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| #1108313 (Received by flhurricane at: 4:59 AM 22.Sep.2022) 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 |
