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Tropical Storm Grace Discussion Number  19
NWS National Hurricane Center Miami FL       AL072021
1100 PM EDT Tue Aug 17 2021
 
NOAA and Air Force Hurricane Hunter aircraft that have been 
investigating Grace this evening have found that the center moved 
off the western tip of Jamaica shortly before 0000 UTC.  Radar and 
conventional satellite imagery, as well as earlier microwave data, 
show that convective banding has increased with a band that wraps 
around the eastern and northern portions of the circulation.  The 
Air Force reconnaissance aircraft measured a peak 850-mb 
flight-level wind of 64 kt, and both aircraft have measured 
believable SFMR winds of 50-52 kt.  Based on those data the maximum 
winds were increased to 50 kt on the 0000 UTC intermediate advisory, 
and the initial intensity remains unchanged for this advisory. 

Grace will be moving over the high ocean heat content waters of the 
northwestern Caribbean Sea during the next day or so. That, along 
with generally low to moderate vertical wind shear conditions, 
should allow for steady strengthening until Grace reaches the coast 
of the Yucatan Peninsula in just under 36 hours. The NHC intensity 
forecast call for Grace to become a hurricane on Wednesday, and the 
storm could peak near category 2 intensity before it reaches the 
coast of Mexico late Wednesday night or early Thursday.  By the 36- 
hour forecast, some weakening is likely to have occurred as that 
point is now inland. Some weakening will likely occur while Grace 
moves over land, but restrengthening is predicted when the storm 
moves over the Bay of Campeche.  The NHC intensity forecast is in 
best agreement with the HCCA and IVCN consensus aids. 

Grace has been moving slightly north of due west, or 280/12 kt. A 
strong deep-layer ridge the extends from the western Atlantic across 
the eastern and central Gulf of Mexico should continue to steer the 
tropical storm westward to west-northwestward for the next several 
days.  While the dynamical models agree on this overall forecast 
scenario, there remains some north-south spread with the HWRF, HMON, 
and GFS along the northern side of the guidance envelope, taking 
Grace near the northeastern portion of the Yucatan Peninsula.  On 
the other hand, the UKMET and its ensemble mean depict a more 
westward track toward the southern portion of the Yucatan Peninsula 
of Mexico.  The NHC forecast splits these differences and lies near 
the various consensus aids, which were just a tad farther south this 
cycle. 

 
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 Wednesday night.
 
2. Heavy rainfall across Jamaica, the Cayman Islands, and the 
Yucatan Peninsula is likely to lead to flash and urban flooding.
Mudslides are possible in Jamaica. 
 
3. Tropical storm conditions are expected over portions of Jamaica
for a few more hours, and over the Cayman Islands later tonight 
and Wednesday.  Hurricane conditions are possible in the Cayman 
Islands by Wednesday 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 tonight through Wednesday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/0300Z 18.4N  79.2W   50 KT  60 MPH
 12H  18/1200Z 18.9N  81.5W   60 KT  70 MPH
 24H  19/0000Z 19.5N  84.7W   75 KT  85 MPH
 36H  19/1200Z 20.0N  87.8W   80 KT  90 MPH...INLAND
 48H  20/0000Z 20.4N  90.7W   50 KT  60 MPH...OVER WATER
 60H  20/1200Z 20.7N  93.2W   65 KT  75 MPH
 72H  21/0000Z 20.8N  95.6W   75 KT  85 MPH
 96H  22/0000Z 20.7N 100.1W   35 KT  40 MPH...INLAND
120H  23/0000Z 20.5N 104.5W   20 KT  25 MPH...POST-TROP/INLAND
 
$$
Forecaster Brown
 
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