and GFS ensemble mean are along the western edge.  The NHC track
foreast remains near the middle of the guidance envelope and is in
closest agreement with UKMET, FSSE, and multi-model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 21.6N  45.5W   55 KT  65 MPH
 12H  26/0600Z 23.4N  47.1W   55 KT  65 MPH
 24H  26/1800Z 25.5N  49.6W   55 KT  65 MPH
 36H  27/0600Z 27.0N  52.3W   60 KT  70 MPH
 48H  27/1800Z 28.1N  54.7W   70 KT  80 MPH
 72H  28/1800Z 30.2N  57.0W   85 KT 100 MPH
 96H  29/1800Z 31.8N  57.7W   95 KT 110 MPH
120H  30/1800Z 34.0N  55.0W   95 KT 110 MPH

$$
Forecaster Brown

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