000
WTNT42 KNHC 230841
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 AM AST TUE AUG 23 2016

Thunderstorm activity has continued to become better organized since
the previous advisory, especially in the inner-core region. In
addition, a 0556Z GPM microwave overpass indicated that a
15-20-nmi-diameter, closed low-level eye feature had developed. The
initial intensity as been increased to 45 kt based on a consensus
Dvorak satellite intensity estimate of T3.0/45 kt from TAFB and SAB,
which is supported by a UW-CIMSS ADT value of T2.9/43 kt.

The initial motion estimate is 290/17 kt. Gaston is expected to
move west-northwestward for the next 72 hours along the southern
periphery of a deep-layer ridge located to the north and northeast
of the cyclone. After that time, a gradual turn toward the
northwest is forecast as Gaston moves into a break in the
subtropical ridge. The models are in excellent agreement on this
track scenario and are closely packed through day 3, with a large
spread in the model tracks occurring after that time due to
differences in the timing of when Gaston will make the poleward turn
through the break in the ridge. The official forecast is similar
to the previous advisory track, and lies a little to the left of the
consensus model TVCN.

The aforementioned eye feature, in conjunction with low shear
conditions, well-defined upper-level anticyclonic outflow, SSTs
near 28 deg C, and mid-level humidity values near 70 percent,
generally would support a period of rapid intensification for the
next 24 hours or so. However, the current forecast follows the trend
of the previous advisory and shows strengthening at a more typical
rate of 20 kt/24h for the next 36 hours due to a narrow band of dry
air that appears to have penetrated into the inner core, which will
take some time to mix out. After that time, the intensity is
leveled off due to the cyclone moving through a band of strong
southwesterly vertical wind shear, which should disrupt both the
convective and upper-level outflow patterns. The initial intensity
is near the IVCN intensity consensus model through 24 hours, and
then a little above that and near the SHIPS model. However, the
new intensity forecast lies well below the GFS model output, which
is forecasting Gaston to become a major hurricane around 120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 13.2N  32.4W   45 KT  50 MPH
 12H  23/1800Z 13.9N  34.8W   55 KT  65 MPH
 24H  24/0600Z 15.2N  37.9W   65 KT  75 MPH
 36H  24/1800Z 16.8N  40.6W   75 KT  85 MPH
 48H  25/0600Z 18.5N  43.0W   80 KT  90 MPH
 72H  26/0600Z 22.6N  47.4W   80 KT  90 MPH
 96H  27/0600Z 26.5N  51.5W   80 KT  90 MPH
120H  28/0600Z 29.4N  53.6W   80 KT  90 MPH

$$
Forecaster Stewart