Tropical Storm Marco Forecast Discussion

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000
WTNT44 KNHC 241445
TCDAT4

Tropical Storm Marco Discussion Number  17
NWS National Hurricane Center Miami FL       AL142020
1000 AM CDT Mon Aug 24 2020

Strong southwesterly shear is taking its toll on Marco, with the 
edge of the nearest deep convection now displaced 50 n mi northeast 
of the exposed circulation center. Data from an Air Force Reserve 
Hurricane Hunter aircraft that investigated the system this morning 
measured 45-kt surface winds with the onboard SFMR instrument in 
that convection, but winds of only 20 to 30 kt were measured 
between this thunderstorm activity and the cyclone's center. 
Sampling by the aircraft has also revealed that winds of tropical 
storm force are likely no longer occurring in any portion of the 
circulation except in that convection. Based on the SFMR data, the 
initial intensity is initialized at 45 kt.

The shear is not forecast to abate in the foreseeable future, and 
the simulated satellite imagery in the GFS and ECMWF suggests that 
Marco will degenerate into a remnant low on Tuesday. The latest NHC 
forecast is near the various consensus aids, which shows the cyclone 
weakening to a tropical depression late tonight. Based on the 
updated intensity forecast, tropical storm conditions are no longer 
expected to be produced by Marco over the central portions of the 
Louisiana coast, and the Tropical Storm Warnings for those 
locations have been discontinued.

Marco has slowed down and has moved a little to the right of the 
forecast track over the past few hours, with an initial motion of 
NNW/7 kt. The track guidance for the entire forecast period has 
shifted back northward, and is just south of where it was this time 
yesterday. The official forecast track was adjusted eastward in the 
near term to accommodate the recent right-of-track bias. After 12 
h, the track forecast lies between the previous one and most of the 
consensus track guidance. On this track, Marco is expected to move 
inland tonight, and remain inland when the system turns 
west-northwestward in the low-level flow by Tuesday morning.


Key Messages:

1. Gusty winds, dangerous storm surge, and heavy rainfall are
expected from Marco along portions of the Gulf Coast beginning
later today. 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  24/1500Z 28.5N  88.5W   45 KT  50 MPH
 12H  25/0000Z 29.2N  89.3W   35 KT  40 MPH...INLAND
 24H  25/1200Z 30.0N  91.3W   30 KT  35 MPH...POST-TROP/REMNT LOW
 36H  26/0000Z 30.5N  93.6W   30 KT  35 MPH...POST-TROP/REMNT LOW
 48H  26/1200Z 30.9N  95.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
 60H  27/0000Z...DISSIPATED

$$
Forecaster Latto



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