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Potential Tropical Cyclone SIX

Potential Tropical Cyclone Six Discussion Number   3
NWS National Hurricane Center Miami FL       AL062021
500 AM AST Tue Aug 10 2021
The disturbance moved through the southern Leeward Islands a few 
hours ago.  Based on surface observations from the islands, the 
system still does not have a well-defined circulation.  Although 
the satellite appearance shows some increase in organization, the 
surface data suggest no significant strengthening has occurred thus 
far.  The current intensity estimate is held at 30 kt in agreement 
with a Dvorak estimate from SAB.  A NOAA Hurricane Hunter aircraft 
is scheduled to investigate the disturbance later this morning and 
should provide an updated intensity estimate, and also determine 
whether the system has become a tropical cyclone. 
The disturbance continues moving west-northwestward, or about 
295/15 kt.  The flow on the south side of a strong subtropical 
ridge should maintain the west-northwestward track for the next 
72 hours or so, with some slowing of forward speed by midweek.  
Beyond that time, a turn toward the northwest with further 
deceleration is likely while the system moves near the western 
periphery of the ridge.  The official track forecast is very 
similar to the previous one and in good agreement with the latest 
simple and corrected multi-model consensus tracks.  Users are 
reminded that the average track errors at days 4 and 5 are 175 miles 
and 200 miles, respectively.

Dynamical conditions appear to be conducive for intensification 
during the next 24 hours or so, with low vertical shear and 
pronounced upper-level outflow over the system.  The main 
hindrance for strengthening today and tonight appears to be some 
dry mid-tropospheric air in the environment, as shown by the global 
models.  Since the projected track takes the disturbance or cyclone 
over the mountainous land mass of Hispaniola, some weakening and 
disruption of the circulation is likely in 36 hours or so.  Later 
in the forecast period, the dynamical guidance shows some increase 
in westerly shear which could also inhibit strengthening.  The 
official intensity forecast closely follows the NOAA corrected 
consensus forecast technique, HCCA.  There is significant 
uncertainty in 4-5 day intensity forecasts, however.
1. The system is forecast to become a tropical storm as it moves
over the northeastern Caribbean Sea today.  Tropical storm 
conditions are expected in portions of the U.S. Virgin Islands and 
Puerto Rico beginning this afternoon, and in the Dominican Republic 
by Wednesday.  Tropical storm conditions are possible in northern 
Haiti and the southeastern Bahamas by late 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 elsewhere in
portions of Hispaniola, the 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.
INIT  10/0900Z 15.7N  62.2W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  10/1800Z 16.7N  64.6W   40 KT  45 MPH...TROPICAL CYCLONE
 24H  11/0600Z 17.9N  67.4W   45 KT  50 MPH
 36H  11/1800Z 19.0N  70.1W   30 KT  35 MPH...INLAND
 48H  12/0600Z 20.1N  72.6W   30 KT  35 MPH
 60H  12/1800Z 21.1N  74.9W   30 KT  35 MPH
 72H  13/0600Z 22.0N  77.0W   35 KT  40 MPH
 96H  14/0600Z 24.4N  80.8W   40 KT  45 MPH
120H  15/0600Z 26.8N  83.1W   50 KT  60 MPH
Forecaster Pasch