[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT43 KNHC 071156
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



$$

Forecaster Brown
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