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Tropical Storm GASTON Forecast Discussion


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000
WTNT42 KNHC 272040
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 PM AST SAT AUG 27 2016

The inner-core convection of Gaston has continued to increase with
the CDO having expanded and become more symmetrical. Subjective
satellite intensity estimates remain T4.0/65 kt and T3.5/55 kt from
TAFB and SAB, respectively, and the latest UW-CIMSS ADT value is
T3.9/63 kt. An average of these estimates supports maintaining the
initial intensity at 60 kt, which could be conservative.

Gaston's initial motion estimate is 320/08 kt. There is no
significant change to the previous forecast track reasoning. Gaston
is expected to continue moving northwestward and gradually
decelerate during the next two days as the cyclone moves into a
break in the Bermuda-Azores ridge. A shortwave trough and associated
frontal system is forecast to approach Gaston by 72 hours and slowly
lift out the tropical cyclone to the north and northeast. On days 4
and 5, Gaston is expected to get caught up in the mid-latitude
westerly flow ahead of the aforementioned frontal system and
accelerate east-northeastward. The new NHC track model guidance is
in much better agreement now that the ECMWF model has made a
significant eastward shift in its track forecast and is much closer
to the previous advisory track. The new official forecast track was
only shifted eastward slightly to come more in line with the
consensus models TVCN and GFEX. However, the track shift also
resulted in a significant decrease in the forward speed on days 3,
4, and 5.

The global and regional models continue to indicate that Gaston
will remain in a very favorable upper-level outflow pattern for at
least the next 72 hours. In fact, the Navy-COAMPS, GFS, and ECMWF
models forecast Gaston to become a major hurricane by 72-120 h, with
the ECMWF forecasting a central pressure of 945 mb on day 5. In
contrast, the HWRF and the statistical-dynamical models SHIPS/LGEM
only bring Gaston up to 85-90 kt. Due to the possibility of
occasional dry air intrusions, with mid-level humidity values in the
40-percent range, weakening the inner-core convection, the official
intensity forecast remains unchanged and on the conservative side
through 72 hours, which is slightly above the consensus model IVCN.
On days 4 and 5, strong westerly shear of about 30 kt should induce
weakening, which also argues against this system becoming a major
hurricane at that time like the ECMWF model is predicting, and the
forecast is a little lower than the consensus on those days.

FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 29.2N  54.0W   60 KT  70 MPH
 12H  28/0600Z 30.0N  54.6W   70 KT  80 MPH
 24H  28/1800Z 30.7N  55.1W   80 KT  90 MPH
 36H  29/0600Z 31.0N  55.3W   85 KT 100 MPH
 48H  29/1800Z 31.4N  54.9W   90 KT 105 MPH
 72H  30/1800Z 32.5N  53.0W   85 KT 100 MPH
 96H  31/1800Z 34.0N  50.0W   75 KT  85 MPH
120H  01/1800Z 37.0N  44.0W   65 KT  75 MPH

$$
Forecaster Stewart