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Tropical Storm Grace Discussion Number   4
NWS National Hurricane Center Miami FL       AL072021
500 AM AST Sat Aug 14 2021
 
The cyclone has become a little better organized overnight, with an 
area of concentrated convection persisting over the center for the 
past several hours. The system remains compact, with the convective 
canopy about 100 n mi in diameter, while earlier ASCAT data revealed 
winds greater than 25 kt extended only about 30 n mi from its 
center, with peak winds of 27 kt. Despite the weaker maximum winds 
sampled earlier this evening, the increase in organization of such 
a small system has likely produced a notable increase in the 
surface winds near the center. In addition the latest Dvorak 
intensity estimates from TAFB, SAB, and the UW-CIMSS ADT and SATCON 
suggest that the intensity is now somewhere between 30 and 45 kt. 
Based on a conservative blend of these values, it is estimated that 
the Tropical Depression Seven has strengthened into 35-kt Tropical 
Storm Grace.
 
Grace continues to move quickly westward, with an initial motion of 
280/19 kt. A strong mid-level ridge to the north of the storm should 
continue to steer it westward for the next few days. Beyond that 
time, the forecast models begin to diverge in their track solutions, 
as they vary in the strength of the ridge. Overall, the models have 
trended toward a stronger, more persistent ridge to the north of 
the cyclone for the latter half of the forecast period, and as such 
the model solutions are generally showing a track that lies a bit 
south of the previous runs for those time periods. The official NHC 
forecast track is little changed through 72 h and lies near the 
middle of the tightly clustered track guidance. Beyond 72 h, the 
forecast track has been shifted slightly to the south, but still 
lies to north of the consensus model tracks. 
 
The earlier bout of wind shear that had entrained surrounding dry 
air into Grace appears to have abated, at least in the short term. 
And, global models indicate that the shear will remain low for about 
the next 24-36 h while the cyclone moves over increasing SSTs. 
Therefore, it is reasonable to assume that strengthening should 
occur over the next day or so. Thereafter, the intensity forecast 
becomes somewhat complicated as the system is forecast to interact 
with an upper-level trough while possibly crossing the Greater 
Antilles at the same time. Slight weakening is indicated in the NHC 
intensity forecast as the system crosses the northern portion of the 
Dominican Republic early next week. Thereafter, moderate to strong 
northerly shear is forecast to impact Grace as the system moves into 
the flow on the western side of an upper-level trough over the 
western Atlantic. This shear should prevent any further 
strengthening through the end of the forecast period. The intensity 
forecast is a little lower than a blend of the NOAA HCCA and IVCN 
consensus, as there is likely a high bias from the HWRF in these 
solutions. The latter portion of the NHC intensity forecast is of 
lower-than-normal confidence due to the potential for a longer 
amount of time over the rugged terrain of the Greater Antilles than 
indicated, and also due to the fragile nature of the small cyclone 
and the possible effects of the stronger shear later in the 
forecast period. 

 
Key Messages:
 
1.  Tropical storm conditions are expected in portions of the 
Leeward Islands by tonight or early Sunday, and the U.S. Virgin 
Islands and Puerto Rico on Sunday. Tropical storm conditions are 
also possible over the British Virgin Islands on Sunday. The risk of 
strong winds will then spread westward to the Dominican Republic 
Sunday night and Monday.
 
2.  Heavy rainfall could lead to flash and urban flooding over the 
Leeward and Virgin Islands. Across Puerto Rico, 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 Haiti, the 
Turks and Caicos Islands, the southeastern Bahamas, Cuba, and 
Florida next week, and interests in those areas should monitor the 
progress of this system.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/0900Z 15.8N  55.6W   35 KT  40 MPH
 12H  14/1800Z 16.2N  58.5W   35 KT  40 MPH
 24H  15/0600Z 16.9N  61.8W   40 KT  45 MPH
 36H  15/1800Z 17.7N  64.8W   45 KT  50 MPH
 48H  16/0600Z 18.3N  67.3W   45 KT  50 MPH
 60H  16/1800Z 18.9N  69.4W   40 KT  45 MPH...INLAND
 72H  17/0600Z 19.8N  71.5W   40 KT  45 MPH...INLAND
 96H  18/0600Z 21.9N  75.6W   40 KT  45 MPH...OVER WATER
120H  19/0600Z 24.5N  79.7W   40 KT  45 MPH
 
$$
Forecaster Latto
 
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