[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 240256
TCDAT4

Tropical Storm Marco Discussion Number  15
NWS National Hurricane Center Miami FL       AL142020
1000 PM CDT Sun Aug 23 2020

Marco is feeling the effect of strong southwesterly shear. The
center, as identified by an Air Force Reserve Hurricane Hunter
aircraft, is displaced to the southwest edge of the convective
canopy. That said, Marco has not quite fully decoupled yet and has
moved well right of the previous forecast. Data from the plane
indicates that Marco has weakened; the highest flight-level winds
were only 58 kt with unflagged SFMR winds just above 50 kt.
Marco's intensity has been lowered slightly to 60 kt, but this is
probably generous.

The intensity guidance remains maddeningly inconsistent. The most
recent runs of the HWRF and HMON show that there is still a chance
that Marco could reach the Gulf Coast as a hurricane, despite the
shear. While this seems unlikely given the current structure of the
cyclone, it also seems slightly premature to rule it out entirely.
After all, the shear lessened enough to allow Marco to briefly
strengthen after it was strongly sheared for a time last night. The
rest of the guidance shows Marco weakening further as it approaches
the coast, and this seems like a more likely solution. The NHC
forecast just holds Marco at 60 kt for the next 24 h, but its
certainly possible it could already be weaker than that. Rapid
weakening is likely by 36 h if it hasn't happened sooner and Marco
is forecast to become a remnant low by early Wednesday.

Marco's track is tied entirely to its intensity and structure. Once
Marco becomes fully decoupled it will slow and turn westward. It is
not out of the question that this could happen before Marco reaches
the coast and the system never makes landfall. However, as long as
the strong southwesterly upper-level flow contributes to the
steering, Marco will continue to move more northward to
north-northwestward. Due to the recent northward movement of Marco's
center, the NHC track forecast has been adjusted in that direction,
but it is blended to the previous forecast by about 36 h and
onward. Confidence in this forecast unfortunately remains low since
the model spread remains usually high.

It is worth noting that Marco is a small tropical cyclone. The large
area of Tropical Storm and Hurricane watches and warnings along the
northern Gulf Coast is a reflection of the unusually high
uncertainty in the forecast, and it is unlikely that all of those
regions will experience tropical-storm-force winds or
life-threatening storm surge associated with Marco. However, impacts
will likely occur in some portions of the watch/warning area
beginning on Monday, and heavy rain is likely across most of the
region during the next couple of days. Changes to the watches and
warnings are likely on Monday and users should consult products from
their local weather forecast office for more information about
potential hazards in their area.

Key Messages:

1. Strong winds, life-threatening storm surge, and heavy rainfall
are expected from Marco along portions of the Gulf Coast beginning
on Monday. Interests in these areas should follow any advice given
by local government officials.

2. Tropical Storm Laura could bring additional storm surge,
rainfall, and wind impacts to portions of the U.S. Gulf Coast by the
middle of the week. This could result in a prolonged period of
hazardous weather for areas that may also 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  24/0300Z 26.8N  87.6W   60 KT  70 MPH
 12H  24/1200Z 28.0N  88.7W   60 KT  70 MPH
 24H  25/0000Z 29.0N  90.2W   60 KT  70 MPH
 36H  25/1200Z 29.6N  92.0W   40 KT  45 MPH...INLAND
 48H  26/0000Z 29.8N  93.6W   30 KT  35 MPH...INLAND
 60H  26/1200Z 29.7N  94.9W   25 KT  30 MPH...POST-TROP/INLAND
 72H  27/0000Z...DISSIPATED

$$
Forecaster Zelinsky
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