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Tropical Storm Grace Discussion Number   7
NWS National Hurricane Center Miami FL       AL072021
1100 PM AST Sat Aug 14 2021

Grace is a poorly organized tropical cyclone. Trying to pinpoint the 
center of the storm over the last 6-12 hours has been a challenge, 
even with the help of earlier NOAA aircraft, surface observations, 
and radar data from Guadalupe. Earlier data from the NOAA aircraft 
reconnaissance Twin Doppler Radar showed that the mid-level center 
was tilted significantly further southeast relative to the 
poorly-defined low-level circulation. Around 0000 UTC, multiple 
reporting stations in Guadalupe showed a wind shift to the west, and 
there were some skeletal bands seen on the nearby Guadalupe radar 
ahead of a convective squall propagating ahead, which is currently 
producing the coldest cloud tops with Grace. Given all of these
data, the center of Grace is estimated to have passed by just north 
of Guadalupe over the last few hours. Despite the higher subjective 
and objective Dvorak estimates, the initial intensity was kept at 35 
kt for this advisory given the lackluster wind data from the earlier 
NOAA aircraft reconnaissance mission.

Grace appears to have reformed a bit further North tonight, and also 
now appears to be on a somewhat slower west-northwest heading, at 
285/17 kt. There remains a large amount of spread in the guidance, 
even in the short-term, which is likely related to the current 
disorganized nature of Grace. In general, Grace is expected to 
maintain a west-northwestward motion and gradually slow down over 
the next 24-36 hours. A strong low- to mid-level ridge poleward of 
Grace should then maintain this west-northwest heading through the 
remainder of the forecast. A lot of the track uncertainty in the 
latter part of the forecast appears to be related to the future 
intensity of the storm, and both the ECMWF and GFS ensembles show a 
large north-to-south spread with stronger members tracking further 
north and weaker members tracking further south. For now, the NHC 
track forecast has been shifted north of the previous track, mostly 
related to the further north initial position, and is in close 
agreement with the HFIP corrected-consensus approach (HCCA) model.

Grace appears to finally be slowing down a bit this evening, but 
will still need to slow down a bit more in order to allow the low- 
to mid-level centers to become better aligned. Because of this 
disjointed structure, only slow intensification is anticipated. The 
latest intensity guidance is a bit lower, and the NHC intensity 
guidance follows suit, with a peak intensity of 45-kt in 24 hours. 
Thereafter, it appears likely that Grace will have to deal with 
significant land interaction over Hispaniola and weakening is 
indicated by 48 hours. If Grace survives, it is possible some modest 
intensification could occur in the latter part of the forecast 
period. The intensity forecast remains closely tied to the track 
forecast, and any deviations to the north or south could allow Grace 
to stay stronger than indicated in this forecast. However, it also 
remains distinctly possible that Grace could dissipate before the 
end of this 5-day forecast due to the forecasted land interaction.

As previously mentioned, the exact track of the center and the 
intensity of the system will likely not be as important as the heavy 
rainfall that is forecast to fall across the Leeward Islands and the 
Greater Antilles during the next few days.
 
Key Messages:
 
1.  Tropical storm conditions are expected over the Virgin Islands 
and Puerto Rico on Sunday.  Tropical storm conditions are possible 
over eastern parts of the Dominican Republic Sunday night and 
Monday.
 
2.  Heavy rainfall could lead to flash and urban flooding over the 
Leeward and Virgin Islands, Haiti and the Dominican Republic. Across 
Puerto Rico, 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, Cuba, and Florida next week, but forecast uncertainty
remains higher than usual. Interests in those areas should monitor
the progress of Grace and updates to the forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  15/0300Z 16.8N  62.4W   35 KT  40 MPH
 12H  15/1200Z 17.3N  64.6W   40 KT  45 MPH
 24H  16/0000Z 18.1N  67.0W   45 KT  50 MPH...NEAR PUERTO RICO
 36H  16/1200Z 18.7N  69.1W   45 KT  50 MPH...INLAND OVER HISPANIOLA
 48H  17/0000Z 19.3N  70.9W   35 KT  40 MPH...INLAND OVER HISPANIOLA
 60H  17/1200Z 20.2N  73.2W   35 KT  40 MPH...OVER WATER
 72H  18/0000Z 21.2N  75.8W   35 KT  40 MPH
 96H  19/0000Z 23.7N  81.0W   40 KT  45 MPH
120H  20/0000Z 25.6N  84.6W   45 KT  50 MPH
 
$$
Forecaster Papin/Brown
 
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