[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 232054
TCDAT4

Hurricane Marco Discussion Number  14
NWS National Hurricane Center Miami FL       AL142020
400 PM CDT Sun Aug 23 2020

Shortly after the 10 AM CDT advisory was issued, the Air Force
Reserve Hurricane Hunter aircraft investigating Marco measured SFMR
winds of 65 to 69 kt for several minutes. A dropsonde around the
same time also measured winds equivalent to 64 kt at the surface.
Based on these data, it was evident that Marco had strengthened
into a hurricane. Since that time, the convective pattern has
changed little, and the final leg of the aircraft mission did
not reveal any notable changes to the cyclone's intensity.
Therefore, the initial intensity has been set to 65 kt, which is
also in agreement with the latest Dvorak intensity estimate from
SAB.

Marco continues to move north-northwest at around 11 kt. Model
guidance is in good agreement on a north-northwest and then
northwest motion into Monday morning as Marco nears the northern
Gulf coast. However, there has been a major shift in the track
guidance beyond Monday morning, and the majority of the forecast
models now keep Marco offshore of the northern Gulf coast for the
next few days rather than moving it inland over Louisiana Monday
afternoon. Since this shift was so abrupt, I would rather split the
difference between the previous official forecast track and the
latest consensus tracks until another round of model runs can
confirm this new suggested track is higher confidence. Based on this
shift in track, tropical storm conditions are expected farther west
along the Louisiana coast, and a Tropical Storm Warning has been
issued from Morgan City to Cameron, Louisiana. It should be noted
that if the trend in the models are correct, some of the impacts
over portions of the north-central Gulf coast could be lessened.

The current shear analysis from UW-CIMSS shows that Marco is
experiencing about 20-25 kt of southwesterly shear, yet Marco has
been able to slowly strengthen in this environment today, likely
due to the presence of very warm waters and plenty of atmospheric
moisture. By late tonight and through Monday, the SHIPS guidance
suggests this shear will increase to over 30 kt. This should limit
any further intensification, and could cause Marco to weaken before
it nears the northern Gulf coast Monday afternoon. By Monday night,
the shear is forecast to increase to close to 40 kt, which should
strip the convection away from the center of the cyclone, causing it
to weaken. After 48 h, Marco is now expected to become a remnant low
devoid of deep convection. The official NHC intensity forecast is
similar to the previous one, but is at the high end of the guidance
through 24 h.

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. 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  23/2100Z 25.8N  87.8W   65 KT  75 MPH
 12H  24/0600Z 27.1N  88.6W   65 KT  75 MPH
 24H  24/1800Z 28.5N  89.7W   65 KT  75 MPH
 36H  25/0600Z 29.3N  91.4W   50 KT  60 MPH...INLAND
 48H  25/1800Z 29.8N  93.2W   35 KT  40 MPH...INLAND
 60H  26/0600Z 29.9N  95.1W   30 KT  35 MPH...POST-TROP/INLAND
 72H  26/1800Z 29.7N  96.8W   25 KT  30 MPH...POST-TROP/INLAND
 96H  27/1800Z...DISSIPATED

$$
Forecaster Latto
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