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Potential Tropical Cyclone Six Discussion Number   1
NWS National Hurricane Center Miami FL       AL062021
500 PM AST Mon Aug 09 2021

Deep convection associated with the area of low pressure located 
northeast of Barbados has continued to consolidate this afternoon, 
with several bands noted in both satellite and radar data from 
Barbados and Martinique.  ASCAT data from this morning revealed a 
sharp trough axis, but the system lacked a well-defined circulation. 
However, recent visible satellite imagery hints that a better 
defined center may be forming just southwest of the primary 
convective mass.  These trends suggest the system is likely to 
become a tropical depression or storm tonight or Tuesday when it 
moves near the Lesser Antilles and into the eastern Caribbean.  
Therefore, advisories are being initiated on the system in order to 
issue Tropical Storm Watches for portions of the Lesser Antilles, 
the U.S. Virgin Islands, Puerto Rico and parts of the Dominican 
Republic. 

Since the system is still in its formative stage the initial motion 
estimate is a somewhat uncertain 290/13 kt. A mid-tropospheric ridge 
anchored over the western Atlantic should steer the system 
west-northwestward through the forecast period.  Although there is 
high confidence in the overall steering pattern over the next 
several days, there is lower than normal confidence in the details 
of the track forecast, especially in the short-range due to the lack 
of a well-defined center.  Exactly where the center forms will have 
some downstream implications on the exact forecast track, especially 
across the eastern Caribbean.  Regardless of the exact track, 
locally heavy rainfall and gusty winds are expected to spread across 
portions of the Leeward Islands, the Virgin Islands, Puerto Rico, 
and the Dominican Republic over the next day or two, hence the need 
for Tropical Storm Watches for portions of those areas. 

The disturbance is embedded within an area of low vertical wind 
shear and SSTs of around 28C. These conditions favor additional 
development, but the occasional entrainment of dry mid-level air 
located just west of the system is likely to prevent more rapid 
organization. By Wednesday, the system is likely to be near 
Hispaniola, where subtle differences in the forecast track could 
have large implications on the intensity of the storm later this 
week. A track directly over Hispaniola would likely significantly 
disrupt the circulation, while a track more poleward of the island 
could allow the system to stay more intact.  An additional caveat 
beyond 48 hours is that vertical wind shear out of the southwest may 
also increase, which could limit the intensity after the system 
moves past Hispaniola, although uncertainty exists in how much the 
shear will increase given differences between the more favorable 
ECMWF and less favorable GFS model solutions. The NHC intensity 
forecast brings the system up to a 45 kt tropical storm before 
potential land interaction with Hispaniola and afterwards is 
conservative given the possibility of additional land interaction 
and less favorable environmental conditions towards the end of the 
forecast period. 

KEY MESSAGES:

1. The system is forecast to become a tropical storm as it moves 
through the Lesser Antilles tonight.  Tropical storm conditions are 
possible in portions of the southern Leeward Islands tonight and 
Tuesday and in the Virgin Islands and Puerto Rico beginning Tuesday 
afternoon, and in the Dominican Republic by Wednesday. 

2. Heavy rainfall could lead to flash, urban, and small stream 
flooding and potential mudslides across the U.S. Virgin Islands and 
Puerto Rico. The greatest threat for flooding impacts will be across 
the eastern and southeastern portions of Puerto Rico.

3. There is a risk of wind and rainfall impacts in portions of 
Hispaniola, the Turks and Caicos, the southeastern Bahamas, and Cuba 
later this week, although the forecast is more uncertain than usual 
since the system is still in its formative stage. Interests in these 
areas should monitor the system's progress and updates to the 
forecast. 

4. Interests in the remainder of the Bahamas and Florida should 
monitor updates to the forecast for this system, but it is too soon 
to determine what if any impacts could occur there by late this week 
or this weekend given the uncertainty in the long-range forecast.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  09/2100Z 14.2N  59.2W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  10/0600Z 15.4N  61.4W   35 KT  40 MPH...TROPICAL CYCLONE
 24H  10/1800Z 16.8N  64.5W   40 KT  45 MPH
 36H  11/0600Z 17.9N  67.3W   45 KT  50 MPH
 48H  11/1800Z 19.2N  70.2W   35 KT  40 MPH...INLAND
 60H  12/0600Z 20.3N  72.8W   35 KT  40 MPH...OVER WATER
 72H  12/1800Z 21.1N  75.0W   35 KT  40 MPH
 96H  13/1800Z 22.8N  78.8W   40 KT  45 MPH
120H  14/1800Z 24.5N  82.0W   40 KT  45 MPH
 
$$
Forecaster Brown/Papin
 
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