Major Hurricane Grace

[Windspeed]

HWRF has seemed to have a high bias. GFS shear at this point is under 4 m/s, about 10 mph.  I think it is possible.

That position and intensity is pretty long range. Anything is possible at this point. I would be reluctant to focus on modeled intensity until we have a better understanding of what Grace is after it clears its interaction with the Tiburon Peninsula.

[WxWatcher007]

HWRF has struggled mightily this season and the NHC has noted it. Quite a shame since it can be an excellent model.

[hlcater]

Grace is almost surely back to TS strength.

With such a persistent convective complex(even if decoupled), I find it hard to imagine that there’s not sustained 35-40kt winds somewhere, especially since it was already basically there this morning.

[Windspeed]

A significant amount of rainfall has been occurring over the Tiburon Peninsula of Haiti this evening. Mudflows are going to be an issue throughout Tuesday and Wednesday. Moreover, the organization of Grace is not degrading but improving significantly tonight. This is going to be problematic for Jamaica over the next few days as well.

[David Reimer]

Inbound recon is finding FL winds up to around 45 knots (a bit higher perhaps, but suspect). I think it is safe to say Grace has regained tropical storm intensity. 

[hlcater]

the center has relocated significantly to the south.

[hlcater]

Well the LLC is not where I thought it was going to be, and the system is *incredibly* isobaric. The large envelope is likely why there's such expansive thunderstorm activity. Bodes for a fairly resilient system if nothing else. 

[Floydbuster]

I think it's possible that Grace becomes a more potent hurricane in the Western Caribbean than we are expecting. It's organizing and slowing down a bit.

[yoda]

Tropical Storm Grace Advisory Number  16
NWS National Hurricane Center Miami FL       AL072021
500 AM EDT Tue Aug 17 2021

...GRACE CONTINUING TO RE-ORGANIZE AS IT HEADS TOWARD JAMAICA...
...HEAVY RAINS CAUSING FLOODING ACROSS PORTIONS OF HISPANIOLA...


SUMMARY OF 500 AM EDT...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...18.2N 75.4W
ABOUT 165 MI...265 KM E OF MONTEGO BAY JAMAICA
ABOUT 385 MI...620 KM E OF GRAND CAYMAN
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...1005 MB...29.68 INCHES


WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY:

The government of Mexico has issued a Hurricane Watch for portions
of the Yucatan Peninsula from Cabo Catoche southward to Punta Allen.

[yoda]

Tropical Storm Grace Discussion Number  16
NWS National Hurricane Center Miami FL       AL072021
500 AM EDT Tue Aug 17 2021

An Air Force Reserve reconnaissance aircraft had difficulty locating 
the center of Grace early this morning. The last few visible 
satellite images along with 0200Z ASCAT wind data indicated that a 
lee-side surface low had formed south of the western tip of Haiti. 
That feature has gradually been pulling northward closer to the deep 
convection that has developed near the well-defined mid-level 
circulation center. The initial position is the midpoint between the 
recon position in the latest vortex message and the mid-level 
circulation noted in passive microwave imagery. The aircraft also 
found some patches of 925-mb flight-level winds of 48-51 kt, which 
equates to surface winds of 36-38 kt. Reliable SFMR surface winds 
obtained by the aircraft were also in the 36-38-kt range. Based on 
these data, Grace was upgraded back to a 35-kt tropical storm at 
0600 UTC. Regardless of Grace's exact intensity right now, the 
immediate threat is still torrential rainfall across western 
Hispaniola today and over Jamaica this afternoon and tonight, which 
will likely cause severe flooding in some locations.

The initial motion estimate is an uncertain 280/14 kt. A strong 
mid-level ridge is forecast to build westward across Florida and the 
Gulf of Mexico during the next several days, which should keep Grace 
on a westward or west-northwestward track through the 120-h forecast 
period. Only a very sight southward shift in the previous forecast 
track was made, which was due primarily to the new estimated 
southerly initial position, and the new NHC forecast track lies 
along the northern edge of model guidance envelope.

