[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 222055
TCDAT4

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
flooding.

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.

FORECAST POSITIONS AND MAX WINDS

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
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