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| #1015537 (Received by flhurricane at: 4:50 AM 07.Sep.2020) TCDAT3 Tropical Depression Eighteen Discussion Number 1 NWS National Hurricane Center Miami FL AL182020 800 AM CVT Mon Sep 07 2020 Shower and thunderstorm activity associated with the area of low pressure over the far eastern Atlantic has continued to increase and become organized in a band overnight. First light visible satellite imagery suggests that the circulation has also become better defined. Based on the above, advisories are being initiated on a new tropical depression, the eighteenth of the 2020 Atlantic hurricane season. Dvorak satellite estimates from both TAFB and SAB support an intensity of 25 kt, but given the continued improvement in organization the initial winds are set at 30 kt for this advisory. ASCAT data later this morning should provide additional information on the intensity of the cyclone. The depression lies within a favorable upper-level environment and the SHIPS guidance indicates that the vertical wind shear will remain 10 kt or less over the next few days while the depression traverses sea surface temperatures of around 27 degrees Celsius. These conditions, along with a moist mid-level atmosphere should allow for steady strengthening over the next few days, and the NHC forecast calls for the system to become a tropical storm later today, and a hurricane in about 72 hours. After that time, the cyclone is forecast to pass over slightly cooler waters and the global models depict an increase in southwesterly upper-level flow over the system by the end of the period. These less conducive factors are expected to slow the intensification process toward the end of the period. The NHC intensity forecast is a blend of the IVCN multi-model consensus and the HFIP corrected consensus. The initial motion of the depression is a somewhat uncertain 280/10 kt. A narrow mid-level ridge to the north of the depression is forecast to build westward over the next few days which is expected to steer the system westward to west-northwestward, and the track guidance is in good agreement through 72 hours. After that time, a weakness is forecast to develop in the ridge near 40 degrees west, and the cyclone is expected to turn west-northwestward and then northwestward between the ridge to its northeast and Tropical Depression Seventeen to its southwest. Since there could be some binary interaction between the two tropical cyclones later in the period, the longer range track guidance is not in as good of agreement, and there is more uncertainty than average in the long-range track forecast. Key Messages: 1. The depression is forecast to strengthen into a tropical storm later today and tropical storm conditions are expected to spread over portions of the Cabo Verde Islands later today and tonight. A Tropical Storm Warning is in effect for those islands. 2. The depression is expected to bring locally heavy rainfall to portions of the Cabo Verde Islands through Tuesday morning. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 15.2N 20.3W 30 KT 35 MPH 12H 07/1800Z 15.6N 22.0W 35 KT 40 MPH 24H 08/0600Z 16.0N 24.5W 40 KT 45 MPH 36H 08/1800Z 16.4N 27.3W 50 KT 60 MPH 48H 09/0600Z 16.8N 30.2W 55 KT 65 MPH 60H 09/1800Z 17.4N 32.9W 60 KT 70 MPH 72H 10/0600Z 18.1N 35.5W 65 KT 75 MPH 96H 11/0600Z 19.7N 39.6W 70 KT 80 MPH 120H 12/0600Z 22.4N 43.2W 70 KT 80 MPH $$ |