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Tropical Depression Twenty Discussion Number   1
NWS National Hurricane Center Miami FL       AL202021
200 PM CVT Wed Sep 29 2021

The area of low pressure located over the far eastern tropical 
Atlantic that NHC has been monitoring has now become a tropical 
depression, the twentieth tropical cyclone of the 2021 Atlantic 
hurricane season. Satellite images indicate that the system is 
producing a large area of showers and thunderstorms that have become 
fairly well organized around the center.  An ASCAT-A pass from a  
few hours ago indicated that the system now has a well-defined 
circulation and that the radius of maximum wind is 30-40 n mi north 
of the center.  The initial intensity is set at 30 kt based on the 
ASCAT data and the T2.0/30 kt Dvorak classifications from TAFB and 
SAB.  The minimum pressure of 1007 mb is partially based on data 
from a ship report that passed near the depression.

Based on satellite fixes, the depression appears to be moving 
west-northwestward at about 12 kt.  The system is currently located 
on the south side of a deep-layer ridge, and that feature should 
keep the cyclone on a west-northwest track during the next couple of 
days.  After that time, the models show a mid- to upper-level low 
developing over the central tropical Atlantic, which erodes the 
western portion of the ridge.  This change in the steering flow 
should cause the system to turn northwestward on Friday and then 
northward toward the end of the forecast period.  Although the 
models generally agree on the evolution of the large-scale pattern, 
there are notable differences in the details, which leads to a fair 
amount of spread concerning when and where the turn to the north 
occurs.  The GFS is farthest east while the ECMWF shows the 
westernmost solution.  The NHC track forecast lies between those 
models and is near the TVCA multi-model consensus.  

The depression is expected to remain in conducive environmental 
conditions for strengthening during the next 2 or 3 days.  During 
that time period, the storm is forecast to move over 28-29 deg C 
waters while embedded within an airmass of high mid-level moisture 
and very low wind shear (less than 10 kt).  Therefore, steady 
strengthening seems likely, and the NHC forecast brings the system 
to a tropical storm by tonight and a hurricane in a couple of days.  
Beyond a few days, however, the models show a pronounced increase 
in southwesterly shear and a notably drier atmosphere. These 
unfavorable conditions should end the opportunity for strengthening 
and promote a weakening trend.  The NHC intensity forecast is a near 
a blend of the HCCA, IVCN, and IVDR consensus models.  

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/1500Z  8.3N  24.6W   30 KT  35 MPH
 12H  30/0000Z  8.8N  25.8W   35 KT  40 MPH
 24H  30/1200Z  9.5N  27.6W   45 KT  50 MPH
 36H  01/0000Z 10.3N  29.9W   55 KT  65 MPH
 48H  01/1200Z 11.2N  31.9W   65 KT  75 MPH
 60H  02/0000Z 12.4N  33.7W   70 KT  80 MPH
 72H  02/1200Z 14.0N  35.4W   70 KT  80 MPH
 96H  03/1200Z 18.4N  38.8W   55 KT  65 MPH
120H  04/1200Z 24.3N  40.9W   45 KT  50 MPH
 
$$
Forecaster Cangialosi
 
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