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Tropical Storm Sally Discussion Number   8
NWS National Hurricane Center Miami FL       AL192020
1100 AM EDT Sun Sep 13 2020
 
Although the center of Sally remains near the northwestern edge of 
the deep convection, there is a large area of convection and some 
banding evident over the southeastern portion of the circulation. An 
Air Force Reserve Hurricane Hunter aircraft investigating the storm 
this morning reported a peak 850-mb flight-level wind of 62 kt, and 
believable SFMR winds of around 50 kt, and these data are the basis 
for the 50-kt initial wind speed. The plane reported minimum 
pressures in the 996-998 mb range, with the higher value being the 
most recent information available.
 
Northwesterly shear continues over the cyclone, but this shear is
expected to decrease later today and tonight as Sally moves beneath
a narrow upper-level ridge axis.  This more conducive upper-level
pattern is expected to allow the tropical storm to strengthen while
it moves over the north-central Gulf of Mexico tonight and Monday.
Most of the intensity guidance calls for Sally to become a hurricane
in about 24 hours and so does the official forecast.  Additional
strengthening is expected after that time and Sally could be
slightly stronger at landfall than indicated below since it is
forecast to reach the coast between the 36 h forecast point and
48 h when it is inland over southeast Louisiana. The NHC intensity
forecast is close to the consensus aids through 24 hours and at or
just above the SHIPS, LGEM and HFIP corrected consensus aids at 36
and 48 hours.
 
Sally is moving west-northwestward at about 10 kt.  The tropical 
storm should continue on that general heading and speed over the 
next 12 to 24 hours as it steered around the southern flank of a 
mid-level ridge.  After 24 hours, Sally is expected to be near the 
western portion of the ridge which should cause the storm to slow 
down and turn northwestward.  The global models have trended toward 
slightly more ridging over the northern Gulf during the next 24 
hours, and the track guidance has edged westward.  The NHC track has 
been adjusted slightly westward and lies near the lastest run of the 
GFS, but is not as far west as the ECMWF and the various consensus 
aids. As Sally rounds the ridge in 48 to 72 h, the steering flow is 
expected to be quite weak, and a slow northward motion is forecast 
at that time.  Afterward, a north-northeastward to northeastward 
motion should commence as the cyclone moves in that direction ahead 
of a short-wave trough.
 
Users are reminded to not focus on the exact details of the track or 
intensity forecasts, as the average NHC track error at 36-48 h is 
60-80 miles and the average intensity error is 10-15 mph. In 
addition, wind, storm surge, and rainfall hazards will extend far 
from the center. Unfortunately, confidence is increasing that 
Sally's expected slow forward speed near the Gulf Coast will 
exacerbate the storm surge and heavy rainfall threats.
 
KEY MESSAGES:
 
1. An extremely dangerous and life-threatening storm surge is now
expected, and a Storm Surge Warning is in effect for areas outside
the southeastern Louisiana Hurricane and Storm Damage Risk Reduction
System from Port Fourchon, Louisiana to the Mississippi/Alabama
border. Residents in these areas should follow any advice given by
local officials.
 
2. Hurricane conditions are expected by late Monday from Grand
Isle, Louisiana to Ocean Springs, Mississippi, including
Metropolitan New Orleans, with tropical storm conditions likely by
Monday. Preparations should be rushed to completion in those areas.
 
3. Sally is expected to produce flash flooding across southwest and 
central Florida and prolong existing minor river flooding across 
west-central Florida through Monday. Widespread significant flash 
flooding and minor to isolated major river flooding is likely across 
portions of the central Gulf Coast Monday through the middle of the 
week, with flooding impacts spreading farther into the Southeast in 
the middle to late parts of the week.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  13/1500Z 27.5N  84.9W   50 KT  60 MPH
 12H  14/0000Z 28.0N  86.6W   55 KT  65 MPH
 24H  14/1200Z 28.5N  88.2W   65 KT  75 MPH
 36H  15/0000Z 28.9N  89.4W   80 KT  90 MPH
 48H  15/1200Z 29.7N  90.2W   65 KT  75 MPH...INLAND
 60H  16/0000Z 30.3N  90.5W   50 KT  60 MPH...INLAND
 72H  16/1200Z 31.5N  90.3W   35 KT  40 MPH...INLAND
 96H  17/1200Z 33.6N  88.3W   25 KT  30 MPH...INLAND
120H  18/1200Z 34.8N  83.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Brown
 
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