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| #872940 (Received by flhurricane at: 10:44 AM 25.Aug.2016) TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST THU AUG 25 2016 The satellite presentation of Gaston has degraded some since the previous advisory, with deep convection bursting northeast of the center due to about 25 kt of southwesterly shear as analyzed by UW-CIMSS. An ASCAT-B pass from 1212 UTC was helpful in locating the center and showed peak winds of around 55 kt, which support an intensity of 60 kt for this advisory, a little above the latest Dvorak estimates of T3.5/55 kt. The strong shear is forecast to continue for the next 24 hours until Gaston moves poleward of an upper-level low currently located to its west. Given the shear and SSTs around 27-28C, little change in intensity, or perhaps some slight weakening, is expected in the first 24 hours. After that time, the shear decreases and Gaston moves over SSTs of 29-30C, which should allow for strengthening through the rest of the forecast period. The NHC intensity forecast is a little higher than the previous one later in the period and is close to the SHIPS model and the IVCN intensity consensus. The initial motion estimate is 315/15. Gaston should continue moving quickly northwestward for the next 24 to 36 hours around a mid-level ridge centered to its northeast. Ridging then builds westward to the north of the tropical cyclone, leading to a decrease in forward speed and a turn toward the west-northwest in 48 hours. Late in the period, Gaston will turn poleward into a weakness along 60W and then begin to recurve into the mid-latitude westerlies. The track model guidance agrees on this general scenario, however, there are some differences in the timing and sharpness of recurvature. The GFS, GFS ensemble mean, and UKMET are on the left side of the guidance envelope with a broader turn, while the ECMWF, HWRF, and GFDL show a sharper turn on the right side of the guidance. Given the uncertainty, the NHC forecast remains near the previous one in the middle of the guidance envelope, and is close to the TVCN multi-model consensus. The initial and forecast 34-kt and 50-kt wind radii were adjusted based on data from the ASCAT pass. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 20.4N 44.4W 60 KT 70 MPH 12H 26/0000Z 22.1N 45.9W 55 KT 65 MPH 24H 26/1200Z 24.4N 48.2W 55 KT 65 MPH 36H 27/0000Z 26.0N 50.9W 60 KT 70 MPH 48H 27/1200Z 27.1N 53.1W 65 KT 75 MPH 72H 28/1200Z 29.4N 56.6W 80 KT 90 MPH 96H 29/1200Z 31.0N 58.0W 90 KT 105 MPH 120H 30/1200Z 33.0N 57.0W 90 KT 105 MPH $$ Forecaster Brennan |
