[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 220842
TCDAT3

Tropical Storm Gaston Discussion Number   8
NWS National Hurricane Center Miami FL       AL082022
900 AM GMT Thu Sep 22 2022

GOES-16 Enhanced BD-curve infrared imagery and a recent GMI
microwave pass showed that Gaston had maintained a deep convective
inner core during the past several hours.  Subsequently, a
primary curved band with -66C cloud tops has developed in the
western semicircle.  A compromise of the subjective satellite
intensity estimates from TAFB and SAB, along with a UW-CIMSS AiDT
estimated, yields an initial intensity of 55 kt for this advisory.

Although the intensity guidance agrees that Gaston will commence a
slow weakening trend soon, predicting when Gaston will become a
post-tropical cyclone or if it transitions into an extratropical low
is problematic. The GFS and ECMWF-SHIPS statistical diagnostics
indicate that Gaston will complete extratropical transition in just
6 hours, which appears unrealistic.  On the other hand, the FSU
Cyclone phase forecast for the GFS and UKMET suggest that the system
will maintain a symmetric lower-tropospheric warm core while moving
south of the upper westerlies on Friday.  The latter scenario seems
more reasonable since Gaston is sustaining inner core convection and
the wind flow aloft becomes a bit more diffluent as it approaches
the Azores Islands. Afterwards, the global simulated IR forecasts
show Gaston becoming a post-tropical cyclone (loss of tropical cloud
pattern characteristics) in 48 hours due to sub-24C SSTs, an
increasing stable/dry surrounding atmosphere, and strong northerly
shear.  The NHC forecast is based on the above mentioned global
model guidance and shows Gaston becoming a post-tropical cyclone on
Saturday.

Gaston's initial motion is estimated to be east-northeastward, or
070/15 kt.  The are no significant changes to the previous track
forecast or synoptic reasoning.  Gaston is expected to move
east-northeastward to eastward through Friday morning.
Afterward, the Azores high is predicted to strengthen and cause
Gaston to gradually turn southward.  Beyond 48 hours, the
global and hurricane models indicate that the cyclone will turn
toward the west-southwest to west in response to the aforementioned
high-pressure building to the north and northeast of the system.
The NHC forecast is based on this scenario and lies between the TVCA
and HCCA consensus models.

Because the latest forecast track of Gaston brings the cyclone near
or over the western and central Azores islands before becoming
post-tropical, The Azores Meteorological Service has issued a
Tropical Storm Warning for their western and central island chains.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 40.3N  35.4W   55 KT  65 MPH
 12H  22/1800Z 40.9N  32.6W   55 KT  65 MPH
 24H  23/0600Z 40.7N  30.0W   55 KT  65 MPH
 36H  23/1800Z 39.8N  28.8W   50 KT  60 MPH
 48H  24/0600Z 38.9N  29.1W   45 KT  50 MPH...POST-TROPICAL
 60H  24/1800Z 38.5N  30.4W   45 KT  50 MPH...POST-TROPICAL
 72H  25/0600Z 38.6N  32.2W   40 KT  45 MPH...POST-TROPICAL
 96H  26/0600Z 38.2N  35.9W   35 KT  40 MPH...POST-TROPICAL
120H  27/0600Z 37.1N  39.8W   30 KT  35 MPH...POST-TROPICAL

$$
Forecaster Roberts
This weather bulletin was brought to you by:
www.nashvilleweather.net