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Tropical Depression Ten Forecast Discussion Number 1
2021-08-28 10:55:08| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Aug 28 2021 000 WTNT45 KNHC 280855 TCDAT5 Tropical Depression Ten Discussion Number 1 NWS National Hurricane Center Miami FL AL102021 500 AM AST Sat Aug 28 2021 The tropical wave and associated low pressure system that the NHC has been tracking for the past several days has finally acquired a well-defined surface circulation and enough organized deep convection to be classified as a tropical depression. The last few visible satellite images yesterday evening indicated a tight swirl in the low-cloud field and a 27/2325Z partial ASCAT-A pass showed the circulation was also well-defined, albeit with only 23-kt surface winds. Since the time of that scatterometer pass, however, a significant increase in deep convection with cloud tops colder than -80 deg C has developed very near and to the northeast of he center, with a few cells also now having developed just to the southwest of the center. Based on the structure noted in the ASCAT data and the pronounced increase in the amount and organization of the convection, the advisory intensity is estimated to be 30 kt. The initial motion estimate is 340/06 kt. The system has slowed down markedly during the past several hours, likely due to the sharp increase in the associated convection. A turn toward the north is forecast to begin later this afternoon as the system moves into a break in the Atlantic subtropical ridge pattern, with a general northward motion continuing through the remainder of the forecast period. The latest NHC model guidance is surprisingly in good agreement on this track scenario, with only minor forward speed differences noted between the models. The 18-20 kt of westerly vertical wind shear affecting the depression is expected to abate somewhat during the next 12-24 hours, which should allow for some slight strengthening to occur while the system moves over 27.5 deg C sea-surface temperatures. By 48 hours, however, the shear is forecast to increase again in excess of 25 kt, which should act to weaken the cyclone, possibly even causing it to degenerate into a remnant low. For now, however, the official intensity forecast calls for the system to remain a tropical depression at days 3 and 4 in the event the cyclone regenerates at day 5 when the shear is forecast to decrease below 10 kt, which may allow for convection to redevelop. The official intensity forecast closely follows a blend of the intensity consensus models IVCN and NOAA-HCCA. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 14.0N 49.9W 30 KT 35 MPH 12H 28/1800Z 14.8N 50.3W 35 KT 40 MPH 24H 29/0600Z 16.1N 50.3W 35 KT 40 MPH 36H 29/1800Z 17.7N 50.2W 35 KT 40 MPH 48H 30/0600Z 19.1N 50.0W 30 KT 35 MPH 60H 30/1800Z 20.7N 49.7W 30 KT 35 MPH 72H 31/0600Z 21.9N 49.7W 30 KT 35 MPH 96H 01/0600Z 24.5N 50.5W 30 KT 35 MPH 120H 02/0600Z 27.5N 52.8W 30 KT 35 MPH $$ Forecaster Stewart
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