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Tropical Depression EIGHTEEN


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 seasons. 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.


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

Forecaster Brown