[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 171452
TCDAT2

Tropical Storm Grace Discussion Number  17
NWS National Hurricane Center Miami FL       AL072021
1100 AM EDT Tue Aug 17 2021

The cloud pattern of Grace is gradually becoming better organized
on satellite images, but the system still lacks distinct banding
features over the eastern semicircle.  Cirrus-level outflow from the
system appears to be well defined.  Flight-level and SFMR-observed
surface winds, as well as dropsonde measurements, support an
intensity of 45 kt for this advisory.

Not much change in intensity is likely through this afternoon while
the circulation of Grace interacts with the island of Jamaica.
Thereafter, since the cyclone will be over the very high heat
content waters of the northwestern Caribbean Sea with modestly low
vertical shear tonight and Wednesday, strengthening is anticipated.
Grace will likely become a hurricane tomorrow and continue to
intensify before moving over the Yucatan Peninsula.  The interaction
with that land mass is expected to temporarily interrupt
strengthening, with some reintensification over the Bay of Campeche
in 3-4 days.  The official intensity forecast is near the model
consensus for the first half of the forecast period, but below the
consensus for the latter part of the period owing to the inherent
uncertainties for that extended time frame.

Grace continues to move just a little north of due west, or at
about 280/13 kt.  A well-defined 500 mb ridge should remain
entrenched to the north of the tropical cyclone for the next several
days.  Therefore, a continued west to west-northwestward track is
likely to continue for most, if not all, of the forecast period.
The official forecast is close to the previous one through 72 hours
and just slightly south of it at 4-5 days.  This is very similar to
the latest multi-model consensus prediction.


Key Messages:

1. Heavy rainfall across Haiti, Cuba, Jamaica, the Cayman Islands,
and the northern Yucatan Peninsula may lead to flash, urban, and
small stream flooding, with the potential for mudslides highest in
Haiti and Jamaica.

2. Tropical storm conditions are expected over portions of Jamaica
today, and over the Cayman Islands later today and tonight.
Tropical storm conditions are expected over portions of the southern
coast of Cuba today in the warning area, spreading westward to
possibly other portions of the southern coast of Cuba this evening
through Wednesday morning.

3. There is an increasing risk of wind, rainfall, and storm surge
impacts on the Yucatan Peninsula of Mexico Wednesday night and
Thursday, and a Hurricane Warning will likely be needed for that
area later today.


FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z 18.3N  76.8W   45 KT  50 MPH
 12H  18/0000Z 18.8N  79.0W   55 KT  65 MPH
 24H  18/1200Z 19.2N  82.2W   65 KT  75 MPH
 36H  19/0000Z 19.7N  85.3W   70 KT  80 MPH
 48H  19/1200Z 20.4N  88.3W   65 KT  75 MPH...INLAND
 60H  20/0000Z 20.9N  91.3W   60 KT  70 MPH...OVER WATER
 72H  20/1200Z 21.1N  94.0W   65 KT  75 MPH
 96H  21/1200Z 21.5N  98.5W   65 KT  75 MPH...INLAND
120H  22/1200Z 21.5N 103.0W   30 KT  35 MPH...POST-TROP/INLAND

$$
Forecaster Pasch
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