[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 130941 CCA
TCDAT4

Tropical Storm Sally Discussion Number   7...Corrected
NWS National Hurricane Center Miami FL       AL192020
500 AM EDT Sun Sep 13 2020

Corrected for inland status at 60 and 72 hourrs

A large burst of convection has formed near the center of Sally this
morning, and Air Force Reserve Hurricane Hunter data indicate that
the storm is strengthening.  Peak flight-level winds were up to 57
kt at 925 mb, with believable SFMR winds of up to 45 kt, and
pressure falling a couple of mb to 1001.  The initial wind speed is
set to 45 kt for this advisory.

While northwesterly shear has been affecting Sally, the global
models suggest that the shear will decrease later today as it moves
under a narrow ridge.  Models all respond to these changing
conditions by showing intensification, but they disagree on the rate
of change.  It is an extremely tricky forecast because of how this
might happen close to landfall, but guidance is generally 5-10 kt
higher than the last cycle so the intensity forecast has been raised
by those amounts up through landfall.  It is worth noting that rapid
intensification is a possibility if the system develops a more solid
inner core on Monday, and stronger solutions like the HWRF model
can't be dismissed.

Sally is moving west-northwestward at a faster speed this morning.
The biggest change to note is that almost every model has shifted
westward (faster) due to narrow ridging developing across the
northern Gulf Coast between the storm and an exiting mid-latitude
trough.  The flow gets quite weak near and after landfall, so the
cyclone should slow down on Tuesday/Wednesday and turn northward.
After that time, the next trough should induce a faster
northeastward motion by day 5. The new track is shifted westward
between 30-45 n mi near the coast of Louisiana, and is still east of
the GFS and ECMWF models.

Users are reminded to not focus on the exact details of the track
or intensity forecasts, as the average NHC track error at 48 h is
around 80-90 miles and the average intensity error is around 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 early Tuesday 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. Tropical Storm Sally is expected to produce flash flooding across
southern and central Florida and prolong existing minor river
flooding across Central Florida through Monday. Significant flash
flooding and minor to isolated major river flooding is likely across
portions of the Central Gulf Coast from Monday through the middle of
the week.

FORECAST POSITIONS AND MAX WINDS

INIT  13/0900Z 27.0N  84.0W   45 KT  50 MPH
 12H  13/1800Z 27.7N  85.4W   50 KT  60 MPH
 24H  14/0600Z 28.3N  87.2W   60 KT  70 MPH
 36H  14/1800Z 28.7N  88.7W   70 KT  80 MPH
 48H  15/0600Z 29.2N  89.7W   85 KT 100 MPH
 60H  15/1800Z 30.1N  90.0W   70 KT  80 MPH...INLAND
 72H  16/0600Z 31.0N  90.0W   50 KT  60 MPH...INLAND
 96H  17/0600Z 32.8N  88.6W   30 KT  35 MPH...INLAND
120H  18/0600Z 34.5N  84.5W   20 KT  25 MPH...INLAND

$$
Forecaster Blake
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