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Tropical Storm MARCO

Tropical Storm Marco Discussion Number  10
NWS National Hurricane Center Miami FL       AL142020
400 PM CDT Sat Aug 22 2020
Cutting to the chase, there have been some big changes among the
model guidance, and subsequently the NHC forecast, for Marco this
afternoon.  While at this point it's a little speculative, the data
collected by this morning's NOAA G-IV flight in the environment
around Marco and across the Gulf of Mexico may have played a key
role in the significant eastward shift seen in nearly all the 12z 
models.  This isn't to say that the uncertainty in the eventual 
track has diminished.  In fact, various ensemble members from some 
of the global models still show a potential risk to the coast 
anywhere from Texas to Alabama, and it's entirely possible that the 
volatile shifts seen in the models could continue.  That being 
said, the new NHC track forecast has been shifted significantly 
eastward and now shows the center of Marco reaching southeastern 
Louisiana in about 2 days, which is the scenario currently shown by 
the GFS, ECMWF, HCCA, Florida State Superensemble, and the TVCN 
multi-model consensus.  After Marco reaches the coast, the western 
Atlantic ridge is expected to build westward and should cause the 
cyclone to move more slowly toward the west-northwest across 
southern portions of Louisiana.

As far as the intensity is concerned, the last fix made by this 
morning's reconnaissance flight indicated that the pressure had 
leveled off, and no higher winds had been observed from what was 
measured earlier in the flight.  The radar presentation from Cuban 
radar has also degraded a bit, so Marco's initial intensity is held 
at 55 kt.  Marco is beginning to move into a zone of moderate 
southwesterly shear, but otherwise favorable conditions of warm 
ocean water and some upper-level divergence are expected to foster 
strengthening during the next day or so.  With the exception of the 
HWRF, the other intensity models show Marco reaching hurricane 
strength, and the NHC foreast continues to show that possibility 
while Marco moves over the central Gulf.  The shear is still 
expected to strengthen in 36-48 hours when the system is approaching 
the northern Gulf Coast, but with the shift in the forecast track, 
now there may not be enough time for Marco to weaken below 
hurricane intensity before it reaches land.  The new NHC intensity 
forecast is near or just above the HCCA and Florida State 
Superensemble models and holds Marco as a hurricane until it reaches 
the coast.
The forecast track changes now bring tropical storm force winds to 
the coast in 36-48 hours, which necessitates the issuance of 
Hurricane and Storm Surge Watches for a portion of the northern 
Gulf Coast.  These watches will likely need to be upgraded to 
warnings later tonight.

Key Messages:

1. Tropical storm conditions will continue over portions of extreme
western Cuba through this evening.  Heavy rainfall is also expected 
in the eastern portions of the Mexican states of Quintana Roo and
Yucatan, and across far western Cuba, which could result in flash

2. Marco is expected to approach the central Gulf Coast as a 
hurricane on Monday.  Hurricane conditions, life-threatening storm 
surge, and heavy rainfall are possible along portions of the Gulf 
Coast beginning on Monday, and Hurricane and Storm Surge watches 
have been issued. Interests in these areas should follow any advice 
given by local government officials.

3. Tropical Storm Laura could bring additional storm surge, 
rainfall, and wind impacts to portions of the U.S. Gulf Coast by 
the middle of next week.  This could result in a prolonged period of 
hazardous weather for areas that could be affected by Marco. 
Interests there should monitor the progress of Marco and Laura and 
updates to the forecast during the next few days.
INIT  22/2100Z 21.9N  85.7W   55 KT  65 MPH
 12H  23/0600Z 23.3N  86.5W   65 KT  75 MPH
 24H  23/1800Z 25.3N  87.3W   75 KT  85 MPH
 36H  24/0600Z 27.5N  88.3W   75 KT  85 MPH
 48H  24/1800Z 29.3N  89.6W   65 KT  75 MPH...ON THE COAST
 60H  25/0600Z 30.4N  91.1W   50 KT  60 MPH...INLAND
 72H  25/1800Z 31.1N  92.7W   40 KT  45 MPH...INLAND
 96H  26/1800Z 32.3N  95.6W   25 KT  30 MPH...INLAND
120H  27/1800Z...DISSIPATED
Forecaster Berg