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Hurricane GASTON Forecast Discussion


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WTNT42 KNHC 250834
TCDAT2

HURRICANE GASTON DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072016
500 AM AST THU AUG 25 2016

For much of yesterday afternoon and evening, the NASA/NOAA Global
Hawk unmanned aircraft flew a mission through Gaston.  On the
aircraft's last pass through the storm, it released a dropsonde at
0243 UTC that measured a mean boundary layer wind of 80 kt and an
average wind of 77 kt in the lowest 150 m of the sounding near the
center of the cyclone.  These numbers both support an intensity of
65 kt, and consequently Gaston was upgraded to a hurricane.  The
sonde reported a minimum pressure of 995 mb with a 69-kt surface
wind, which equates to a central pressure of about 988 mb.

Despite Gaston becoming a hurricane, microwave data indicate that
the cyclone is tilted due to 20-25 kt of west-southwesterly shear.
The shear is expected to increase further during the next 12-24
hours when Gaston moves around the eastern side of an upper-level
low, and as a result, the hurricane is forecast to weaken back to a
tropical storm later today.  The shear is then forecast to subside
in about 48 hours, and with sea surface temperatures expected to
increase, Gaston is likely to reintensify during the latter part of
the forecast period.  The updated NHC forecast is higher than the
previous forecast during the first 48 hours primarily to account for
the higher initial intensity.  After 48 hours, the official forecast
is unchanged and closely follows the SHIPS model and the ICON
intensity consensus.

Gaston continues to move northwestward, or 310 degrees at 15 kt,
along the southwestern edge of a mid-tropospheric high.  A fairly
quick northwestward motion should continue for the next 36 hours,
with a turn toward the west-northwest expected by 48 hours when
Gaston moves around the north side of the aforementioned mid- to
upper-level low.  Gaston is then expected to recurve toward the
north-northeast by day 5 as it moves through a break in the ridge
and toward the mid-latitude westerlies.  The track models are in
agreement on this general scenario, although the ECMWF model is a
little bit slower and to the east of the other models, showing a
sharper turn by day 5.  The new NHC track forecast is therefore a
little east of the previous forecast on days 4 and 5 and lies
closest to the HWRF and the TVCN multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 19.5N  43.3W   65 KT  75 MPH
 12H  25/1800Z 21.1N  44.8W   60 KT  70 MPH
 24H  26/0600Z 23.4N  46.9W   55 KT  65 MPH
 36H  26/1800Z 25.3N  49.4W   55 KT  65 MPH
 48H  27/0600Z 26.7N  52.0W   60 KT  70 MPH
 72H  28/0600Z 29.1N  56.0W   70 KT  80 MPH
 96H  29/0600Z 31.0N  58.0W   80 KT  90 MPH
120H  30/0600Z 32.5N  57.5W   85 KT 100 MPH

$$
Forecaster Berg