Tropical Storm GRACE Advisory 8月14日

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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