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Tropical Depression Seven Discussion Number   2
NWS National Hurricane Center Miami FL       AL072021
500 PM AST Fri Aug 13 2021
 
Over the past few hours, the system moved just to the north of a 
buoy owned by the Woods Hole Oceanographic Institute.  The 
buoy's winds backed from northwest to west to south, indicating 
that the system has a closed surface circulation.  In addition, 
Dvorak estimates from TAFB and SAB were a unanimous T2.0, and the 
system is therefore being designated as a tropical depression on 
this advisory.  Maximum winds remain 30 kt based on earlier ASCAT 
data and the Dvorak estimates.

The circulation has closed off despite the depression moving 
quickly westward (275 degrees) at about 19 kt.  The track guidance 
is tightly clustered during the first 48-60 hours, with the 
depression being driven quickly westward across the Leeward Islands 
and toward the Greater Antilles by ridging to the north.  After 60 
hours, there is considerably more spread, with the regional 
dynamical models keeping the system farther south over the 
Caribbean Sea, and most of the other models indicating a turn 
toward the west-northwest, following a track similar to Tropical 
Depression Fred.  The NHC track forecast generally favors the 
latter scenario and is very close to the HCCA and TVCN consensus 
aids.

During the next 2 days, the depression is expected to move beneath 
an upper-level ridge axis, which should allow the deep-layer shear 
to fall below 10 kt, with the system also moving over warmer waters 
and through an environment of increased moisture.  However, the 
depression's fast motion, as well as the possible development of 
some mid-level westerly shear, could stunt the rate of 
strengthening.  Due to these conflicting factors, the NHC intensity 
forecast remains on the conservative side and is not quite as high 
as the solutions shown by the SHIPS and HCCA models.  The HWRF model 
is quite aggressive, bringing the depression to hurricane strength 
by day 3, but that model is an extreme outlier compared to the other 
guidance.  After 48 hours, the current forecast takes the center of 
the depression over Puerto Rico and Hispaniola, a scenario that 
would cause weakening and suppress the system's intensity.  As is 
typically the case, the system could get stronger than shown in the 
official forecast if it ends up moving over less land, or dissipate 
entirely if it moves over land for too long.
 
Key Messages:
 
1.  Tropical storm conditions are possible in portions of the
Leeward Islands late Saturday or early Sunday, and over the 
Virgin Islands and Puerto Rico on Sunday.  The risk of strong winds 
will then spread westward to 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, 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 Haiti, the 
Turks and Caicos Islands, the southeastern Bahamas, and Cuba next 
week, and interests in those areas should monitor the progress of 
this system.

FORECAST POSITIONS AND MAX WINDS
 
INIT  13/2100Z 15.4N  51.8W   30 KT  35 MPH
 12H  14/0600Z 15.9N  54.7W   35 KT  40 MPH
 24H  14/1800Z 16.4N  58.5W   35 KT  40 MPH
 36H  15/0600Z 17.0N  61.9W   40 KT  45 MPH
 48H  15/1800Z 17.7N  65.0W   45 KT  50 MPH
 60H  16/0600Z 18.4N  67.7W   45 KT  50 MPH
 72H  16/1800Z 19.0N  70.3W   40 KT  45 MPH...OVER HISPANIOLA
 96H  17/1800Z 20.9N  74.7W   35 KT  40 MPH...OVER WATER
120H  18/1800Z 23.6N  79.1W   40 KT  45 MPH
 
$$
Forecaster Berg
 
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