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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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