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Tropical Storm Grace Discussion Number  20
NWS National Hurricane Center Miami FL       AL072021
500 AM EDT Wed Aug 18 2021
 
An Air Force Reserve reconnaissance aircraft investigating Grace 
earlier this morning observed a peak 850-mb flight-level wind of 61 
kt in the northeastern quadrant along with reliable SFMR surface 
wind speeds of 50-52 kt. The aircraft measured a central pressure of 
about 1000 mb, a reading that had been steady for about 3 hours. 
Those data were the basis for maintaining the 0600 UTC intensity at 
50 kt. Since the last recon mission, however, deep convection has 
increased markedly over the low-level center, with cloud tops 
exceeding -75 deg C. Although the center is just inside the western 
edge of convective cloud canopy due to modest west-northwesterly 
vertical wind shear, Dvorak satellite intensity estimates from TAFB 
and SAB are a consensus T3.5/55 kt. These intensity estimates, along 
with the increase in convection over the center is the justification 
for increasing the 0900 UTC advisory intensity to 55 kt.

Grace has been maintaining a steady course north of due west, or 
280/14 kt for more than 18 hours. A strong deep-layer ridge to the 
north of the cyclone is expected to keep Grace moving in direction 
between west and west-northwest through the 96-hour forecast period. 
The latest NHC model guidance remain in strong agreement on Grace 
making landfall along the northeastern coast of the Yucatan 
Peninsula of Mexico shortly after the 24-hour forecast period, 
followed by a motion across the peninsula and emerging over the Bay 
of Campeche around 36 hours. Thereafter, the model guidance begins 
to diverge, with the HWRF, HMON, and COAMPS-TC regional models along 
the northern side of the guidance envelope, the GFS and UKMET 
global models down the middle, and the ECMWF model along the 
southern edge of the guidance suite. The new NHC forecast lies close 
to the previous advisory track and the consensus models TVCA and 
HCCA, which are a little south of the FSSE consensus model.

Although the upper-level wind environment isn't ideal for rapid 
strengthening, it is conducive enough for steady strengthening to 
occur due to the flow forecast to be strongly diffluent. Sea-surface 
temperatures are expected to be near 30 deg C and the water is also 
deep and warm, which should prevent any cold upwelling from 
occurring beneath the cyclone. As a result, a peak intensity of 75 
kt just prior to landfall has been carried over from the previous 
advisory, which is a little above all of the 0600 UTC intensity 
guidance. Although Grace will weaken substantially after passing 
over the Yucatan peninsula, restrengthening is expected over the 
warm waters of thew Bay of Campeche in the 48-72- hour period where 
the upper-level flow regime is forecast to be more conducive for 
intensification. Dissipation is now expected over the mountains of 
central Mexico by 120 hours. The NHC intensity forecast is in best 
agreement with the HCCA and IVCN consensus aids at 36 hours and 
beyond.
 
Key Messages:
 
1. Hurricane conditions and a dangerous storm surge are expected
over portions of the Hurricane Warning area in the eastern Yucatan
Peninsula of Mexico beginning late tonight.
 
2. Over the next few days, heavy rainfall across the Cayman Islands
as well as portions of the Yucatan Peninsula and Vera Cruz State
should lead to flash and urban flooding. Mudslides will be possible
in Jamaica.
 
3. Tropical storm conditions are expected over the Cayman Islands
through this morning. Hurricane conditions are possible in the
Cayman Islands later this morning.  Tropical storm conditions are
expected to spread westward from portions of the southern coast of
Cuba in the warning area to, possibly, other portions of the
southern coast of Cuba in the watch area today.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/0900Z 18.8N  80.9W   55 KT  65 MPH
 12H  18/1800Z 19.3N  83.2W   60 KT  70 MPH
 24H  19/0600Z 19.9N  86.4W   75 KT  85 MPH
 36H  19/1800Z 20.3N  89.4W   50 KT  60 MPH...INLAND
 48H  20/0600Z 20.6N  92.1W   50 KT  60 MPH...OVER WATER
 60H  20/1800Z 20.7N  94.4W   65 KT  75 MPH
 72H  21/0600Z 20.7N  96.8W   75 KT  85 MPH
 96H  22/0600Z 20.6N 101.4W   35 KT  40 MPH...INLAND
120H  23/0600Z...DISSIPATED
 
$$
Forecaster Stewart
 
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