Hurricane Sally Discussion Number  18
NWS National Hurricane Center Miami FL       AL192020
400 PM CDT Tue Sep 15 2020

Sally has been inching its way toward the north-central Gulf Coast
today.  The overall structure of the storm has remained about the
same during the day with a large ragged eye in apparent in radar
imagery.  A NOAA P-3 aircraft that has been sampling the storm
since late this morning has reported peak flight-level winds of 
76 kt, and NWS Doppler radar has shown velocities of 75-80 kt at 
around 7000 ft.  The intensity was reduced to 70 kt on the 1800 UTC 
intermediate advisory and remains at that value for this advisory.  

Radar and aircraft fixes show that Sally has been moving very slowly 
toward the north or 350/2 kt.  The track forecast philosophy remains 
unchanged from the previous advisory.  Sally is currently located 
within an area of weak steering flow between a couple of mid-level 
ridges to its east and west.  A weak mid-level trough over the 
south-central United States is forecast to slide eastward over the 
next few days, which should cause Sally to turn north-northeastward 
and then northeastward over the next 24-36 hours.  Sally's forward 
speed is expected to remain quite slow over the next 24-48 hours, 
but the guidance has trended slightly faster after that time. The 
slow forward speed is likely to result in a historical rainfall 
event for the north-central Gulf Coast.  It may sound like a broken 
record, but the track guidance has again shifted eastward during the 
first 24-36 hours, and the NHC forecast has been adjusted 
accordingly.  Since Sally has a large wind field, and storm surge 
and rainfall hazards extend far from the eye, users should not focus 
on the exact forecast track or specific location and timing of 
landfall as strong winds and bands of heavy rainfall are already 
affecting the Gulf Coast and will continue to do so for quite some 
time.

The combination of upwelling and moderate westerly shear is likely 
to result in little change in strength prior to Sally moving 
onshore.  Once the center of the hurricane moves onshore, rapid 
weakening is expected and the global models indicate that the 
circulation will becoming elongated along a frontal boundary in 3 
to 4 days.

KEY MESSAGES:

1. Historic life-threatening flash flooding due to rainfall is 
likely through Wednesday along and just inland of the coast from the 
Florida Panhandle west of the Apalachicola River to far southeastern 
Mississippi.  Widespread moderate to major river flooding 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 and Alabama, and into Georgia and the western Carolinas 
this week. 

2. Life-threatening storm surge is expected along portions of the 
coastline from Alabama to the western Florida Panhandle, including 
Mobile Bay. 

3. Hurricane conditions are expected this evening and overnight  
within portions of the Hurricane Warning area along the Mississippi 
and Alabama coastlines and the western Florida Panhandle.

FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 29.5N  88.1W   70 KT  80 MPH
12H  16/0600Z 29.9N  88.1W   70 KT  80 MPH
24H  16/1800Z 30.6N  87.7W   60 KT  70 MPH...INLAND
36H  17/0600Z 31.5N  86.9W   40 KT  45 MPH...INLAND
48H  17/1800Z 32.5N  85.3W   30 KT  35 MPH...INLAND
60H  18/0600Z 33.1N  83.3W   25 KT  30 MPH...INLAND
72H  18/1800Z 33.5N  81.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
96H  19/1800Z...DISSIPATED

$$
Forecaster Brown