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| #873111 (Received by flhurricane at: 4:44 PM 27.Aug.2016) TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST SAT AUG 27 2016 The inner-core convection of Gaston has continued to increase with the CDO having expanded and become more symmetrical. Subjective satellite intensity estimates remain T4.0/65 kt and T3.5/55 kt from TAFB and SAB, respectively, and the latest UW-CIMSS ADT value is T3.9/63 kt. An average of these estimates supports maintaining the initial intensity at 60 kt, which could be conservative. Gaston's initial motion estimate is 320/08 kt. There is no significant change to the previous forecast track reasoning. Gaston is expected to continue moving northwestward and gradually decelerate during the next two days as the cyclone moves into a break in the Bermuda-Azores ridge. A shortwave trough and associated frontal system is forecast to approach Gaston by 72 hours and slowly lift out the tropical cyclone to the north and northeast. On days 4 and 5, Gaston is expected to get caught up in the mid-latitude westerly flow ahead of the aforementioned frontal system and accelerate east-northeastward. The new NHC track model guidance is in much better agreement now that the ECMWF model has made a significant eastward shift in its track forecast and is much closer to the previous advisory track. The new official forecast track was only shifted eastward slightly to come more in line with the consensus models TVCN and GFEX. However, the track shift also resulted in a significant decrease in the forward speed on days 3, 4, and 5. The global and regional models continue to indicate that Gaston will remain in a very favorable upper-level outflow pattern for at least the next 72 hours. In fact, the Navy-COAMPS, GFS, and ECMWF models forecast Gaston to become a major hurricane by 72-120 h, with the ECMWF forecasting a central pressure of 945 mb on day 5. In contrast, the HWRF and the statistical-dynamical models SHIPS/LGEM only bring Gaston up to 85-90 kt. Due to the possibility of occasional dry air intrusions, with mid-level humidity values in the 40-percent range, weakening the inner-core convection, the official intensity forecast remains unchanged and on the conservative side through 72 hours, which is slightly above the consensus model IVCN. On days 4 and 5, strong westerly shear of about 30 kt should induce weakening, which also argues against this system becoming a major hurricane at that time like the ECMWF model is predicting, and the forecast is a little lower than the consensus on those days. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 29.2N 54.0W 60 KT 70 MPH 12H 28/0600Z 30.0N 54.6W 70 KT 80 MPH 24H 28/1800Z 30.7N 55.1W 80 KT 90 MPH 36H 29/0600Z 31.0N 55.3W 85 KT 100 MPH 48H 29/1800Z 31.4N 54.9W 90 KT 105 MPH 72H 30/1800Z 32.5N 53.0W 85 KT 100 MPH 96H 31/1800Z 34.0N 50.0W 75 KT 85 MPH 120H 01/1800Z 37.0N 44.0W 65 KT 75 MPH $$ Forecaster Stewart |