Once Grace passes Jamaica in 12-18 hours, the cyclone will be 
passing over water temperatures near 30 deg C and into a more 
divergent upper-level wind flow pattern. These two favorable 
conditions should allow for Grace to slowly intensify and possibly 
be near hurricane strength by the time the cyclone reaches the 
Yucatan peninsula in about 48 hours. Thereafter, Grace will weaken 
as it pass over Yucatan, and then re-strengthen some over the Bay 
of Campeche before reaching mainland Mexico in about 96 hours. The 
NHC intensity forecast is close to a blend of the HCCA and IVCN 
consensus models.

Key Messages:

1. Heavy rainfall across the Dominican Republic, 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 the Dominican Republic.

2. Tropical storm conditions are possible over portions of
Hispaniola and over Jamaica on today. Tropical storm conditions are 
expected over portions of the southern coast of Cuba today, 
spreading westward to the Cayman Islands and possibly other 
portions of the southern coast of Cuba this evening through
Wednesday morning.

3.  There is a increasing risk of wind, rainfall, and storm surge
impacts on the Yucatan Peninsula of Mexico Wednesday night and
Thursday.  Interests there areas should monitor the progress of
Grace and updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  17/0900Z 18.2N  75.4W   35 KT  40 MPH
 12H  17/1800Z 18.6N  77.6W   40 KT  45 MPH
 24H  18/0600Z 19.1N  80.7W   45 KT  50 MPH
 36H  18/1800Z 19.7N  83.9W   50 KT  60 MPH
 48H  19/0600Z 20.4N  87.0W   60 KT  70 MPH
 60H  19/1800Z 21.0N  90.0W   50 KT  60 MPH...INLAND
 72H  20/0600Z 21.4N  92.7W   55 KT  65 MPH...OVER WATER
 96H  21/0600Z 22.0N  97.2W   60 KT  70 MPH
120H  22/0600Z 22.3N 101.4W   30 KT  35 MPH...INLAND

$$
Forecaster Stewart

[Iceresistance]

Saved loop

[Windspeed]

[Windspeed]

Curved banding structure is getting established. All Grace requires now is development of core banding, which may be within progress. Some disruption with a close pass of Jamaica may occur today, but I now also think Grace will become a hurricane prior to Yucatán landfall on Thursday.

[Windspeed]

Yeah, SSMIS really shows that banding organization well. If not for Jamaica, I would expect a core to form relatively quickly with such a strong spiral band curving into that center. But Jamaica likely will have some success slowing down that process at least until Grace has cleared the island to the west.

[Windspeed]

Appears the LLC has reorganized off the NE coast of Jamaica where the spiral band on SSIMS had suggested.

[Windspeed]

[Wannabehippie]

That ULL ahead of Grace will be what keeps it from developing in to a hurricane before it crosses the Yucatan Peninsula IMHO.

 

[qg_omega]

21 hours ago, qg_omega said:

Thoughts on Tulum and Grace, have friend leaving Thursday for Tulum

Looks like direct hit

[Windspeed]

That ULL ahead of Grace will be what keeps it from developing in to a hurricane before it crosses the Yucatan Peninsula IMHO.
 

Not really seeing a pronounced upper level trough or low feature. There is an upper ridge/anticyclone over the eastern to southeastern GOM that could impart some northeasterly flow over Grace in the NW Caribbean, but it is forecast to retrograde WSW. It's not terribly strong shear though as Grace will be moving in a western vector of motion with easterly mid-level steering flow. I'm not sure it would be strong enough shear to prevent strengthening at least to lower-end hurricane status by Yucatán landfall. That is if Grace can finally get its act together upon leaving Jamaica, which remains to be seen.

[Windspeed]

RE: Upper level pattern over the next 72 hrs: Watch as the upper ridge over the EGOM pivots and slides WSW while another upper ridge develops and follows Grace over the WCARIB and Yucatán. I've seen worse patterns for WCARIB hurricanes, including a couple of majors there last season. But we shall see...

[Windspeed]

Last pass found some 55kt flight level winds on the north side of the reorganized LLC. This is just off the coast of Jamaica. Let's see if Grace finally starts developing a core now despite its close proximity traversing near-to the northern shoreline of Jamaica today.

