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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.
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
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
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