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Tropical Storm Sally Discussion Number   6
NWS National Hurricane Center Miami FL       AL192020
1100 PM EDT Sat Sep 12 2020
 
Sally's structure remains somewhat disorganized this evening. While 
there has been a recent increase in deep convection near the low- 
level center, there is a large CDO feature with -70 to -80C tops 
displaced 50 to 60 miles to the south and southeast. This structure 
could be due to the 10 to 15 kt of northwesterly shear analyzed by 
the SHIPS model and UW-CIMSS. The initial intensity remains 35 kt 
based on the latest Dvorak estimates from TAFB and SAB, and the 
central pressure of 1003 mb is based on surface observations in the 
Naples area as the center passed offshore before 00Z.  An Air Force 
Reserve Hurricane Hunter aircraft will be investigating Sally 
overnight to provide more information on the storm's intensity and 
structure.
 
Given the current structure, only gradual strengthening is expected 
in the short term. However, once the cyclone develops more of an 
inner core, it should be able to take advantage of warm SSTs and low 
shear over the Gulf of Mexico during the next couple of days. Steady 
intensification to hurricane strength is forecast through 48 hours, 
with a leveling off of the intensity in 60-72 hours as the shear 
is expected to increase. Overall, the intensity guidance is a 
little less bullish this cycle, with the HWRF more aggressive while 
the remainder of the guidance shows a slower rate of 
intensification. The NHC intensity forecast through landfall remains 
unchanged from the previous advisory, but now lies a little above 
HCCA and the intensity consensus. 

The initial motion estimate is a somewhat uncertain 285/07. Sally is 
currently being steered by a mid-level ridge centered over the 
Carolinas that will shift eastward and weaken as a longwave trough 
moves into the eastern United States over the next 24 to 36 hours. 
During this time, Sally should move generally west-northwestward or 
northwestward with a decrease in forward speed on Monday. By 48 
hours, the trough lifts out and a narrow ridge builds in north and 
east of Sally, which should result in the tropical cyclone turning 
north-northwestward and then northward at less than 5 kt as it 
approaches the north-central Gulf Coast and moves inland over the 
Mississippi Valley. After that time, the next trough should induce a 
faster northeastward motion by day 5. There is a fair bit of spread 
in the track guidance after 24 hours, with the COAMPS-TC well to the 
right and the GEFS ensemble mean and UKMET to the left. However, 
the GFS and ECMWF are in good agreement through 60 hours near the 
middle of the guidance envelope. The NHC track is similar to the 
previous one and lies in the middle of the guidance and is near the 
TVCA multi-model consensus and HCCA. 
 
Users are reminded to not focus on the exact details of the track 
or intensity forecasts, as the average NHC track error at 72 h is 
around 110 miles and the average intensity error is around 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. Life-threatening storm surge is possible along the Gulf Coast
beginning on Monday, and a Storm Surge Watch is in effect for areas
outside the southeastern Louisiana Hurricane and Storm Damage Risk
Reduction System from the Mouth of the Mississippi River to the
Alabama/Florida border. Residents in these areas should follow any
advice given by local officials.
 
2. Hurricane conditions are possible by early Tuesday from Grand
Isle, Louisiana to the Alabama/Florida border, including
Metropolitan New Orleans, with tropical storm conditions possible by
Monday.
 
3. Sally is expected to produce flash flooding across portions of 
southern Florida and prolong existing minor river flooding across 
central Florida through Sunday. Flash and urban flooding and minor 
to moderate river flooding are likely across portions of the 
central Gulf Coast from Sunday through the middle of next week. 
 
4. Tropical storm conditions are possible early next week in
portions of the Florida Panhandle, where a Tropical Storm Watch is
in effect.  Wind gusts to tropical storm force could occur over
portions of the lower Florida Keys overnight.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  13/0300Z 26.3N  82.9W   35 KT  40 MPH
 12H  13/1200Z 27.0N  84.4W   40 KT  45 MPH
 24H  14/0000Z 27.7N  86.0W   50 KT  60 MPH
 36H  14/1200Z 28.3N  87.5W   60 KT  70 MPH
 48H  15/0000Z 28.8N  88.6W   70 KT  80 MPH
 60H  15/1200Z 29.4N  89.2W   75 KT  85 MPH
 72H  16/0000Z 30.1N  89.5W   75 KT  85 MPH
 96H  17/0000Z 31.7N  89.1W   50 KT  60 MPH...INLAND
120H  18/0000Z 33.5N  86.5W   25 KT  30 MPH...INLAND
 
$$
Forecaster Brennan
 
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