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Tropical Depression Twenty-one Forecast Discussion Number 1
2020-09-14 11:50:27| National Hurricane Center (Atlantic)
Issued at 900 AM CVT Mon Sep 14 2020 000 WTNT41 KNHC 140950 TCDAT1 Tropical Depression Twenty-One Special Discussion Number 1 NWS National Hurricane Center Miami FL AL212020 900 AM CVT Mon Sep 14 2020 First-light visible satellite imagery indicates that the low pressure system that moved off the coast of Africa a few days ago has continued to become better organized overnight. Scatterometer surface wind data from around 13/2200 UTC indicated that the circulation had become better defined, and that surface winds of 25-28 kt existed in the southwestern quadrant. Since then, deep convection has increased, and it is presumed that the surface winds and circulation definition have increased in response, which justifies the initiation of advisories on TD-21. The initial motion estimate is an uncertain 355/05 kt. The cyclone is forecast to gradually move northward today and northwestward tonight as the depression moves around the eastern end of the eastern Atlantic/west African monsoon trough. By Tuesday night and continuing into Wednesday and Thursday, a west-northwestward to westward motion along the southern periphery of a deep-layer ridge is expected. The NHC track forecast lies down the middle of the surprisingly tightly packed guidance envelope, and is similar to the consensus model TVCA. The depression is expected to be short-lived as a tropical cyclone. Having said that, there is a narrow window of opportunity today and tonight for the cyclone to strengthen into a tropical storm before strong westerly shear in excess of 30 kt is forecast to induce rapid weakening on Tuesday. The cyclone is expected to degenerate into a remnant low by Tuesday night, and then dissipate over water on Friday, if not sooner. FORECAST POSITIONS AND MAX WINDS INIT 14/1000Z 18.5N 28.3W 30 KT 35 MPH 12H 14/1800Z 19.6N 28.6W 35 KT 40 MPH 24H 15/0600Z 20.5N 29.6W 30 KT 35 MPH 36H 15/1800Z 21.3N 30.9W 25 KT 30 MPH 48H 16/0600Z 21.9N 32.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 16/1800Z 22.4N 34.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 17/0600Z 22.8N 36.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 18/0600Z 23.4N 41.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 19/0600Z...DISSIPATED $$ Forecaster Stewart/Beven
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