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Tropical Storm Grace Discussion Number   8
NWS National Hurricane Center Miami FL       AL072021
500 AM AST Sun Aug 15 2021
 
Radar data from the Leeward Islands and Puerto Rico, as well as
satellite images, indicate that Grace is still not a well-organized
tropical cyclone, although over the past few hours banding features
have become more evident and the outflow has improved. An earlier
ASCAT-C overpass showed peak winds of 30 kt associated with Grace,
and the latest Dvorak intensity estimate from TAFB was 35 kt.
Assuming some undersampling from the ASCAT instrument, the initial 
intensity estimate remains 35 kt for this advisory.
 
Grace continues to move fairly quickly to the west-northwest, or 
285/18 kt to the south of a strong mid-level ridge. Aside from the 
HWRF, which is a northern outlier and has not performed particularly 
well for Fred or Grace, and also the Canadian model, which is a 
southern outlier, the track guidance has come into better agreement 
on the future path of Grace. The cyclone is expected to slow its 
forward speed today, and maintain a west-northwestward motion to the 
south of the ridge for the next several days. The NHC track forecast 
is little changed from the previous one through 72 h, and then was 
adjusted a little to the south of the previous one thereafter. On 
this path, Grace would pass just south of Puerto Rico later today, 
cross Hispaniola tonight through Monday night, then move along the 
northern coast of Cuba Tuesday and Wednesday.
 
Grace is in an environment favorable for intensification, and the 
NHC intensity forecast calls for modest strengthening to 45 kt 
before it reaches the Dominican Republic Monday morning. This 
portion of the forecast is in good agreement with the various 
consensus models. The intensity forecast becomes highly uncertain 
thereafter, and is dependent on how much of the Greater Antilles the 
cyclone interacts with. Based on the current track forecast, Grace 
would cross a large portion of the rugged terrain of Hispaniola, 
then interact with the landmass of Cuba for a couple of days. There 
is a decent chance that the low-level center of Grace could 
dissipate over Hispaniola as the system opens back into a tropical 
wave. However, due to the possibility of the center remaining intact 
after crossing that landmass, the NHC forecast calls for weakening 
followed by little change in strength thereafter as it moves along 
the Cuban coastline. The latest NHC intensity forecast beyond 24 h 
is little changed from the previous one, and is close to the FSU 
Superensemble.
 
Key Messages:
 
1.  Tropical storm conditions are expected over portions of the
Leewards Islands this morning and the Virgin Islands and Puerto Rico
today.  Tropical storm conditions are expected over eastern parts of
the Dominican Republic tonight and Monday. Tropical storm conditions
are possible over western portions of the Dominican Republic and
Haiti Monday and Monday night.
 
2.  Heavy rainfall could lead to flash and urban flooding over the
Leeward and Virgin Islands, Haiti and the Dominican Republic. 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 the rest of
the Dominican Republic, Haiti, the Turks and Caicos Islands, the
Bahamas, Cuba, and Florida, but forecast uncertainty remains higher
than usual. Interests in those areas should monitor the progress of
Grace and updates to the forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  15/0900Z 16.9N  64.4W   35 KT  40 MPH
 12H  15/1800Z 17.5N  66.6W   40 KT  45 MPH
 24H  16/0600Z 18.3N  68.9W   45 KT  50 MPH
 36H  16/1800Z 18.9N  70.6W   35 KT  40 MPH...INLAND OVER HISPANIOLA
 48H  17/0600Z 19.8N  72.6W   35 KT  40 MPH...INLAND OVER HISPANIOLA
 60H  17/1800Z 20.9N  75.3W   35 KT  40 MPH...OVER WATER
 72H  18/0600Z 22.0N  78.0W   35 KT  40 MPH...NEAR CUBA COAST
 96H  19/0600Z 23.9N  82.5W   40 KT  45 MPH
120H  20/0600Z 25.6N  86.2W   45 KT  50 MPH
 
$$
Forecaster Latto
 
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