817
WTNT44 KNHC 132040
TCDAT4

Tropical Storm Sally Discussion Number 9
NWS National Hurricane Center Miami FL AL192020
400 PM CDT Sun Sep 13 2020

The overall structure of Sally has not changed much since this
morning, but there has been a recent increase in convection near
and to the east of the center this afternoon. It appears that
the northwesterly shear is beginning to relax, and the increase in
convection near the center may be a harbinger of the expected
strengthening phase. Air Force and NOAA Hurricane Hunter aircraft
that have been in the storm since the previous advisory have
reported a fairly stable central pressure of 996-998 mb, and recent
data from the NOAA aircraft still supports an initial intensity of
50 kt. Earlier ASCAT data have been helpful in analyzing the radius
of 34-kt winds, which has expand over the eastern semicircle.

Sally is forecast to move beneath a narrow upper-level ridge axis
through Monday and the expected decrease in shear should allow the
storm to strengthen. Since Sally is forecast to decelerate on its
approach to the northern Gulf coast, the system still has at least
another 36 h to take advantage of the expected conducive
environmental conditions. As a result, the NHC intensity forecast
continues to call for Sally to become a hurricane on Monday, with
additional strengthening likely until landfall. The intensity
guidance has trended slightly lower this cycle, with the HMON model
now at the upper end of the guidance envelope. The reduction in the statistical guidance is likely due to the fact that Sally has not
strengthened today and there is a persistence component to the
forecast from those models. The latest NHC intensity forecast is
similar to the previous advisory and lies a little above the latest
HFIP corrected consensus aid.

Recent satellite and aircraft fixes show that Sally continues to
move west-northwestward at about 10 kt. The tropical storm is
currently being steered around the southern portion of a mid-level
ridge to its northeast. Sally is forecast to reach the western
extent of the ridge on Monday, and a slower northwestward motion is
expected when the storm is near the north-central Gulf coast. The
steering currents are forecast to weaken further in a couple of days
as Sally rounds the western extent of the ridge and a slow northward
motion is expected during that time. By 72 hours, the cyclone
should begin to move somewhat faster toward the northeast as a
trough deepens to its west. As often occurs, there has been some
run-to-run variability among the various track models, and the
latest iterations of them have shifted eastward with a slower
forward speed. Despite the shifts of the individual models, the
latest consensus aids are only slightly north and east of the
previous NHC track through 60 hours, so only a small adjustment has
been made to the earlier official forecast through that time. It is
important not to focus too much on these small track changes and
to the exact forecast track itself, as impacts are expected to
extend far from the center. Also, since there is still quite a
bit of model spread in both the location and timing of when the
center of Sally reaches the northern Gulf Coast, additional
adjustments to the track forecast are possible.

Regardless of Sally's exact landfall location and intensity, the
cyclone is expected to bring wind, storm surge, and rainfall hazards
to a large part of the north-central Gulf Coast. In particular,
Sally's slow forward speed near the coast will exacerbate the storm
surge and heavy rainfall threats.

KEY MESSAGES:

1. An extremely dangerous and life-threatening storm surge is
expected for areas outside the southeastern Louisiana Hurricane and
Storm Damage Risk Reduction System from Port Fourchon, Louisiana, to
the Mississippi/Alabama border, where a Storm Surge Warning is in
effect. Residents in these areas should follow any advice given by
local officials.

2. Hurricane conditions are expected by late Monday within portions
of the Hurricane Warning area from Morgan City, Louisiana, to Ocean
Springs, Mississippi, including Metropolitan New Orleans, with
tropical storm conditions likely to begin Monday. Preparations
should be rushed to completion in those areas.

3. Sally could continue to produce flash flooding across central and
northern 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 southeastern Louisiana, Mississippi, and Alabama through the middle
of the week. Flooding impacts are expected to spread farther across
the Southeast U.S. through the week.


FORECAST POSITIONS AND MAX WINDS

INIT 13/2100Z 27.8N 85.9W 50 KT 60 MPH
12H 14/0600Z 28.3N 87.3W 60 KT 70 MPH
24H 14/1800Z 28.7N 88.7W 70 KT 80 MPH
36H 15/0600Z 29.2N 89.6W 80 KT 90 MPH
48H 15/1800Z 29.8N 90.1W 60 KT 70 MPH...INLAND
60H 16/0600Z 30.7N 90.1W 45 KT 50 MPH...INLAND
72H 16/1800Z 31.8N 89.6W 30 KT 35 MPH...INLAND
96H 17/1800Z 33.5N 87.5W 25 KT 30 MPH...INLAND
120H 18/1800Z 34.5N 83.0W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown
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