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Tropical Storm GASTON Forecast Discussion


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000
WTNT42 KNHC 271440
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
1100 AM AST SAT AUG 27 2016

A 0954Z WindSAT microwave image indicated that Gaston had developed
a 15-nmi-diameter low-level eye that was embedded in the center of
the nearly circular central dense overcast. Subjective satellite
intensity estimates are T4.0/65 kt and T3.5/55 kt from TAFB and
SAB, respectively, and the latest UW-CIMSS ADT value is T3.7/59 kt.
An average of these estimates supports increasing the initial
intensity to 60 kt.

As anticipated, Gaston has slowed down and the motion estimate is
now 310/09 kt. The cyclone is forecast to maintain a northwestward
motion and continue to decelerate during the next 48 hours as Gaston
moves into a break in the subtropical ridge located to its north. A
mid-level shortwave trough is forecast to dig southeastward from
Canada and the northeastern United States by 72 hours, and force
Gaston to make a sharp turn toward the north and northeast when the
cyclone is located several hundred miles east of Bermuda. On days 4
and 5, Gaston is expected to get caught up in the deep-layer mid-
latitude westerlies and accelerate east-northeastward over the North
Atlantic. The new official forecast track is a little slower than
the previous advisory track and has been shifted a little to the
east, but not nearly as far east as the consensus model TVCN out of
respect for the ECWMF model, which is close to the previous
forecast track.

Gaston has maintained two pronounced upper-level outflow jets to its
east and southwest. These jets are flowing into large upper-level
lows that are acting as significant mass sinks, a pattern that
favors intensification.  Although the mid-level environment is
expected to be characterized by low humidity values of 40-45 percent
during the next 72 hours, the cyclone will be moving over SSTs of
29C and remain in a low vertical wind shear regime. Those latter
conditions, along with the small eye feature and the outflow jet
pattern should allow Gaston to overcome the dry conditions and
result in strengthening at a typical rate of about 20 kt per day. By
days 4 and 5, the vertical shear is forecast to increase sharply and
become westerly at more than 30 kt, which should induce a weakening
trend. The NHC intensity forecast is similar to the previous
advisory, and lies close to the consensus model IVCN through 36
hours, and a little above IVCN and close to the SHIPS/LGEM models
after that.

FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 28.4N  53.2W   60 KT  70 MPH
 12H  28/0000Z 29.3N  54.3W   65 KT  75 MPH
 24H  28/1200Z 30.1N  55.3W   75 KT  85 MPH
 36H  29/0000Z 30.7N  56.0W   85 KT 100 MPH
 48H  29/1200Z 31.3N  56.3W   90 KT 105 MPH
 72H  30/1200Z 32.6N  54.4W   90 KT 105 MPH
 96H  31/1200Z 34.2N  49.6W   80 KT  90 MPH
120H  01/1200Z 36.9N  43.7W   70 KT  80 MPH

$$
Forecaster Stewart