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Tropical Storm Grace Discussion Number   6
NWS National Hurricane Center Miami FL       AL072021
500 PM AST Sat Aug 14 2021
 
Grace is a poorly organized tropical storm this afternoon.  The
NOAA Hurricane Hunters were unable to close off a circulation, at
least at their flight level of 5000 feet, but dropsonde observations
and reports from some of the islands in the Lesser Antilles suggest
that there's at least a broad cyclonic circulation at the surface.
The various data also indicate that the center has sped up, or
re-formed, and is located farther southwest than previously
estimated.  Based on the aircraft data and earlier ASCAT data, the
initial intensity is set at 35 kt.
 
Both the future track and intensity of Grace have a high level of 
uncertainty.  For the track, the forecast is likely to be 
complicated by the fact that the system doesn't currently have a 
tight low-level circulation, and the center could always re-form at 
any time, especially with convection ongoing farther to the north. 
In addition, the storm has not yet slowed down, and in fact, the 
initial motion is estimated to be toward the west (275 degrees) at 
23 kt.  The guidance envelope has made a notable southward shift due 
to the adjustment of the initial position, and the models insist 
that Grace will primarily have a west-northwestward heading for much 
of the forecast period with the speed gradually decreasing during 
the next 48 hours or so.  The NHC track forecast has been shifted 
southward accordingly, although any re-formations of the center 
could cause this track to shift again in future advisory cycles.
 
If Grace slows down as forecast--which is obviously not a sure
thing--environmental conditions should be conducive to allow for
some strengthening before the system reaches the Greater Antilles.
The southward adjustment in the official forecast now takes Grace
over the Greater Antilles for a longer period of time, and the
official intensity forecast is therefore lowered beyond 48 hours.
This is a middle-of-the-road solution, and actually lower than most
of the intensity guidance.  If the forecast track shifts north or
south, the system could strengthen further over water. 
Alternatively, Grace could go the way of Fred and dissipate before
the end of the 5-day period.
 
In the end, the exact track of the center and the intensity of the
system will likely not be as important as the heavy rainfall that
is forecast to fall across the Leeward Islands and the Greater
Antilles during the next few days.

Key Messages:
 
1.  Tropical storm conditions are expected in portions of the
Leeward Islands tonight, and the Virgin Islands and Puerto Rico on
Sunday.  Tropical storm conditions are possible over eastern parts
of 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, the Dominican 
Republic, and Haiti, 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 next week, 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  14/2100Z 15.9N  60.7W   35 KT  40 MPH
 12H  15/0600Z 16.5N  63.3W   40 KT  45 MPH
 24H  15/1800Z 17.1N  66.1W   45 KT  50 MPH
 36H  16/0600Z 17.7N  68.5W   50 KT  60 MPH
 48H  16/1800Z 18.3N  70.6W   50 KT  60 MPH...NEAR DOMINICAN REP.
 60H  17/0600Z 19.2N  72.8W   40 KT  45 MPH...NEAR HAITI
 72H  17/1800Z 20.2N  75.3W   40 KT  45 MPH...OVER CUBA
 96H  18/1800Z 22.4N  80.1W   35 KT  40 MPH...OVER CUBA
120H  19/1800Z 24.8N  84.3W   45 KT  50 MPH...OVER WATER
 
$$
Forecaster Berg
 
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