[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 230858
TCDAT4

Tropical Storm Marco Discussion Number  12
NWS National Hurricane Center Miami FL       AL142020
400 AM CDT Sun Aug 23 2020

Thunderstorm bursts continue to fire near the center of Marco, then
weaken an hour or two later due to persistent shear. Overall, the
storm's appearance hasn't changed with nearly all of the deep
convection very near or northeast of the center. Although the
satellite presentation is somewhat lacking at the moment, the
earlier Air Force Reserve Hurricane Hunter mission found believable
SFMR values to 60 kt, so that value remains the initial wind speed.

The intensity forecast remains tricky with Marco due to its small
size and marginal environment.  There are some models that briefly
relax the shear today, which will likely be enough of a change to
allow Marco to reach hurricane strength.  Later on, while the
cyclone is near the coast of Louisiana, the shear is forecast to
increase, but it is unknown exactly how close to landfall this will
occur.  Our best forecast at this time is that the strongest winds
will be confined to the coast, and that Marco will then weaken
faster than most hurricanes do over the swamps of Louisiana due to
the shear.  No significant changes were made to the intensity
forecast, which is very close to the model consensus.  The new
forecast necessitates the issuance of hurricane warnings for
portions of southeastern Louisiana.

Marco continues moving north-northwestward or 335/11 kt.  This
general track and speed is likely today, with a turn to the
northwest and decrease in forward speed expected as the storm
weakens late Monday.  While the track forecast is essentially
unchanged from the previous one, there is still a fair bit of
spread in the model guidance, likely tied to the intensity
forecast.  The stronger guidance is near or northeast of the new
model consensus, due to the upper-level flow, and the NHC forecast
leans in that direction, close to the previous official prediction.


Key Messages:

1. Hurricane conditions, life-threatening storm surge, and heavy
rainfall are expected from Marco along portions of the Gulf Coast
beginning on Monday, and Hurricane and Storm Surge Warnings have
been issued. 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 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  23/0900Z 23.7N  87.0W   60 KT  70 MPH
 12H  23/1800Z 25.3N  87.6W   65 KT  75 MPH
 24H  24/0600Z 27.3N  88.6W   65 KT  75 MPH
 36H  24/1800Z 29.0N  89.7W   65 KT  75 MPH
 48H  25/0600Z 29.9N  91.5W   45 KT  50 MPH...INLAND
 60H  25/1800Z 30.5N  93.1W   30 KT  35 MPH...INLAND
 72H  26/0600Z 31.0N  95.0W   25 KT  30 MPH...INLAND
 96H  27/0600Z...DISSIPATED

$$
Forecaster Blake
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