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| #872752 (Received by flhurricane at: 4:44 AM 23.Aug.2016) TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST TUE AUG 23 2016 Thunderstorm activity has continued to become better organized since the previous advisory, especially in the inner-core region. In addition, a 0556Z GPM microwave overpass indicated that a 15-20-nmi-diameter, closed low-level eye feature had developed. The initial intensity as been increased to 45 kt based on a consensus Dvorak satellite intensity estimate of T3.0/45 kt from TAFB and SAB, which is supported by a UW-CIMSS ADT value of T2.9/43 kt. The initial motion estimate is 290/17 kt. Gaston is expected to move west-northwestward for the next 72 hours along the southern periphery of a deep-layer ridge located to the north and northeast of the cyclone. After that time, a gradual turn toward the northwest is forecast as Gaston moves into a break in the subtropical ridge. The models are in excellent agreement on this track scenario and are closely packed through day 3, with a large spread in the model tracks occurring after that time due to differences in the timing of when Gaston will make the poleward turn through the break in the ridge. The official forecast is similar to the previous advisory track, and lies a little to the left of the consensus model TVCN. The aforementioned eye feature, in conjunction with low shear conditions, well-defined upper-level anticyclonic outflow, SSTs near 28 deg C, and mid-level humidity values near 70 percent, generally would support a period of rapid intensification for the next 24 hours or so. However, the current forecast follows the trend of the previous advisory and shows strengthening at a more typical rate of 20 kt/24h for the next 36 hours due to a narrow band of dry air that appears to have penetrated into the inner core, which will take some time to mix out. After that time, the intensity is leveled off due to the cyclone moving through a band of strong southwesterly vertical wind shear, which should disrupt both the convective and upper-level outflow patterns. The initial intensity is near the IVCN intensity consensus model through 24 hours, and then a little above that and near the SHIPS model. However, the new intensity forecast lies well below the GFS model output, which is forecasting Gaston to become a major hurricane around 120 hours. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 13.2N 32.4W 45 KT 50 MPH 12H 23/1800Z 13.9N 34.8W 55 KT 65 MPH 24H 24/0600Z 15.2N 37.9W 65 KT 75 MPH 36H 24/1800Z 16.8N 40.6W 75 KT 85 MPH 48H 25/0600Z 18.5N 43.0W 80 KT 90 MPH 72H 26/0600Z 22.6N 47.4W 80 KT 90 MPH 96H 27/0600Z 26.5N 51.5W 80 KT 90 MPH 120H 28/0600Z 29.4N 53.6W 80 KT 90 MPH $$ Forecaster Stewart |
