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Tropical Storm Grace Discussion Number 5
NWS National Hurricane Center Miami FL AL072021
1100 AM AST Sat Aug 14 2021
Grace is sending mixed signals on its intensity this morning. The
storm has been producing persistent convective bursting since
overnight, which would suggest that some strengthening has occurred.
The latest subjective data-T numbers are 2.5 (35 kt) from TAFB and
SAB, while objective satellite estimates are higher, roughly between
45-50 kt. Then, an ASCAT pass from 1302 UTC showed winds between
35-40 kt. Given these data, the initial intensity is set at 40 kt.
A NOAA Hurricane Hunter aircraft is scheduled to investigate Grace
this afternoon to provide a better estimate of the storm's
Grace is speeding along toward the west (280 degrees) at 19 kt.
Mid-level ridging, entrenched over the western Atlantic, is
expected to weaken slightly during the next couple of days. This
evolution should cause Grace to slow down to 10-15 kt by Sunday
night and Monday and take on a west-northwestward heading. That
general trajectory should continue through the end of the forecast
period. The track guidance is tightly clustered during the first
48-60 hours or so, showing Grace moving near or across the Leeward
Islands, Puerto Rico, and the Dominican Republic. On days 3-5,
there is a little more spread, with a few models keeping the system
on a southern track across Cuba while others show tracks across the
Bahamas. The NHC track forecast splits this difference and
continues to show a track running between Cuba and the Bahamas,
very close to the HCCA consensus solution.
Deep-layer shear is forecast to be 10 kt or less for the next 36
hours or so while Grace is approaching the islands of the Greater
Antilles. The thermodynamic environment also appears conducive for
strengthening, but the system's continued fast motion is likely to
be an inhibiting factor on the rate of intensification. The NHC
intensity forecast remains on the conservative side, bringing
Grace's intensity up to 50 kt by the time the storm reaches the
Virgin Islands and Puerto Rico, but it should be noted that this
forecast is on the lower side of the guidance. Only one model, the
HWRF, brings Grace to hurricane strength, but it does this by
having the storm move farther north and avoid land interaction
altogether. The intensity forecast is highly uncertain on days 3-5
since it depends on exactly how much Grace moves over the Greater
Antilles, and there is some model signal that increasing
northeasterly upper-level winds over the Bahamas and Florida could
become a negative factor.
1. Tropical storm conditions are expected in portions of the
Leeward Islands tonight and early Sunday, and the Virgin Islands
and Puerto Rico on Sunday. Tropical storm conditions are possible
over eastern parts of the Dominican Republic by early Monday.
2. Heavy rainfall could lead to flash and urban flooding over the
Leeward and Virgin Islands. Across Puerto Rico and the Dominican
Republic, 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, and Florida next week, and interests in those areas should
monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS
INIT 14/1500Z 16.2N 57.9W 40 KT 45 MPH
12H 15/0000Z 16.8N 60.6W 45 KT 50 MPH
24H 15/1200Z 17.5N 63.8W 50 KT 60 MPH
36H 16/0000Z 18.1N 66.4W 50 KT 60 MPH...OVER PUERTO RICO
48H 16/1200Z 18.6N 68.6W 50 KT 60 MPH...OVER DOMINICAN REP.
60H 17/0000Z 19.4N 70.7W 45 KT 50 MPH...OVER DOMINICAN REP.
72H 17/1200Z 20.5N 72.8W 45 KT 50 MPH...OVER WATER
96H 18/1200Z 22.8N 77.3W 45 KT 50 MPH
120H 19/1200Z 25.4N 81.2W 45 KT 50 MPH