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Hurricane Sally Special Discussion Number  20
NWS National Hurricane Center Miami FL       AL192020
130 AM CDT Wed Sep 16 2020

Justification for this special advisory is to increase the 
initial intensity and the 12-h forecast intensity at landfall.
 
Observations from an Air Force Hurricane Hunter aircraft and Mobile 
Alabama WSR-88D Doppler radar data indicate that Sally has continued 
to strengthen this morning. Radar data show that Sally's eye has 
become better defined and Doppler velocities in the northern eyewall 
have reached average values of at least 110 kt between 5000-6000 ft 
ASL, which supports an equivalent surface wind speed of about 89 kt. 
The reconnaissance aircraft has recently measured peak 700-mb 
flight-level winds of 103 kt, which equates to about 93 kt using a 
standard 90 percent adjustment factor, while peak SFMR surface wind 
speed have been 85 kt. In addition, a recent dropsonde in the 
northeast eyewall measured an average wind speed of 113 kt in the 
lowest 150 meters, which equates to about a 94-kt surface wind. 
However, those winds appeared to be possibly contaminated by wind 
gusts. Based on the above data, the initial intensity has been 
increased to 90 kt.

Some additional slight strengthening is possible until landfall 
occur, and Sally could peak at 95 kt. Rapid weakening will occur 
after the center moves inland, and the system should become a 
remnant low in a couple of days.  This is consistent with the 
latest model guidance.
 
Radar and aircraft center fixes indicate that Sally's motion is 
north-northeastward, or 030/02 kt. No changes were made to the 
previous track forecast. Sally should continue to move  
north-northeastward this morning, and then turn northeastward with 
a gradual increase in forward speed by tonight. That motion will 
then continue for the next day or so. Then, as Sally approaches the 
westerly flow at higher latitudes, the cyclone should turn toward 
the east-northeast with a slight further increase in forward speed 
until becoming a dissipating remnant low near the southeast U.S. 
coast in 2-3 days.  The official forecast is close to the latest 
corrected dynamical model consensus, HCCA, prediction.
 
 
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 the
Alabama/Mississippi border.  Widespread moderate to major river
flooding is forecast across the Florida Panhandle and southern
Alabama.  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 morning and then continue 
into this afternoon within portions of the Hurricane Warning area 
along the Mississippi and Alabama coastlines and the western Florida 
Panhandle.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/0630Z 29.9N  87.8W   90 KT 105 MPH
 12H  16/1200Z 30.3N  87.5W   95 KT 110 MPH
 24H  17/0000Z 31.2N  86.8W   40 KT  45 MPH...INLAND
 36H  17/1200Z 32.0N  85.7W   30 KT  35 MPH...INLAND
 48H  18/0000Z 32.9N  83.9W   25 KT  30 MPH...INLAND
 60H  18/1200Z 33.6N  81.5W   20 KT  25 MPH...INLAND
 72H  19/0000Z 34.0N  79.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  20/0000Z...DISSIPATED
 
$$
Forecaster Stewart
 
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