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Tropical Depression Eight Forecast Discussion Number 2
2017-08-13 10:52:45| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Aug 13 2017 000 WTNT43 KNHC 130852 TCDAT3 Tropical Depression Eight Discussion Number 2 NWS National Hurricane Center Miami FL AL082017 500 AM EDT Sun Aug 13 2017 Satellite-derived winds from ASCAT passes at 0122Z and 0208Z indicated that the tropical depression had a closed surface circulation with peak winds of 33 kt in the southeastern quadrant. However, those peak winds were occurring in the same area of the circulation where NOAA Buoy 41046, which several hours ago measured a sustained wind of 35 kt and a peak gust to 43 kt, due to what appears to have been cold, downdraft-driven wind gusts based on a sharp decrease in the temperature and dew point values. As a result, the intensity will be held at 30 kt for this advisory, in line with a consensus 30-kt estimate from TAFB, SAB, and UW-CIMSS ADT. The initial motion estimate is 335/12 kt. The depression is forecast to move north-northwestward to northward for the next 36 h around the western portion a deep-layer subtropical ridge that is oriented east-west along 30-32N latitude. By 48 h and beyond, a vigorous shortwave trough currently located over the northern Plains is expected to dig east-southeastward and weaken the ridge along the east coast of the United States from the Carolinas northward to New England. The increasing southwesterly flow ahead of the trough is forecast to accelerate the cyclone northeastward, on an optimal track about midway from the U.S. and Bermuda and keeping it away from those two land masses. The new NHC tack forecast is similar to the previous advisory, and lies close to a blend of the HCCA and TVCN consensus track models. The depression is expected to remain in a relatively low vertical wind shear environment for the next 24-36 hours, but also within a fairly dry mid-level moisture environment for the duration of the forecast. As a result, only modest strengthening is expected during the next couple of days. By 48 h and beyond, southwesterly vertical wind shear is expected to steadily increase to more than 20 kt ahead of the aforementioned shortwave trough, reaching 40-50 kt by 96-120 h, which should act to cap any additional intensification. Owing to the strong shear, the cyclone is forecast to become extratropical by days 4 and 5. The official intensity forecast is similar to the previous advisory, and lies close to the consensus models IVCN and ICON. The intensity forecast at 96 and 120 h reflects guidance provided by the NOAA Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 26.5N 70.9W 30 KT 35 MPH 12H 13/1800Z 28.1N 71.7W 35 KT 40 MPH 24H 14/0600Z 29.9N 72.2W 40 KT 45 MPH 36H 14/1800Z 31.5N 72.0W 45 KT 50 MPH 48H 15/0600Z 33.4N 71.0W 50 KT 60 MPH 72H 16/0600Z 37.7N 64.1W 50 KT 60 MPH 96H 17/0600Z 42.4N 50.8W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 18/0600Z 46.3N 36.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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