Tropical Storm GASTON Forecast Discussion (Text)

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WTNT42 KNHC 241457

1100 AM AST WED AUG 24 2016

Corrected for initial intensity

Gaston's overall cloud pattern has not changed much in organization
since the last advisory.  The low-level center is located underneath
a small central dense overcast (CDO), with hints of a prototype eye
within the convective mass.  A solid band, attached to the CDO,
wraps around the eastern half of the circulation.  A 1234 UTC ASCAT
pass supports keeping the initial wind estimate at 60 kt.

Gaston could still reach hurricane strength today.  However, the
shear associated with a potent mid- to upper-level trough along 52W
is forecast to greatly increase over Gaston and peak in 36 to 48
hours.  Even though the cyclone will gradually be moving over warmer
waters at that time, the shear should be enough to arrest its
development or perhaps even result in weakening.  By 72 hours,
Gaston's involvement with the trough should lessen and the shear
should decrease.  Much lower westerly shear is forecast toward the
end of the forecast period, which should give Gaston an opportunity
to re-strengthen, and perhaps significantly, as it moves over waters
of around 29 deg C.  The new NHC intensity forecast is reduced some
compared to the previous one through 48 hours, but shows slightly
greater intensification by 120 hours.  The official forecast is in
generally in good agreement with the multi-model consensus.

The latest fixes indicate that Gaston's heading has a much greater
northerly component, and the long-term initial motion estimate is
305/14.  Gaston is expected to move northwestward between a
subtropical ridge over the eastern Atlantic and the aforementioned
trough to the west during the next 3 days.  After 72 hours, Gaston
should gradually turn north-northwestward and northward with a
considerable decrease in forward speed, when it encounters a more
substantial break in the Atlantic subtropical ridge along 60W.  The
ECMWF shows less ridging and makes Gaston vulnerable to the mid-
latitude westerlies earlier, with the model showing recurvature
before 120 hours.  The bulk of the guidance, however, lies farther
west, with these models showing more ridging north of Gaston through
day 5.  The new track forecast is a little right of the previous
one through 72 hours in response to the cyclone's abrupt change in
heading since the last advisory and is close to a blend of the
ECMWF and GFS solutions.  The track beyond 72 hours was adjusted
westward and remains close to a ECMWF/GFS model blend.


INIT  24/1500Z 16.1N  39.4W   60 KT  70 MPH
 12H  25/0000Z 17.5N  41.2W   65 KT  75 MPH
 24H  25/1200Z 19.6N  43.5W   60 KT  70 MPH
 36H  26/0000Z 21.9N  45.6W   55 KT  65 MPH
 48H  26/1200Z 24.3N  47.9W   55 KT  65 MPH
 72H  27/1200Z 27.8N  53.1W   65 KT  75 MPH
 96H  28/1200Z 30.5N  55.9W   80 KT  90 MPH
120H  29/1200Z 32.2N  56.5W   90 KT 105 MPH

Forecaster Kimberlain

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Page last modified: Wednesday, 24-Aug-2016 14:58:04 UTC