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