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Tropical Storm GASTON Forecast Discussion

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WTNT42 KNHC 261454

1100 AM AST FRI AUG 26 2016

Gaston continues to be adversely impacted by about 20 kt of
southwesterly vertical shear because of a strong upper low to the
southwest.  The shear is forcing the diminished deep convection to
be primarily located north of the center.  The TAFB and SAB
current intensities indicated 55 kt at 12Z, which is supported by a
1330Z ASCAT-B scatterometer pass which just became available.

As Gaston moves away from the upper-level low, the shear should drop
fairly dramatically to between 5 and 10 kt by tomorrow.  The shear
should stay low through about 72 h while the cyclone traverses over
quite warm waters.  After about three days, Gaston should encounter
strong mid-latitude upper-level westerlies and a return of hostile
shear at the same time that 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 the
IVCN intensity model consensus and is very similar to that from the
previous advisory.

Gaston's initial position is very well known, due to the center
being along the edge of the deep convection.  The tropical storm is
moving toward the north-northwest at a rapid 15-kt clip, as it's
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 the
tightly clustered members of the TVCN track model consensus and is
just slightly north of the previous advisory due to Gaston's initial
position being more north than previously anticipated.

The initial 34- and 50-kt wind radii were made slightly larger
based upon an AMSU satellite pass and the wind radii forecast is
similar to the RVCN consensus technique.


INIT  26/1500Z 25.2N  48.3W   55 KT  65 MPH
 12H  27/0000Z 26.7N  50.1W   55 KT  65 MPH
 24H  27/1200Z 27.9N  52.5W   65 KT  75 MPH
 36H  28/0000Z 29.0N  54.3W   75 KT  85 MPH
 48H  28/1200Z 29.9N  55.5W   85 KT 100 MPH
 72H  29/1200Z 31.2N  56.1W   95 KT 110 MPH
 96H  30/1200Z 32.5N  54.0W   90 KT 105 MPH
120H  31/1200Z 34.5N  50.0W   80 KT  90 MPH

Forecaster Landsea