[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 221445
TCDAT3

Tropical Storm Gaston Discussion Number   9
NWS National Hurricane Center Miami FL       AL082022
Issued by the NWS Weather Prediction Center College Park MD
300 PM GMT Thu Sep 22 2022

Gaston has been relatively steady in strength during the past
several hours.  Recent visible and microwave images indicate that
the storm has a fairly well-defined inner core.  However, an SSMI
overpass shows that the system is tilted in the vertical due to
west-southwesterly shear.  The 12Z Dvorak estimates from TAFB and
SAB were unchanged, and the initial intensity remains 55 kt at the
high end of the classifications.  An ASCAT-B pass showed peak winds
of around 45 kt in the southeastern quadrant and given the low bias
of the instrument for these cases, this data also supports the
55-kt initial wind speed.  The 34-kt wind radii has been expanded
outward based on the scatterometer data.

The storm is still moving east-northeastward at 18 kt on the
northwestern periphery of a subtropical ridge.  A turn to the east
and a reduction in forward speed are expected tonight, followed by a
clockwise loop near or over the central and western Azores on Friday
and Saturday as the storm moves on the south side of another ridge.
The models have generally changed little this cycle, and the NHC
track forecast is very similar to the previous one.

Despite moderate to strong shear, relatively dry air, and cool
SSTs, Gaston has been maintaining its strength.  Given that these
conditions are only expected to gradually worsen for the system, a
slow weakening trend seems likely.  The models all show a similar
theme, and the NHC intensity forecast lies near the middle of the
guidance envelope.  Gaston is expected to become post-tropical in a
couple of days when it merges with an approaching shortwave trough.

Key Messages:

1. Tropical storm conditions are expected to begin tonight in the
western Azores and will likely spread to the central Azores on
Friday.

2. Gaston is expected to produce heavy rainfall over the western
and central Azores beginning tonight and continuing into Saturday.

FORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z 40.9N  33.8W   55 KT  65 MPH
 12H  23/0000Z 41.2N  31.2W   55 KT  65 MPH
 24H  23/1200Z 40.7N  29.3W   55 KT  65 MPH
 36H  24/0000Z 39.6N  29.0W   50 KT  60 MPH
 48H  24/1200Z 38.9N  30.0W   50 KT  60 MPH...POST-TROPICAL
 60H  25/0000Z 38.9N  31.7W   45 KT  50 MPH...POST-TROPICAL
 72H  25/1200Z 39.1N  33.6W   45 KT  50 MPH...POST-TROPICAL
 96H  26/1200Z 38.3N  37.2W   35 KT  40 MPH...POST-TROPICAL
120H  27/1200Z 37.2N  40.7W   35 KT  40 MPH...POST-TROPICAL

$$
Forecaster Cangialosi/Weiss/Taylor
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