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Potential Tropical Cyclone SEVEN

Potential Tropical Cyclone Seven Discussion Number   1
NWS National Hurricane Center Miami FL       AL072021
1100 AM AST Fri Aug 13 2021
A tropical wave and small area of low pressure moving quickly
westward across the central tropical Atlantic has been producing
bursts of deep convection near and to the west of the wave axis.
Scatterometer data from last evening indicated that the system did
not quite have a closed circulation, but it is close.  In addition,
the convective organization has increased a bit since yesterday,
with TAFB and SAB providing Dvorak classification of T1.5 and T2.0.
Therefore, only an additional slight increase in convective
organization and the low-level circulation closing off would make
the system a tropical depression, and it could be at tropical storm
strength when it approaches the Leeward Islands during the next
couple of days.  Therefore, advisories are being initiated on the
system in order to issue Tropical Storm Watches for portions of the
Leeward Islands at this time.
Subtropical ridging extending across nearly the entire Atlantic is
pushing the low along quickly toward the west, or 280/18 kt.  In
general this ridging is expected to persist into the middle part of
next week.  However, around the Sunday-Monday time frame, some
mid-level troughing over the western Atlantic could allow the
system to gain a little more latitude before it runs into
additional ridging located near the east coast of the United States
at the end of the forecast period.  The GFS, which has perhaps one 
of the strongest solutions, is a northern outlier and shows the
system turning farther into the weak troughing, while the ECMWF has
a weaker solution and keeps the system on a westward track across
the Lesser and Greater Antilles.  At this time, the NHC official
forecast lies close to the TVCN and HCCA consensus aids, and it is
also relatively close to the ECMWF ensemble mean, which is farther
north than its parent model.

The biggest negative to the system becoming a tropical cyclone and 
strengthening is its fast motion of 15-20 kt.  However, that motion 
is expected to gradually decrease in 2 to 3 days.  At that same 
time, deep-layer shear has decreased a bit, and the system will be 
moving over warmer waters and toward a slightly more moist 
environment.  Therefore, gradual strengthening is anticipated, and 
the NHC intensity forecast closely follows the HCCA and IVCN 
consensus aids through 72 hours, up until the system is near 
Hispaniola.  After that time, land interaction is likely to disrupt 
the circulation, and the official forecast is below the model 
consensus aids on days 4 and 5.

Key Messages:

1.  Tropical storm conditions are possible in portions of the 
Leeward Islands late Saturday or early Sunday, and the risk of 
tropical storm conditions will spread westward to the Virgin 
Islands and Puerto Rico late Sunday and Sunday night.

2.  Heavy rainfall with this system may produce areas of scattered 
flash flooding over the northern Leeward Islands Saturday into 

3.  There is a risk of wind and rainfall impacts in portions of 
Hispaniola, the Turks and Caicos Islands, the southeastern Bahamas, 
and Cuba next week, and interests in those areas should monitor the 
progress of this system.
INIT  13/1500Z 15.3N  49.3W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  14/0000Z 15.7N  52.0W   30 KT  35 MPH...TROPICAL DEPRESSION
 24H  14/1200Z 16.2N  55.9W   35 KT  40 MPH
 36H  15/0000Z 16.7N  59.6W   35 KT  40 MPH
 48H  15/1200Z 17.2N  63.0W   40 KT  45 MPH
 60H  16/0000Z 17.8N  66.0W   40 KT  45 MPH
 72H  16/1200Z 18.4N  68.7W   45 KT  50 MPH...OVER HISPANIOLA
 96H  17/1200Z 20.0N  73.5W   35 KT  40 MPH...OVER WATER
120H  18/1200Z 23.0N  77.0W   40 KT  45 MPH
Forecaster Berg