Tropical Storm Marco Forecast Discussion

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WTNT44 KNHC 220254
TCDAT4

Tropical Storm Marco Discussion Number   7
NWS National Hurricane Center Miami FL       AL142020
1100 PM EDT Fri Aug 21 2020

An Air Force Reserve Hurricane Hunter aircraft has investigated the 
system over the northwest Caribbean during the past few hours. The 
plane reported a number of unflagged SFMR winds between 35 and 40 kt 
and max flight level winds of 41 kt. A blend of these data supports 
an intensity of 35 kt, and therefore, this system has been 
designated as Tropical Storm Marco. Deep convection has increased 
near and to the east of Marco's center during the past few hours. 
Although there still isn't much evidence of inner-core banding, the 
data from the plane does indicate that the center of Marco has 
become better defined since the afternoon and that the minimum 
pressure has dropped.

Unfortunately the intensity forecast has not become any clearer and 
confidence in that aspect of the forecast is quite low. Marco is 
embedded within an environment that could support a fast rate of 
strengthening. However, recent microwave data does not indicate that 
the system has developed an inner-core, and only gradual 
strengthening is likely until it does. The intensity guidance spread 
is quite high, with the GFS and ECMWF global models both showing 
little further strengthening, while the HMON regional model rapidly 
makes Marco a hurricane before it reaches the northeast tip of the 
Yucatan peninsula. That possibility can not be ruled out, but a 
majority of the intensity guidance favors the weaker solution of the 
global models. Even with the HMON outlier included, the NHC 
intensity forecast is above the model consensus. Once Marco moves 
over the central Gulf of Mexico, a rapid increase in wind shear 
associated with an upper-level trough should limit the potential for 
further strengthening, and weakening is still anticipated before 
Marco nears the northern Gulf Coast, as shown in the previous 
official forecast.

Confidence in the track forecast is also lower than normal, as the 
models spread remains quite high. Only small adjustments were made 
to the NHC forecast which heavily favors the GFS and ECMWF solutions 
on the left side of the track guidance. It is worth noting that the 
NHC track forecast is near middle of the GFS and ECMWF ensembles. 
Marco is currently forecast to move northwestward toward a weakness 
in the subtropical ridge over the central Gulf of Mexico for the 
next day or two, before the ridge builds and turns the tropical 
cyclone farther west. Near the end of the period, Marco's track and 
intensity could be also influenced by Tropical Storm Laura which is 
also forecast to be over the Gulf of Mexico, however the details of 
that interaction are highly uncertain at this time. Given the high 
uncertainty in the forecast, larger than normal changes could be 
required to future advisories.


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  22/0300Z 18.7N  84.9W   35 KT  40 MPH
 12H  22/1200Z 19.7N  85.7W   45 KT  50 MPH
 24H  23/0000Z 21.1N  86.7W   50 KT  60 MPH
 36H  23/1200Z 22.7N  88.0W   55 KT  65 MPH
 48H  24/0000Z 24.4N  89.5W   60 KT  70 MPH
 60H  24/1200Z 25.9N  91.0W   60 KT  70 MPH
 72H  25/0000Z 27.5N  92.8W   60 KT  70 MPH
 96H  26/0000Z 29.0N  95.4W   45 KT  50 MPH...INLAND
120H  27/0000Z 30.0N  97.4W   30 KT  35 MPH...INLAND

$$
Forecaster Zelinsky



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