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Atlantic Tropical Cyclone Discussion

732
WTNT42 KNHC 240835
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016
500 AM AST WED AUG 24 2016

A 0414 UTC AMSR2 microwave image revealed that Gaston's structure
has improved with the development of a well-defined low-level cloud
ring and a mid-level microwave eye. However, the mid-level center
is displaced about 25 n mi to the northeast of the low-level
center. Dvorak intensity estimates from TAFB and SAB are both
T4.0/65 kt, but objective numbers are still between 55-60 kt. Given
the tilted structure of the cyclone, the initial intensity is only
raised to 60 kt on this advisory.

University of Wisconsin CIMSS shear analyses indicate that 10 kt
of southwesterly shear is affecting Gaston, which could explain
the cyclone's tilted structure. The shear is expected to remain low
enough during the next 24 hours to allow Gaston to strengthen to a
hurricane later today. However, the intensification trend should be
interrupted after 24 hours, continuing through day 3, when Gaston
moves into a higher shear zone to the east of a mid-/upper-level
low. Some strengthening is then probable on days 4 and 5 when
Gaston moves north of the upper low into a lower-shear environment.
The intensity models are in very good agreement on this general
scenario, and the official NHC forecast was only lowered a bit at
36 and 48 hours to be more in line with the IVCN consensus.

Tracking the low-level center observed in microwave data yields a
motion of 290/15 kt. Gaston is approaching a break in the
subtropical ridge caused by the aforementioned mid-/upper-level
low, and the steering currents should cause the motion to become
northwestward later today and continue along that heading for the
next four days. By day 5, Gaston is expected to slow down and turn
northward to the west of a mid-level high. There is lower-than-
normal spread among the track guidance for the entire five-day
forecast period, and the NHC forecast continues to closely follow
the TVCN multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT 24/0900Z 14.9N 38.6W 60 KT 70 MPH
12H 24/1800Z 16.2N 40.6W 65 KT 75 MPH
24H 25/0600Z 18.2N 42.9W 70 KT 80 MPH
36H 25/1800Z 20.4N 45.1W 70 KT 80 MPH
48H 26/0600Z 22.9N 47.2W 70 KT 80 MPH
72H 27/0600Z 26.9N 52.3W 70 KT 80 MPH
96H 28/0600Z 29.5N 55.5W 80 KT 90 MPH
120H 29/0600Z 31.5N 55.5W 85 KT 100 MPH

$$
Forecaster Berg


Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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