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| #872920 (Received by flhurricane at: 4:41 AM 25.Aug.2016) TCDAT2 HURRICANE GASTON DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST THU AUG 25 2016 For much of yesterday afternoon and evening, the NASA/NOAA Global Hawk unmanned aircraft flew a mission through Gaston. On the aircraft's last pass through the storm, it released a dropsonde at 0243 UTC that measured a mean boundary layer wind of 80 kt and an average wind of 77 kt in the lowest 150 m of the sounding near the center of the cyclone. These numbers both support an intensity of 65 kt, and consequently Gaston was upgraded to a hurricane. The sonde reported a minimum pressure of 995 mb with a 69-kt surface wind, which equates to a central pressure of about 988 mb. Despite Gaston becoming a hurricane, microwave data indicate that the cyclone is tilted due to 20-25 kt of west-southwesterly shear. The shear is expected to increase further during the next 12-24 hours when Gaston moves around the eastern side of an upper-level low, and as a result, the hurricane is forecast to weaken back to a tropical storm later today. The shear is then forecast to subside in about 48 hours, and with sea surface temperatures expected to increase, Gaston is likely to reintensify during the latter part of the forecast period. The updated NHC forecast is higher than the previous forecast during the first 48 hours primarily to account for the higher initial intensity. After 48 hours, the official forecast is unchanged and closely follows the SHIPS model and the ICON intensity consensus. Gaston continues to move northwestward, or 310 degrees at 15 kt, along the southwestern edge of a mid-tropospheric high. A fairly quick northwestward motion should continue for the next 36 hours, with a turn toward the west-northwest expected by 48 hours when Gaston moves around the north side of the aforementioned mid- to upper-level low. Gaston is then expected to recurve toward the north-northeast by day 5 as it moves through a break in the ridge and toward the mid-latitude westerlies. The track models are in agreement on this general scenario, although the ECMWF model is a little bit slower and to the east of the other models, showing a sharper turn by day 5. The new NHC track forecast is therefore a little east of the previous forecast on days 4 and 5 and lies closest to the HWRF and the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 19.5N 43.3W 65 KT 75 MPH 12H 25/1800Z 21.1N 44.8W 60 KT 70 MPH 24H 26/0600Z 23.4N 46.9W 55 KT 65 MPH 36H 26/1800Z 25.3N 49.4W 55 KT 65 MPH 48H 27/0600Z 26.7N 52.0W 60 KT 70 MPH 72H 28/0600Z 29.1N 56.0W 70 KT 80 MPH 96H 29/0600Z 31.0N 58.0W 80 KT 90 MPH 120H 30/0600Z 32.5N 57.5W 85 KT 100 MPH $$ Forecaster Berg |