[NorthHillsWx]

Grace has struggled mightily at organizing a coherent core it’s entire life. Even this recon mission shows the center of circulation is weak and embedded in what by all accounts looks like a wave envelope. Would not shock me to see several center reformations as it interacts with Jamaica. Until that happens, the very gradual strengthening we’ve seen last 24hrs would be all I’d forecast heading into the Yucatán. Strange system, overall environment would usually indicate a stronger system than what we’re seeing. 

[Windspeed]

Grace has struggled mightily at organizing a coherent core it’s entire life. Even this recon mission shows the center of circulation is weak and embedded in what by all accounts looks like a wave envelope. Would not shock me to see several center reformations as it interacts with Jamaica. Until that happens, the very gradual strengthening we’ve seen last 24hrs would be all I’d forecast heading into the Yucatán. Strange system, overall environment would usually indicate a stronger system than what we’re seeing. 

Land interactions have certainly not been kind to Grace. But things can change pretty fast if a core does get established. Obviously the thermodynamic potential is out the roof tomorrow if it can get going.

[NorthHillsWx]

15 minutes ago, Windspeed said:

20 minutes ago, NorthHillsWx said:

Grace has struggled mightily at organizing a coherent core it’s entire life. Even this recon mission shows the center of circulation is weak and embedded in what by all accounts looks like a wave envelope. Would not shock me to see several center reformations as it interacts with Jamaica. Until that happens, the very gradual strengthening we’ve seen last 24hrs would be all I’d forecast heading into the Yucatán. Strange system, overall environment would usually indicate a stronger system than what we’re seeing. 

Land interactions have certainly not been kind to Grace. But things can change pretty fast if a core does get established. Obviously the thermodynamic potential is out the roof tomorrow if it can get going.

Yep. Guidance is quite split on this one too, especially before it gets to the Yucatán. About half the guidance forecasts a slow strengthening pattern to continue, I’m assuming due to lack of a cohesive core, while the other half makes grace a hurricane at a more rapid pace. Really tough forecast, I think NHC is playing it correctly forecasting grace to be on the cusp of hurricane strength at landfall. If the system had a more well defined core this would be a strengthening hurricane. Relatively low shear, extremely moist envelop, and due to the thermodynamic potential and what we’ve already seen, grace has not lacked intense convection through its journey in the Caribbean 

[Windspeed]

Grace is now forecasted to become a hurricane in the NW Caribbean prior to landfall along the Riviera Maya coastline, Yucatán.

[Windspeed]

000
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

 

[Cayman Tracker]

Which quadrant of the storm is likely to pack the strongest winds?

Fairly concerned Grand Cayman resident here!

[ldub23]

10 minutes ago, Cayman Tracker said:

Which quadrant of the storm is likely to pack the strongest winds?

Fairly concerned Grand Cayman resident here!

Normally its the  northeast.

[Amped]

Models all over the place with this, now they've got way stronger the last 3 runs.    There is a lot of heat potential over this part of the Caribbean so a small amount of shear makes a huge difference in potential outcomes. HMON went from 1002mb to 978mb in one run. HWRF did almost the same thing.   Best bet is to average the two and get 990mb like the GFS has. Probably cat1  at Yucatan landfall.

[andyhb]

SHIPS RI probs starting to take off with Grace especially beyond 24 hours.

 SHIPS Prob RI for 20kt/ 12hr RI threshold=  10% is   2.1 times climatological mean ( 4.9%)
 SHIPS Prob RI for 25kt/ 24hr RI threshold=  33% is   3.0 times climatological mean (10.9%)
 SHIPS Prob RI for 30kt/ 24hr RI threshold=  22% is   3.2 times climatological mean ( 6.8%)
 SHIPS Prob RI for 35kt/ 24hr RI threshold=  11% is   2.9 times climatological mean ( 3.9%)
 SHIPS Prob RI for 40kt/ 24hr RI threshold=   8% is   3.3 times climatological mean ( 2.4%)
 SHIPS Prob RI for 45kt/ 36hr RI threshold=  21% is   4.5 times climatological mean ( 4.6%)
 SHIPS Prob RI for 55kt/ 48hr RI threshold=  27% is   5.8 times climatological mean ( 4.7%)
 SHIPS Prob RI for 65kt/ 72hr RI threshold=  33% is   6.2 times climatological mean ( 5.3%)