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Tropical Storm Grace Discussion Number  16
NWS National Hurricane Center Miami FL       AL072021
500 AM EDT Tue Aug 17 2021
 
An Air Force Reserve reconnaissance aircraft had difficulty locating 
the center of Grace early this morning. The last few visible 
satellite images along with 0200Z ASCAT wind data indicated that a 
lee-side surface low had formed south of the western tip of Haiti. 
That feature has gradually been pulling northward closer to the deep 
convection that has developed near the well-defined mid-level 
circulation center. The initial position is the midpoint between the 
recon position in the latest vortex message and the mid-level 
circulation noted in passive microwave imagery. The aircraft also 
found some patches of 925-mb flight-level winds of 48-51 kt, which 
equates to surface winds of 36-38 kt. Reliable SFMR surface winds 
obtained by the aircraft were also in the 36-38-kt range. Based on 
these data, Grace was upgraded back to a 35-kt tropical storm at 
0600 UTC. Regardless of Grace's exact intensity right now, the 
immediate threat is still torrential rainfall across western 
Hispaniola today and over Jamaica this afternoon and tonight, which 
will likely cause severe flooding in some locations.
 
The initial motion estimate is an uncertain 280/14 kt. A strong 
mid-level ridge is forecast to build westward across Florida and the 
Gulf of Mexico during the next several days, which should keep Grace 
on a westward or west-northwestward track through the 120-h forecast 
period. Only a very sight southward shift in the previous forecast 
track was made, which was due primarily to the new estimated 
southerly initial position, and the new NHC forecast track lies 
along the northern edge of model guidance envelope.
 
Once Grace passes Jamaica in 12-18 hours, the cyclone will be 
passing over water temperatures near 30 deg C and into a more 
divergent upper-level wind flow pattern. These two favorable 
conditions should allow for Grace to slowly intensify and possibly 
be near hurricane strength by the time the cyclone reaches the 
Yucatan peninsula in about 48 hours. Thereafter, Grace will weaken 
as it pass over Yucatan, and then re-strengthen some over the Bay 
of Campeche before reaching mainland Mexico in about 96 hours. The 
NHC intensity forecast is close to a blend of the HCCA and IVCN 
consensus models.
 
Key Messages:
 
1. Heavy rainfall across the Dominican Republic, Haiti, Cuba,
Jamaica, the Cayman Islands, and the northern Yucatan Peninsula 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 and over Jamaica on today. Tropical storm conditions are 
expected over portions of the southern coast of Cuba today, 
spreading westward to the Cayman Islands and possibly other 
portions of the southern coast of Cuba this evening through
Wednesday morning.
 
3.  There is a increasing risk of wind, rainfall, and storm surge
impacts on the Yucatan Peninsula of Mexico Wednesday night and
Thursday.  Interests there areas should monitor the progress of
Grace and updates to the forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  17/0900Z 18.2N  75.4W   35 KT  40 MPH
 12H  17/1800Z 18.6N  77.6W   40 KT  45 MPH
 24H  18/0600Z 19.1N  80.7W   45 KT  50 MPH
 36H  18/1800Z 19.7N  83.9W   50 KT  60 MPH
 48H  19/0600Z 20.4N  87.0W   60 KT  70 MPH
 60H  19/1800Z 21.0N  90.0W   50 KT  60 MPH...INLAND
 72H  20/0600Z 21.4N  92.7W   55 KT  65 MPH...OVER WATER
 96H  21/0600Z 22.0N  97.2W   60 KT  70 MPH
120H  22/0600Z 22.3N 101.4W   30 KT  35 MPH...INLAND
 
$$
Forecaster Stewart
 
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