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| #873033 (Received by flhurricane at: 4:35 PM 26.Aug.2016) TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST FRI AUG 26 2016 The hostile vertical shear from an upper low to Gaston's southwest appears to be dropping with the SHIPS and CIMSS shear analyses down to about 15 kt. The deep convection, however, is still mainly north of the center and lacks significant banding features. SAB and TAFB Dvorak current intensity estimates range from 45 to 55 kt, down slightly from earlier today. In deference to the 1330Z ASCAT scatterometer pass and the very robust low-level circulation evident in the visible imagery during the day, the maximum winds are kept at 55 kt. As Gaston moves away from the upper-level low, the shear should continue to drop to values between 5 and 10 kt by tomorrow. The shear should then stay low through about 72 h while the cyclone traverses over quite warm waters. A key uncertainty in the intensity forecast is from the environmental low-level moisture, which may decrease substantially during the next few days. This could reduce the otherwise quite conducive conditions that Gaston should soon experience. After about three days, Gaston is likely to encounter strong mid-latitude upper-level westerlies and a return of hostile shear while SSTs steadily drop. The official NHC forecast is for steady intensification between days one and three, with gradual weakening thereafter. This forecast is based upon a blend of the tightly packed HWRF/COAMPS/SHIPS/LGEM models and is the same as in the previous advisory. Gaston's center is just tucked in on the southern edge of the deep convection, allowing for a confident assessment of the initial position. The tropical storm is moving toward the northwest at a 15-kt clip, as it is being steered between the strong upper low and a subtropical ridge to its northeast. Around 72 h, Gaston should slow to a crawl as it reaches a weak steering pattern. But by the end of the forecast period, the system should be accelerating northeastward as it enters the mid-latitude westerlies. The track forecast is based upon a blend of the GFS and ECMWF global models and is nearly the same from that in the previous advisory. The initial 34- and 50-kt wind radii were only slightly tweaked based upon the ASCAT scatterometer pass and the wind radii forecast is similar to the RVCN consensus technique. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 26.2N 49.7W 55 KT 65 MPH 12H 27/0600Z 27.4N 51.5W 55 KT 65 MPH 24H 27/1800Z 28.6N 53.5W 65 KT 75 MPH 36H 28/0600Z 29.7N 54.8W 75 KT 85 MPH 48H 28/1800Z 30.4N 55.8W 85 KT 100 MPH 72H 29/1800Z 31.5N 56.0W 95 KT 110 MPH 96H 30/1800Z 32.5N 53.5W 90 KT 105 MPH 120H 31/1800Z 34.5N 49.0W 80 KT 90 MPH $$ Forecaster Landsea |
