Tropical Storm Marco Discussion Number  11
NWS National Hurricane Center Miami FL       AL142020
1000 PM CDT Sat Aug 22 2020

Marco has taken on distinctly sheared appearance. Reports from an 
Air Force Reserve Hurricane Hunter plane, microwave imagery, and 
radar imagery from Cuba all indicate that deep convection is limited 
to the east side of the tropical storm and that it no longer has a 
nearly closed eyewall. The degradation of Marco's structure appears 
to be primarily due to strong upper-level southwesterly flow. 
Despite the shear, the plane still measured SFMR winds near 55 kt 
and the intensity is held at that value.

Marco is a small tropical storm and will be susceptible to rapid 
changes in structure and intensity until it reaches the northern 
Gulf Coast. Such systems are often not very resilient in a 
high-shear environment, however even a brief relaxation of the shear 
could result in quick strengthening. It would not be surprising if 
Marco's intensity evolves in step-wise fashion consisting of periods 
of arrested development followed by fast strengthening if/when the 
shear relaxes. While the statistical models still show Marco 
becoming a hurricane within 24 h, the run-to-run consistency of the 
dynamical guidance remains poor. The latest HWRF, HMON and GFS 
forecasts show Marco weakening as it approaches the northern Gulf 
Coast, and this remains a distinct possibility if the shear remains 
consistently high. The NHC intensity forecast has not been changed 
substantially, in large part due to the low confidence of the 
forecast, and is consequently above all of the guidance at 36 and 48 
h when Marco is forecast to be near the northern Gulf Coast. 
Additional adjustments to the forecast are likely on Sunday.

In sharp contrast to earlier today, no large changes were made to 
the track forecast, though that should not be interpreted as an 
increase in forecast confidence. Marco is forecast to move 
north-northwestward and approach the northern Gulf Coast on Monday. 
As it moves inland and weakens, a turn toward the west at a slower 
forward speed is anticipated. This turn could occur before or after 
Marco moves inland, and will be tied in part to exactly when Marco 
begins to weaken since a stronger, deeper storm should continue to 
feel the affects of the upper-level southwesterly flow and move 
farther north  while a weaker system will be steered westward by a 
low- to mid-level ridge extending over the southeastern US. The NHC 
forecast is nearly on top of the multi-model consensus, but the 
spread in the guidance is still higher than normal.

Key Messages:

1. Tropical storm conditions will continue over portions of extreme
western Cuba for a few more hours. Heavy rainfall is also expected
overnight 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 be at or near hurricane strength when it
approaches 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 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  23/0300Z 22.8N  86.3W   55 KT  65 MPH
12H  23/1200Z 24.3N  87.1W   65 KT  75 MPH
24H  24/0000Z 26.3N  87.9W   70 KT  80 MPH
36H  24/1200Z 28.3N  88.9W   70 KT  80 MPH
48H  25/0000Z 29.6N  90.4W   55 KT  65 MPH...INLAND
60H  25/1200Z 30.5N  92.0W   35 KT  40 MPH...INLAND
72H  26/0000Z 30.9N  93.5W   30 KT  35 MPH...INLAND
96H  27/0000Z...DISSIPATED

$$
Forecaster Zelinsky