[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 160241
TCDAT2

Tropical Depression Grace Discussion Number  11
NWS National Hurricane Center Miami FL       AL072021
1100 PM AST Sun Aug 15 2021

An Air Force Reserve Hurricane Hunter Aircraft investigated Grace
earlier this evening and found maximum flight level winds just less
than 40 kt at 925 mb, which supports Grace's estimated 30 kt
intensity. The plane also found that the depression still has a
slightly elongated but closed surface circulation. During the past
few hours, satellite imagery has shown a slight increase in
convective banding features and associated heavy rain associated
with Grace. That activity highlights the primary threat from Grace
during the next 24 hours: prolonged heavy rainfall that could lead
to flash and urban flooding along with the potential for mudslides
over Puerto Rico and Hispaniola.

The forecast for Grace is incredibly challenging. Imminent
interactions with the high terrain of Hispaniola and Cuba could
cause Grace to dissipate as soon as Monday evening. However, a track
south of Cuba, as shown by recent runs of the GFS and COAMPS-TC, may
allow Grace to maintain its tropical cyclone status and possibly
even intensify. The HWRF even shows it becoming a hurricane over the
western Caribbean, with the caveat that the model has produced
several poor forecasts for Grace thus far. Although it is not
explicitly forecast, slight intensification is still possible
tonight or tomorrow morning before the center of Grace moves inland.
After that time, the NHC forecast assumes Grace will continue as a
tropical depression through 72 h.  Once/if Grace makes it to the
western Caribbean Sea and Gulf of Mexico in 3-4 days, it could have
an opportunity to reorganize and intensify, and this is again shown
in the official intensity forecast. That said, users are encouraged
to not focus on the exact track or intensity forecasts at days 4 and
5.

The track guidance has shifted south for this advisory, and
generally calls for Grace to move westward to west-northwestward
through the forecast period. The official track forecast has been
shifted a little south once again, but is north of the most recent
multi-model consensus.

Key Messages:

1. Heavy rainfall across Puerto Rico, the Dominican Republic, and
Haiti may lead to flash, urban, and small stream flooding, along
with the potential for mudslides.

2. Tropical storm conditions are possible over portions of
Hispaniola Monday and Monday night.

3. There is a risk of some wind and rainfall impacts across Cuba
beginning Tuesday morning, but forecast uncertainty is much higher
than usual. Interests there areas should monitor the progress of
Grace and updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 17.3N  68.6W   30 KT  35 MPH
 12H  16/1200Z 17.7N  70.3W   30 KT  35 MPH
 24H  17/0000Z 18.7N  72.8W   30 KT  35 MPH
 36H  17/1200Z 19.5N  75.3W   30 KT  35 MPH
 48H  18/0000Z 20.4N  78.1W   30 KT  35 MPH
 60H  18/1200Z 21.2N  81.0W   30 KT  35 MPH
 72H  19/0000Z 22.2N  84.0W   30 KT  35 MPH...INLAND
 96H  20/0000Z 23.5N  89.0W   35 KT  40 MPH...OVER WATER
120H  21/0000Z 24.5N  93.5W   45 KT  50 MPH

$$
Forecaster Zelinsky
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