[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 220850
TCDAT4

Tropical Storm Marco Discussion Number   8
NWS National Hurricane Center Miami FL       AL142020
400 AM CDT Sat Aug 22 2020

SFMR data from the last leg of the 53rd Air Force Reserve
reconnaissance aircraft indicated a couple of 48-50 kt winds, but
were associated with a significant spike in the rainfall rate.
Additionally, the flight-level winds were only 36 kt in that area
and the central pressure has increased a little to 1005 mb.  The
cloud pattern has improved during the past several hours with deep
convection developing near the center and spiral bands forming over
the eastern portion of cyclone.  Based on a blend the hurricane
hunter data and current subjective satellite intensity estimates,
the initial intensity is raised to 40 kt.

Although the UW-CIMSS shear product and global models indicate a
favorable upper wind pattern, GOES-16 sounder analysis data show a
higher than normal vertically stable environment in the western
Caribbean.  This negative contribution maybe what's hampering
significant development.  Consequently, as shown in the previous
advisory, only modest strengthening is indicated in the NHC
forecast through 48 hours.  Only the Decay SHIPS and LGEM intensity
models indicate that Marco will become a hurricane in a couple of
days.  Both the HMON and HWRF have backed off of showing a
hurricane in the Gulf.  Afterward, increasing west-southwesterly
shear should lead to weakening as the cyclone turns toward the
northwestern Gulf coast.  The NHC forecast lies between the higher
LGEM/Decay SHIPS solution and the lower NOAA HCCA intensity model
which indicates a peak intensity of 55 kt.

The initial motion is estimated to be west-northwestward, or 335/10
kt.  Marco is being steered toward the northwest by a mid- to
upper-level cut-off low and associated trough extending into the
southwestern Gulf from the Mississippi Valley.  By Sunday, the
aforementioned feature is forecast to lift northeastward allowing
the subtropical ridge to build back over the central gulf.  This
should cause Marco to turn northwestward and west-northwestward
early next week toward the the northwestern Gulf coast.
Large-scale guidance generally agree that any binary direct or
indirect interaction, while both systems are in the gulf and at
near equal latitude, is unlikely at this point.  The official track
forecast has been adjusted to the right of the previous advisory
and lies just to the left of the various multi-model consensus aids.

The 34 kt wind radii have been adjusted based on the aircraft
reconnaissance data and a 0224 UTC scatterometer pass.



Key Messages:

1. Marco is forecast to strengthen over the northwestern Caribbean
Sea through Saturday as it approaches the northeast coast of the
Yucatan Peninsula. A Hurricane Watch and a Tropical Storm Warning
are in effect for portions of that region.

2. The system is expected to move into the south-central Gulf of
Mexico as a tropical storm on Sunday. Although some strengthening
is anticipated on Sunday, weakening is forecast as the system
approaches the northwestern Gulf coast on Tuesday. It is still too
soon to know exactly the location and magnitude of impacts the
system will produce along the central or northwestern Gulf Coast,
and interests in that area should continue monitoring the progress
of this system over the next few days.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 19.6N  85.4W   40 KT  45 MPH
 12H  22/1800Z 20.9N  86.1W   45 KT  50 MPH
 24H  23/0600Z 22.5N  87.1W   50 KT  60 MPH
 36H  23/1800Z 24.1N  88.3W   55 KT  65 MPH
 48H  24/0600Z 25.7N  89.9W   60 KT  70 MPH
 60H  24/1800Z 27.3N  91.3W   55 KT  65 MPH
 72H  25/0600Z 28.3N  93.4W   50 KT  60 MPH
 96H  26/0600Z 29.1N  96.2W   40 KT  45 MPH...INLAND
120H  27/0600Z 29.6N  97.8W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Roberts
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