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Tropical Storm Grace Discussion Number  18
NWS National Hurricane Center Miami FL       AL072021
500 PM EDT Tue Aug 17 2021
 
Radar imagery and earlier Hurricane Hunter aircraft and ASCAT 
observations indicate that the center of Grace has been moving over 
Jamaica.  The aircraft and scatterometer data indicate that the 
maximum winds are near 45 kt.  Cirrus-level outflow from the 
system remains quite pronounced, suggestive of weak vertical shear 
at this time.

Grace will be moving over the waters of the northwestern Caribbean 
Sea beginning later tonight.  These waters are of very high oceanic 
heat content.  Although mid-level environmental humidities are 
shown by the SHIPS diagnoses to be only marginally moist for the 
next couple of days, other factors should be conducive for 
strengthening.  The official intensity forecast, which shows Grace 
becoming a hurricane tomorrow, is in reasonable agreement with the 
latest multi-model consensus.  Some slight weakening should occur 
due to passage over the Yucatan Peninsula, followed by 
re-intensification over the Bay of Campeche.  There is, however, 
significant uncertainty in the intensity prediction for this latter 
part of the forecast period.

The storm continues on a heading just north of west, or 280/13 kt.  
A strong 500-mb ridge should be maintained to the north of Grace 
for essentially the entire forecast period.  There is good 
agreement among the track models on a continued generally westward 
to west-northwestward motion for the next 5 days.  Little change 
has been made to the official track forecast in comparison to the 
previous one.  The new NHC track is, again, very close to the 
dynamical model consensus. 

 
Key Messages:
 
1.  Hurricane conditions and a dangerous storm surge are expected 
over portions of the Hurricane Warning area in the eastern Yucatan 
Peninsula of Mexico late Wednesday night.
 
2. Heavy rainfall across Haiti, Cuba, Jamaica, the Cayman Islands, 
and the Yucatan Peninsula is likely to cause to flash, urban, and 
small stream flooding, with the potential for mudslides highest in 
Haiti.
 
3. Tropical storm conditions are expected over portions of Jamaica
through this evening, and over the Cayman Islands tonight and 
early Wednesday.  Tropical storm conditions are expected to spread 
westward from portions of the southern coast of Cuba in the warning 
area to, possibly, other portions of the southern coast of Cuba in 
the watch area tonight through Wednesday.


FORECAST POSITIONS AND MAX WINDS
 
INIT  17/2100Z 18.4N  77.9W   45 KT  50 MPH
 12H  18/0600Z 18.9N  80.3W   60 KT  70 MPH
 24H  18/1800Z 19.4N  83.4W   70 KT  80 MPH
 36H  19/0600Z 20.0N  86.7W   75 KT  85 MPH
 48H  19/1800Z 20.5N  89.7W   60 KT  70 MPH...INLAND
 60H  20/0600Z 20.9N  92.2W   65 KT  75 MPH...OVER WATER
 72H  20/1800Z 21.2N  94.8W   75 KT  85 MPH
 96H  21/1800Z 21.0N  99.5W   40 KT  45 MPH...INLAND
120H  22/1800Z 21.0N 103.0W   20 KT  25 MPH...POST-TROP/INLAND
 
$$
Forecaster Pasch
 
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