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Tropical Storm GRACE

Tropical Storm Grace Discussion Number   5
NWS National Hurricane Center Miami FL       AL072021
1100 AM AST Sat Aug 14 2021
Grace is sending mixed signals on its intensity this morning.  The 
storm has been producing persistent convective bursting since 
overnight, which would suggest that some strengthening has occurred. 
The latest subjective data-T numbers are 2.5 (35 kt) from TAFB and 
SAB, while objective satellite estimates are higher, roughly between 
45-50 kt.  Then, an ASCAT pass from 1302 UTC showed winds between 
35-40 kt.  Given these data, the initial intensity is set at 40 kt.  
A NOAA Hurricane Hunter aircraft is scheduled to investigate Grace 
this afternoon to provide a better estimate of the storm's 

Grace is speeding along toward the west (280 degrees) at 19 kt.  
Mid-level ridging, entrenched over the western Atlantic, is 
expected to weaken slightly during the next couple of days.  This 
evolution should cause Grace to slow down to 10-15 kt by Sunday 
night and Monday and take on a west-northwestward heading.  That 
general trajectory should continue through the end of the forecast 
period.  The track guidance is tightly clustered during the first 
48-60 hours or so, showing Grace moving near or across the Leeward 
Islands, Puerto Rico, and the Dominican Republic.  On days 3-5, 
there is a little more spread, with a few models keeping the system 
on a southern track across Cuba while others show tracks across the 
Bahamas.  The NHC track forecast splits this difference and 
continues to show a track running between Cuba and the Bahamas, 
very close to the HCCA consensus solution.

Deep-layer shear is forecast to be 10 kt or less for the next 36 
hours or so while Grace is approaching the islands of the Greater 
Antilles.  The thermodynamic environment also appears conducive for 
strengthening, but the system's continued fast motion is likely to 
be an inhibiting factor on the rate of intensification.  The NHC 
intensity forecast remains on the conservative side, bringing 
Grace's intensity up to 50 kt by the time the storm reaches the 
Virgin Islands and Puerto Rico, but it should be noted that this 
forecast is on the lower side of the guidance.  Only one model, the 
HWRF, brings Grace to hurricane strength, but it does this by 
having the storm move farther north and avoid land interaction 
altogether.  The intensity forecast is highly uncertain on days 3-5 
since it depends on exactly how much Grace moves over the Greater 
Antilles, and there is some model signal that increasing 
northeasterly upper-level winds over the Bahamas and Florida could 
become a negative factor.

Key Messages:
1.  Tropical storm conditions are expected in portions of the
Leeward Islands tonight and early Sunday, and the Virgin Islands 
and Puerto Rico on Sunday.  Tropical storm conditions are possible 
over eastern parts of the Dominican Republic by early Monday.
2.  Heavy rainfall could lead to flash and urban flooding over the 
Leeward and Virgin Islands.  Across Puerto Rico and the Dominican 
Republic, heavy rainfall may lead to flash, urban and small stream 
flooding, along with the potential for mudslides.
3.  There is a risk of wind and rainfall impacts across the rest of 
the Dominican Republic, Haiti, the Turks and Caicos Islands, the 
Bahamas, and Florida next week, and interests in those areas should 
monitor the progress of Grace and updates to the forecast. 

INIT  14/1500Z 16.2N  57.9W   40 KT  45 MPH
 12H  15/0000Z 16.8N  60.6W   45 KT  50 MPH
 24H  15/1200Z 17.5N  63.8W   50 KT  60 MPH
 36H  16/0000Z 18.1N  66.4W   50 KT  60 MPH...OVER PUERTO RICO
 48H  16/1200Z 18.6N  68.6W   50 KT  60 MPH...OVER DOMINICAN REP.
 60H  17/0000Z 19.4N  70.7W   45 KT  50 MPH...OVER DOMINICAN REP.
 72H  17/1200Z 20.5N  72.8W   45 KT  50 MPH...OVER WATER
 96H  18/1200Z 22.8N  77.3W   45 KT  50 MPH
120H  19/1200Z 25.4N  81.2W   45 KT  50 MPH
Forecaster Berg