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TROPICAL STORM GASTON DISCUSSION NUMBER  17
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 PM AST FRI AUG 26 2016

The hostile vertical shear from an upper low to Gaston's southwest
appears to be dropping with the SHIPS and CIMSS shear analyses down
to about 15 kt.  The deep convection, however, is still mainly north
of the center and lacks significant banding features.  SAB and TAFB
Dvorak current intensity estimates range from 45 to 55 kt, down
slightly from earlier today.  In deference to the 1330Z ASCAT
scatterometer pass and the very robust low-level circulation evident
in the visible imagery during the day, the maximum winds are kept at
55 kt.

As Gaston moves away from the upper-level low, the shear should
continue to drop to values between 5 and 10 kt by tomorrow.  The
shear should then stay low through about 72 h while the cyclone
traverses over quite warm waters.  A key uncertainty in the
intensity forecast is from the environmental low-level moisture,
which may decrease substantially during the next few days.  This
could reduce the otherwise quite conducive conditions that Gaston
should soon experience. After about three days, Gaston is likely to
encounter strong mid-latitude upper-level westerlies and a return of
hostile shear while SSTs steadily drop. The official NHC forecast is
for steady intensification between days one and three, with gradual
weakening thereafter.  This forecast is based upon a blend of the
tightly packed HWRF/COAMPS/SHIPS/LGEM models and is the same as in
the previous advisory.

Gaston's center is just tucked in on the southern edge of the deep
convection, allowing for a confident assessment of the initial
position. The tropical storm is moving toward the northwest at a
15-kt clip, as it is being steered between the strong upper low and
a subtropical ridge to its northeast.  Around 72 h, Gaston should
slow to a crawl as it reaches a weak steering pattern.  But by the
end of the forecast period, the system should be accelerating
northeastward as it enters the mid-latitude westerlies.  The track
forecast is based upon a blend of the GFS and ECMWF global models
and is nearly the same from that in the previous advisory.

The initial 34- and 50-kt wind radii were only slightly tweaked
based upon the ASCAT scatterometer pass and the wind radii forecast
is similar to the RVCN consensus technique.


FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 26.2N  49.7W   55 KT  65 MPH
 12H  27/0600Z 27.4N  51.5W   55 KT  65 MPH
 24H  27/1800Z 28.6N  53.5W   65 KT  75 MPH
 36H  28/0600Z 29.7N  54.8W   75 KT  85 MPH
 48H  28/1800Z 30.4N  55.8W   85 KT 100 MPH
 72H  29/1800Z 31.5N  56.0W   95 KT 110 MPH
 96H  30/1800Z 32.5N  53.5W   90 KT 105 MPH
120H  31/1800Z 34.5N  49.0W   80 KT  90 MPH

$$
Forecaster Landsea

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