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Potential Tropical Cyclone FIVE

Potential Tropical Cyclone Five Discussion Number   1
NWS National Hurricane Center Miami FL       AL052021
500 PM AST Wed Jun 30 2021

Satellite imagery indicates that the convection associated with the 
low pressure area over the central tropical Atlantic is becoming 
better organized.  However, earlier scatterometer data showed that 
the circulation was elongated and not well defined.  Since the 
system is about 48 h away from the Lesser Antilles and is expected 
to become a tropical storm before reaching the islands, advisories 
are being initiated at this time on Potential Tropical Cyclone Five. 
The initial intensity is set to 30 kt based on the scatterometer 
data and satellite intensity estimates from TAFB and SAB.

The initial motion is 285/18.  The disturbance is currently on the 
south side of a strong subtropical ridge, and for the next 72 h the 
system  should move rapidly west-northwestward.  Indeed, the forward 
speed may exceed 25 kt as the system moves through the Lesser 
Antilles.  There is excellent agreement in the guidance on this 
part of the forecast track.  After 72 h, the system is expected to 
approach a weakness in the ridge caused by a mid-latitude trough 
over the eastern United States.  The should cause the system to slow 
its forward speed and turn more northwestward.  The track guidance 
become more diverse during this time in regards to how far north 
the system will turn, and this part of the NHC forecast track is 
most similar to the GFS and UKMET forecasts.  Note that the track 
forecast is more uncertain than usual since the system does not yet 
have a well-defined center.

The disturbance is currently in a favorable upper-level wind 
pattern.  However, most Atlantic systems in this area moving near 25 
kt have trouble strengthening, and there are indications that the 
fast forward speed may produce some westerly shear.  Thus, the NHC 
intensity forecast for the first 72 h is near the lower edge of the 
intensity guidance.  After 72 h, the intensity forecast becomes 
highly uncertain due to possible land interaction and differences in 
the model forecasts of the upper-level winds.  This portion of the 
NHC intensity forecast maintains a constant 50-kt intensity due to 
the level of uncertainty.

Key Messages:

1. The system is expected to become a tropical storm before reaching 
the Lesser Antilles and tropical storm conditions are possible 
beginning Friday in portions of the Windward and southern Leeward 

2. Heavy rainfall will move quickly across the Windward and southern 
Leeward Islands, including Barbados, on Friday.  Isolated flash 
flooding and mudslides are possible.

3. There is a risk of wind and rainfall impacts in portions of the 
Virgin Islands, Puerto Rico, Hispaniola, Cuba, the Turks and Caicos 
and the southeastern Bahamas through early next week, although the 
forecast is more uncertain than usual since the system does not have 
a well-defined center. Interests in these areas should monitor the 
system's progress and updates to the forecast. 

4. Interests in Florida should monitor updates to the forecast for 
this system, but it is too soon to determine what if any impacts 
could occur there next week given the uncertainty in the long-range 

INIT  30/2100Z  9.6N  43.7W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  01/0600Z 10.2N  46.6W   35 KT  40 MPH...TROPICAL CYCLONE
 24H  01/1800Z 11.1N  51.0W   40 KT  45 MPH
 36H  02/0600Z 12.4N  56.0W   45 KT  50 MPH
 48H  02/1800Z 13.8N  61.2W   45 KT  50 MPH
 60H  03/0600Z 15.4N  66.3W   50 KT  60 MPH
 72H  03/1800Z 17.0N  70.9W   50 KT  60 MPH
 96H  04/1800Z 20.0N  77.0W   50 KT  60 MPH...INLAND
120H  05/1800Z 22.5N  81.5W   50 KT  60 MPH...INLAND
Forecaster Beven