[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 150914 CCB
TCDAT4

Hurricane Sally Discussion Number  16...Corrected
NWS National Hurricane Center Miami FL       AL192020
400 AM CDT Tue Sep 15 2020

Corrected Key Messages 2 and 3

There has been little change overall in Sally's convective
structure in both satellite and Doppler radar data. An eye has tried
to close off several times this morning, but after less than 30
minutes the southern eyewall has eroded. Until just recently, the
central pressure had been steady for the past several hours at 986
mb. However, the most recent Air Force Reserve reconnaissance pass
through Sally's center reported a dropsonde pressure of 984 mb and
13 kt winds, which equals a pressure of 983 mb. Maximum 700-mb
flight-level winds observed have only been 63 kt and peak SFMR
winds have been 58 kt. Also, reports from nearby oil rigs have
dropped off significantly since yesterday are are now in the 40-50
kt range. Based on these data, the initial intensity has been
lowered to 75 kt.

The initial motion estimate to 300/02 kt. After a brief jog due
west, it appears that Sally has resumed a slow drift toward the
west-northwest. Sally is embedded within weak steering flow based
on 0000Z upper-air data indicating 500-mb heights of 5900 meters
and slightly higher surrounding the cyclone from Florida northward
into the Tennessee Valley and then westward into the central and
southern Plains. This weak steering pattern is expected to persist
for the next few days, with a weak mid-level trough forecast to
move into the Missouri and Tennessee Valleys by Wednesday and
Thursday, which will gradually lift Sally northward and then
northeastward. Sally is forecast to merge with a frontal system by
day 4 or 5. The new NHC forecast track is similar to the previous
advisory and lies down the middle of the rather divergent model
guidance envelope.

Sally is now expected to remain in a moderate to high mid-to
upper-level wind shear environment. Ina addition, some modest
upwelling is likely occurring in the inner-core region based a SST
decrease of nearly 2 deg F during the past 24 hours based on data
from buoy 42012. After the Sally makes landfall, rapid weakening is
forecast and Sally should become post-tropical in 3 days or less.

Users are reminded to not focus on the exact forecast track or the
specific timing and location of landfall.  Hurricane-force winds,
dangerous storm surge, and flooding rainfall will affect a large
portion of the north-central Gulf Coast during the next few days.

KEY MESSAGES:

1. It is still too early to determine where Sally's center will move
onshore given the uncertainty in the timing and location of Sally's
northward turn near the central Gulf Coast.  Users should not focus
on the details of the official forecast track, since NHC's average
forecast error at 36 hours is around 60 miles, and dangerous storm
surge, rainfall, and wind hazards will extend well away from the
center.

2. An extremely dangerous and life-threatening storm surge is
expected for areas outside the southeastern Louisiana Hurricane and
Storm Damage Risk Reduction System from the Mouth of the Mississippi
River to the Okaloosa/Walton County Line in the Florida Panhandle,
where a Storm Surge Warning is in effect.  Residents in these areas
should follow any advice given by local officials.

3. Hurricane conditions are expected today within the Hurricane
Warning area along the Mississippi and Alabama coastlines and the
western Florida Panhandle. Tropical storm conditions are already
occurring in some of these areas.

4. Historic flooding is possible with extreme life-threatening flash
flooding likely through Wednesday along and just inland of the
central Gulf Coast from the western Florida Panhandle to far
southeastern Mississippi.  Widespread moderate to major flooding on
area rivers is forecast along and just inland of the central Gulf
Coast. Significant flash and urban flooding, as well as widespread
minor to moderate river flooding is likely across inland portions of
Mississippi, Alabama, northern Georgia, and the western Carolinas
through the week.


FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 28.9N  88.1W   75 KT  85 MPH
 12H  15/1800Z 29.2N  88.4W   75 KT  85 MPH
 24H  16/0600Z 29.9N  88.5W   75 KT  85 MPH
 36H  16/1800Z 30.6N  88.3W   60 KT  70 MPH...INLAND
 48H  17/0600Z 31.4N  87.5W   35 KT  40 MPH...INLAND
 60H  17/1800Z 32.2N  86.2W   25 KT  30 MPH...INLAND
 72H  18/0600Z 32.8N  84.6W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  19/0600Z 33.2N  81.3W   20 KT  25 MPH...POST-TROP/EXTRATROP
120H  20/0600Z...DISSIPATED

$$
Forecaster Stewart
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