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Tropical Storm GASTON Forecast Discussion Number 20
2016-08-27 16:41:28| National Hurricane Center (Atlantic)
Issued at 1100 AM AST SAT AUG 27 2016 000 WTNT42 KNHC 271440 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST SAT AUG 27 2016 A 0954Z WindSAT microwave image indicated that Gaston had developed a 15-nmi-diameter low-level eye that was embedded in the center of the nearly circular central dense overcast. Subjective satellite intensity estimates are T4.0/65 kt and T3.5/55 kt from TAFB and SAB, respectively, and the latest UW-CIMSS ADT value is T3.7/59 kt. An average of these estimates supports increasing the initial intensity to 60 kt. As anticipated, Gaston has slowed down and the motion estimate is now 310/09 kt. The cyclone is forecast to maintain a northwestward motion and continue to decelerate during the next 48 hours as Gaston moves into a break in the subtropical ridge located to its north. A mid-level shortwave trough is forecast to dig southeastward from Canada and the northeastern United States by 72 hours, and force Gaston to make a sharp turn toward the north and northeast when the cyclone is located several hundred miles east of Bermuda. On days 4 and 5, Gaston is expected to get caught up in the deep-layer mid- latitude westerlies and accelerate east-northeastward over the North Atlantic. The new official forecast track is a little slower than the previous advisory track and has been shifted a little to the east, but not nearly as far east as the consensus model TVCN out of respect for the ECWMF model, which is close to the previous forecast track. Gaston has maintained two pronounced upper-level outflow jets to its east and southwest. These jets are flowing into large upper-level lows that are acting as significant mass sinks, a pattern that favors intensification. Although the mid-level environment is expected to be characterized by low humidity values of 40-45 percent during the next 72 hours, the cyclone will be moving over SSTs of 29C and remain in a low vertical wind shear regime. Those latter conditions, along with the small eye feature and the outflow jet pattern should allow Gaston to overcome the dry conditions and result in strengthening at a typical rate of about 20 kt per day. By days 4 and 5, the vertical shear is forecast to increase sharply and become westerly at more than 30 kt, which should induce a weakening trend. The NHC intensity forecast is similar to the previous advisory, and lies close to the consensus model IVCN through 36 hours, and a little above IVCN and close to the SHIPS/LGEM models after that. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 28.4N 53.2W 60 KT 70 MPH 12H 28/0000Z 29.3N 54.3W 65 KT 75 MPH 24H 28/1200Z 30.1N 55.3W 75 KT 85 MPH 36H 29/0000Z 30.7N 56.0W 85 KT 100 MPH 48H 29/1200Z 31.3N 56.3W 90 KT 105 MPH 72H 30/1200Z 32.6N 54.4W 90 KT 105 MPH 96H 31/1200Z 34.2N 49.6W 80 KT 90 MPH 120H 01/1200Z 36.9N 43.7W 70 KT 80 MPH $$ Forecaster Stewart
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