Issued at 1000 AM CDT Sun Aug 23 2020

000
WTNT44 KNHC 231457
TCDAT4

Tropical Storm Marco Discussion Number 13
NWS National Hurricane Center Miami FL

AL142020
1000 AM CDT Sun Aug 23 2020

Deep convection with cloud tops of -75 to -80 degrees C has
persisted over the center of Marco for the past several hours. An
Air Force Reserve Hurricane Hunter aircraft currently investigating
the cyclone measured winds equivalent to 62 kt both with the SFMR
and in a dropsonde in the northeastern eyewall. However, the
aircraft reported peak 850-mb flight level winds of only 64 kt.
Based on a blend of these data it appears that Marco is on the cusp
of becoming a hurricane, but is not quite there yet. Therefore, the
initial intensity remains 60 kt.

Based on the SHIPS guidance, Marco has about a 12-24 hour window to
intensify in an environment characterized by moderate southwesterly
shear, very warm waters, and plenty of atmospheric moisture. After
that time, the vertical wind shear is expected to increase and this
should begin to dominate the cyclone's environment. The latest NHC
intensity forecast is unchanged from the previous one, forecasting
Marco to become a hurricane later today, and maintaining hurricane
intensity up until landfall in agreement with the latest LGEM
intensity guidance. While it is possible that Marco will weaken just
prior to landfall due to the increasing shear, there is little
difference in the impacts between a 60 and 65 kt system.

Marco is moving north-northwestward or 340/12 kt. Although the
overall guidance has not changed much since the previous advisory,
there remains considerable spread in this guidance by the time Marco
reaches the northern Gulf coast. This spread could be attributed to
the varying ways the models handle the system as it encounters the
more hostile environment near the coast. Since the track consensus
aids have changed little through 36 h, the official forecast is
essentially the same as the previous one through that time. The
model guidance has shifted a little northward beyond 36 h, so the
official forecast was nudged to the north during that time as well.

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 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 23/1500Z 24.7N 87.3W

60 KT 70 MPH
12H 24/0000Z 26.4N 88.1W

65 KT 75 MPH
24H 24/1200Z 28.2N 89.1W

65 KT 75 MPH
36H 25/0000Z 29.5N 90.4W

55 KT 65 MPH...INLAND
48H 25/1200Z 30.5N 92.1W

35 KT 40 MPH...INLAND
60H 26/0000Z 31.2N 93.8W

30 KT 35 MPH...INLAND
72H 26/1200Z 31.8N 95.4W

25 KT 30 MPH...INLAND
96H 27/1200Z...DISSIPATED

$$
Forecaster Latto

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