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Tropical Depression Grace Discussion Number  12
NWS National Hurricane Center Miami FL       AL072021
500 AM EDT Mon Aug 16 2021
 
The structure and organization of Tropical Depression Grace has 
changed little over the past several hours. An earlier ASCAT-B 
overpass showed that the circulation is elongated from the NE-SW, 
with peak winds of 30 kt. An Air Force Reserve Hurricane Hunter 
Aircraft investigated Grace a few hours ago, and indicated that the 
cyclone refuses to make the decrease in forward speed and turn to 
the west-northwest that most of the guidance has been calling for. 
The aircraft also measured peak flight level winds of 37 kt at 925 
mb, which equates to about 28 kt at the surface. Although there were 
some SFMR vectors of tropical storm force, those winds were sampled 
in heavy rainfall and were likely not representative of the actual 
surface winds.  Based on this data, the initial intensity is kept at 
30 kt for this advisory. 

Grace is moving toward the west, or 275/13 kt. The forecast models 
continue to make a shift to the south, as they are coming into very 
good agreement on maintaining a strong mid-level ridge to the north 
of the cyclone throughout the forecast period. The latest track 
forecast was adjusted a little south of the previous one through the 
first couple of days of the forecast, with a larger shift to the 
south farther out in time. This forecast track remains to the north 
of the track consensus, so if the track models remain consistent, 
additional adjustments to the south will likely be made in the next 
forecast cycle. 

The evolution of the forecast track of Grace is having major 
implications on the future intensity of the cyclone. It is becoming 
more likely that the system will track over only the southern 
portion of Hispaniola, which would result in a lesser disruption 
of Grace's circulation. Furthermore, the more southern track would 
bring the cyclone over the warm waters of the northwestern Caribbean 
beginning on Tuesday. The environment over that portion of the 
Caribbean is not all that hostile, with about 10-15 kt of northerly 
shear being the main inhibiting factor for intensification. By 
midweek, Grace may interact with the Yucatan Peninsula, which 
should limit strengthening or perhaps weaken the system depending on 
the track. Once over the western Gulf of Mexico, late in the 
forecast period, some additional strengthening is expected, 
although the wind shear is forecast to increase slightly during 
that time frame. The latest NHC intensity forecast was raised 
slightly to reflect the additional time expected over water. 
However, this forecast is a little lower than most of the guidance. 

 
Key Messages:
 
1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, 
Jamaica, and the Cayman Islands may lead to flash, urban, and small 
stream flooding, with the potential for mudslides highest in Haiti 
and the Dominican Republic.
 
2. Tropical storm conditions are possible over portions of
Hispaniola today and tonight. 
 
3. There is a risk of some wind and rainfall impacts across 
southeastern Cuba and Jamaica beginning Tuesday morning, and the 
Cayman Islands by Tuesday night. Interests there areas should 
monitor the progress of Grace and updates to the forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/0900Z 17.4N  70.2W   30 KT  35 MPH
 12H  16/1800Z 17.9N  72.2W   30 KT  35 MPH
 24H  17/0600Z 18.6N  74.8W   30 KT  35 MPH
 36H  17/1800Z 19.3N  77.4W   35 KT  40 MPH
 48H  18/0600Z 20.1N  80.2W   35 KT  40 MPH
 60H  18/1800Z 20.8N  83.3W   40 KT  45 MPH
 72H  19/0600Z 21.6N  86.3W   45 KT  50 MPH
 96H  20/0600Z 22.6N  91.8W   45 KT  50 MPH
120H  21/0600Z 23.4N  96.2W   50 KT  60 MPH
 
$$
Forecaster Latto
 
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