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Tropical Storm Gabrielle Forecast Discussion Number 15
2019-09-07 10:37:20| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Sep 07 2019 000 WTNT43 KNHC 070837 TCDAT3 Tropical Storm Gabrielle Discussion Number 15 NWS National Hurricane Center Miami FL AL082019 500 AM AST Sat Sep 07 2019 Gabrielle's cloud pattern has temporarily degraded over the past several hours. With the vertical shear pattern shifting from the east, the shrinking, shapeless, convective mass is displaced to the west of the surface circulation. A blend of the Dvorak subjective satellite T-numbers yields a current intensity estimate of 35 kt, and a UW-CIMSS ADT estimate as well as an AMSU-A analysis come in at 39 kt. The initial intensity is generously above these data and is set at 45 kt out of respect for the earlier scatterometer wind analysis. Gabrielle has moved to the northeast of the upper cut-off low which has been producing a moderate to strong southerly shear environment over the past several days. The upper wind pattern in this quadrant of the upper low is more easterly, and although still quite strong, a little more diffluent. This slightly more favorable flow aloft, along with increasingly warmer sea surface temperatures in the region, should allow Gabrielle to gradually strengthen during the next 2 to 3 days. Toward the end of the forecast period, weakening is forecast to commence as the cyclone turns northeastward and quickly moves within deep-layer strong southwesterly flow associated with an approaching mid-latitude baroclinic frontal zone. Interaction with the frontal system as well as the cyclone traversing significantly decreasing oceanic temperatures (less than 20C), should induce a rather rapid extratropical transition around day 5. The NHC intensity forecast is an update of the previous one is above all of the guidance with the exception of the HWRF. The initial motion is estimated to be northwestward, or 310/15 kt. Azores high pressure to the northeast of Gabrielle is forecast to build over the north central Atlantic, causing the cyclone to turn west-northwestward later today. Afterward, southwesterly flow downstream from the aforementioned front should force Gabrielle to turn northward and northeastward on Sunday and Monday, respectively. A slight adjustment to the left of the previous forecast was made through the entire forecast in order to align more with the NOAA HFIP HCCA model. Some adjustments to the wind radii were made based on a 06 Sep 2320 UTC ASCAT-A/B ambiguity analysis. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 30.9N 42.1W 45 KT 50 MPH 12H 07/1800Z 31.9N 44.3W 50 KT 60 MPH 24H 08/0600Z 33.3N 46.6W 55 KT 65 MPH 36H 08/1800Z 35.4N 47.9W 60 KT 70 MPH 48H 09/0600Z 38.0N 47.3W 65 KT 75 MPH 72H 10/0600Z 43.6N 40.3W 75 KT 85 MPH 96H 11/0600Z 50.7N 26.7W 65 KT 75 MPH 120H 12/0600Z 59.4N 7.9W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Roberts
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