Official Discussion issued by the National Hurricane Center

Marco (AL142020) DATA RELEASED: 8/22/2020 10:00:00 AM UTC

Copy of official data

Tropical Storm Marco Discussion Number   9
NWS National Hurricane Center Miami FL       AL142020
1000 AM CDT Sat Aug 22 2020

Reconnaissance data, geostationary and microwave satellite imagery,
and radar data from Cuba all indicate that Marco is strengthening
quickly this morning.  The Air Force Reserve Hurricane Hunters
measured a peak 850-mb flight-level wind of 69 kt, and there were
several SFMR measurements of 50-55 kt to the northeast of the
center.  These data support raising Marco's initial intensity to 55
kt, and the central pressure based on dropsonde data has fallen to
992 mb.  The crew on the plane reported the formation of a partial
eyewall, which agrees with what we've seen on recent microwave and
radar images.

The track forecast has been complicated by the fact that the plane
has fixed Marco's center to the east of the previous forecast
track, and that makes the current motion north-northwestward, or
340/10 kt.  The subtropical ridge currently located over the
southwestern Atlantic is forecast to build westward along the
northern Gulf Coast during the next few days, and this expanding
ridge is expected to push Marco northwestward and then eventually
westward while the cyclone moves across the Gulf of Mexico.  This
general thinking has not changed, but the adjusted initial position
ended up shifting the track guidance to the north and east on this
cycle.  In response, the new NHC track forecast has been adjusted
eastward and northward during the first 3 days and is generally
between the HCCA and TVCN consensus aids.

Marco has finally tapped into the favorable conditions over the
northwestern Caribbean Sea, and the cyclone's overall small size and
small radius of maximum winds makes it susceptible to quick changes
in intensity. The tropical storm is just beginning to move into a
higher zone of shear to its north, but conditions should remain
conducive enough for Marco to intensity to a hurricane during the
next 24 hours. After that time, southwesterly shear is expected to
increase over 20 kt by day 2 and then over 30 kt by day 3, and those
conditions, along with the cyclone's small size, should cause
weakening as Marco gets closer to the central and northwestern Gulf
coast. The updated NHC intensity forecast has been increased during
the first 2 days and lies above the HCCA/Florida State
Superensemble solutions but below the SHIPS/LGEM scenarios. The
intensity forecast comes back in line with the previous forecast by
day 3 during the expected weakening phase, and Marco is ultimately
expected to dissipate over Texas by the end of the forecast period.

The updated track forecast suggests that watches could be required
for a portion of the central Gulf Coast later today.

Key Messages:

1. Marco is forecast to strengthen to a hurricane as it moves into
the southeastern Gulf of Mexico by tonight, and tropical storm
conditions are expected over the northeastern coast of the Yucatan
Peninsula and in extreme western Cuba.  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 move across the central Gulf of Mexico as a
hurricane Sunday and approach the central Gulf Coast on Monday.
There is an increasing risk of impacts from storm surge, winds, and
heavy rainfall from the upper Texas coast to Louisiana early next
week, and interests there should monitor the progress of Marco, as
storm surge, tropical storm, and/or hurricane watches could be
issued later today.


INIT  22/1500Z 20.9N  85.3W   55 KT  65 MPH
12H  23/0000Z 22.1N  86.0W   65 KT  75 MPH
24H  23/1200Z 23.9N  87.1W   75 KT  85 MPH
36H  24/0000Z 25.6N  88.4W   75 KT  85 MPH
48H  24/1200Z 27.2N  89.9W   70 KT  80 MPH
60H  25/0000Z 28.5N  91.6W   60 KT  70 MPH
72H  25/1200Z 29.1N  93.6W   50 KT  60 MPH
96H  26/1200Z 29.3N  96.6W   35 KT  40 MPH...INLAND
120H  27/1200Z...DISSIPATED

Forecaster Berg