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Tropical Storm Ida Discussion Number  17
NWS National Hurricane Center Miami FL       AL092021
400 AM CDT Mon Aug 30 2021
 
Ida is now located well inland over southwestern Mississippi and
weakening rapidly. However, Doppler velocity data from the Slidell,
Louisiana, WSR-88D radar before it went down was still indicating
velocity values near 90 kt between 4,000-4,500 ft, while the latest
velocity data from Jackson, Mississippi, have been in the 70-75 kt
range between 7,500 and 8,000 ft well east of the center. Thus, Ida
will still be capable of producing damaging wind gusts in some of
the stronger showers and thunderstorms for the next few hours. The
intensity at 0600 UTC was 65 kt, which was based on a decay rate of
about 10 kt per hour. A slightly slower decay rate of about 5 kt
per hour has been used since then, which is the basis for the 50-kt
advisory intensity at 0900 UTC. The estimated pressure of 990 mb is
based on surface observation data, especially from McComb,
Mississippi (KMCB), which has been reporting pressures near 995 mb
with 25-30 kt of wind the past couple of hours.
 
The initial motion estimate is now 355/07 kt. Ida should continue 
moving northward today around the western periphery of a deep-layer 
ridge situated over the southeastern United States. A faster motion 
toward the northeast is expected by Tuesday as a mid- to upper-level 
trough approaches the cyclone, with that motion continuing through 
the remainder of the week.  The new NHC track forecast is just a tad 
to the south of the previous advisory, and lies down the middle off 
the tightly packed consensus models.
 
Rapid weakening will continue as Ida moves farther inland over 
Mississippi due to land interaction and southwesterly vertical wind 
shear of more than 20 kt. However, damaging winds, especially in 
gusts, are expected to continue over southeastern Louisiana and 
southwestern Mississippi through through this morning.  To account 
for this, the gust factor in the Forecast/Advisory has been adjusted 
accordingly.  Ida is likely to weaken to a tropical depression this 
evening. Some slight restrengthening as an extratropical storm is 
possible when Ida moves over the western Atlantic in the day 4-5 
period.  In addition, heavy rains will spread northward and then 
northeastward along the forecast track.

 
Key Messages:
 
1. Dangerous storm surge inundation will continue through this
morning along portions of the coast between Grand Isle, Louisiana,
to Bay St. Louis, Mississippi. Overtopping of local levees outside
of the Hurricane and Storm Damage Risk Reduction System is possible
where local inundation values may be higher.
 
2. Damaging winds, especially in gusts, will continue to spread
inland near the track of Ida's center into southwestern Mississippi
through this morning and early afternoon. These winds will likely
lead to widespread tree damage and power outages.
 
3. Ida will continue to produce heavy rainfall tonight through
Tuesday morning across portions of southeast Louisiana, coastal
Mississippi, and southwestern Alabama resulting in considerable to
life-threatening flash and urban flooding and significant riverine
flooding impacts. As Ida moves inland, considerable flooding impacts
are possible across portions of the Lower Mississippi Valley,
Tennessee Valley, Ohio Valley, Central and Southern Appalachians,
and Mid-Atlantic through Wednesday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/0900Z 31.0N  90.8W   50 KT  60 MPH...INLAND
 12H  30/1800Z 32.2N  90.6W   30 KT  35 MPH...INLAND
 24H  31/0600Z 33.7N  89.4W   25 KT  30 MPH...INLAND
 36H  31/1800Z 35.1N  87.2W   25 KT  30 MPH...INLAND
 48H  01/0600Z 36.6N  84.3W   20 KT  25 MPH...POST-TROP/INLAND
 60H  01/1800Z 38.1N  80.8W   20 KT  25 MPH...POST-TROP/INLAND
 72H  02/0600Z 39.0N  77.4W   25 KT  30 MPH...POST-TROP/EXTRATROP
 96H  03/0600Z 40.1N  71.5W   30 KT  35 MPH...POST-TROP/EXTRATROP
120H  04/0600Z 40.5N  67.2W   30 KT  35 MPH...POST-TROP/EXTRATROP
 
$$
Forecaster Stewart
 
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