000
WTNT42 KNHC 242034
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 PM AST WED AUG 24 2016

Gaston is being affected by southwesterly vertical shear associated
with a strong mid- to upper-level trough and cut-off low seen in
water vapor imagery near 26n 51w.  The shear has caused the
low-level center to become partially exposed while much of the
deep convection has been shunted to eastern half of the circulation.
In spite of the degraded satellite presentation, dropsonde data
from the unmanned NASA Global Hawk aircraft investigating Gaston
support keeping the intensity at 60 kt.  In fact, additional
observations from the ongoing mission might reveal that the system
is even a little stronger than this estimate.

Gaston is in for a prolonged period of strong southwesterly shear
for the next 36 to 48 hours, with the shear possibly peaking around
30 kt during that time.  The considerable shear is expected to
induce weakening, and it is possible that more weakening could occur
than indicated in the forecast despite the cyclone's moving over
warmer waters.  Once Gaston's interaction with the trough lessens in
about 2 days, the cyclone should reach 29 deg C water when the
shear diminishes.  This should give Gaston an opportunity to
re-intensify for at least a couple of days before the models
indicate an increase in shear at the end of the forecast period.
The intensity forecast is reduced relative to the previous one and
is a little below the multi-model consensus through 48 hours, but
then reverses and is above the consensus aids from 72-120 hours.

Gaston's heading is now definitively northwestward, and the initial
motion estimate is 315/14.  A continued northwestward motion is
likely for the next few days as Gaston moves between one cell of the
subtropical ridge centered over the eastern Atlantic and the cut-off
low to the northwest.  After 72 hours, Gaston should approach a more
significant weakness in the Atlantic subtropical ridge around 60w
and slow while turning north-northwestward to northward.  There are
substantial model differences again this cycle, with the ECMWF
indicating a weaker subtropical ridge.  The weaker ridge makes
Gaston more vulnerable to the mid-latitude westerly flow over
the North Atlantic, which results in an earlier recurvature.
However, the bulk of the guidance has a stronger ridge, and thus
have solutions that go much farther west.  No major changes have
been made to the previous forecast in the short term but the track
has been adjusted much farther to the left after 72 hours, in the
direction of but not as far left as the multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 17.4N  40.6W   60 KT  70 MPH
 12H  25/0600Z 18.8N  42.3W   55 KT  65 MPH
 24H  25/1800Z 21.0N  44.5W   45 KT  50 MPH
 36H  26/0600Z 23.5N  46.6W   45 KT  50 MPH
 48H  26/1800Z 25.7N  49.2W   50 KT  60 MPH
 72H  27/1800Z 28.3N  54.6W   65 KT  75 MPH
 96H  28/1800Z 30.1N  57.4W   80 KT  90 MPH
120H  29/1800Z 31.6N  58.2W   85 KT 100 MPH

$$
Forecaster Kimberlain/Franklin