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Hurricane Ida Discussion Number  12
NWS National Hurricane Center Miami FL       AL092021
400 AM CDT Sun Aug 29 2021
 
Ida has undergone some dramatic inner-core structural changes since 
the previous advisory. The eye between 25,000-45,000 ft has become 
circular with a diameter of about 15 nmi now, and at least two 
eyewall mesocyclones have been noted rotating cyclonically around 
the eyewall in both radar and high-resolution 1-minute GOES-16 
satellite imagery. The result has been rapid strengthening of at 
least 30 kt during the past 6 hours, along with a pressure drop of 
more than 15 mb during that same time, with a 6-mb decrease having 
occurred in the 1-hr period between about 0500-0600 UTC based on Air 
Force Reserve reconnaissance aircraft eye dropsonde data. The 
aircraft also measured a maximum 700-mb flight-level wind speed of 
133 kt in the northeastern quadrant, along with a peak SFMR surface 
wind speed of 116 kt. Furthermore, NWS Doppler radar velocity data 
from Slidell, Louisiana, has recently been measuring velocities of 
120-130 kt between 25,000-30,000 ft, which is quite rare, and 
indicates that Ida is a vertically deep and intense hurricane. Ida 
was initialized with 115 kt at 0600 UTC, but the 0900 UTC advisory 
intensity has been increased to 120 kt based on the 133-kt 
flight-level wind and the improved structure in both radar data and 
satellite imagery since the 0609 UTC time of that aircraft 
observation.
 
The initial motion remains northwestward, or 315/13 kt. There is no 
significant change to the previous forecast track or synoptic 
reasoning. The subtropical ridge oriented east-west along 30N-31N 
across the southeastern U.S. is forecast to remain intact through 
the forecast period with only minor shifts in the location and 
strength of the ridge. As a result, Ida should continue to move 
northwestward toward the southeastern Louisiana coast today, 
followed by a gradual turn toward the north tonight after landfall. 
On Monday, the hurricane is expected to  move northeastward across 
the Tennessee Valley when Ida moves north of the ridge axis. 
Impacts and hazards will arrive well before the eye of the hurricane 
makes landfall. Tropical-storm-force winds are likely to begin later 
this morning. Therefore, all preparations to protect life and 
property must be rushed to completion. The new track forecast is 
basically just an update of the previous advisory track.
 
Ida will remain over waters with high oceanic heat content for 
another 6 hours or so. Thereafter, the heat content will drop 
sharply to less than half of the current value of more than 100 
units. However, some additional strengthening is expected until 
landfall occurs. After Ida moves inland tonight, rapid weakening is 
forecast due to a combination of land interaction, entrainment 
of drier air, and some increase in westerly vertical wind shear.
 
Users are again reminded to not focus on the exact details of the
track forecast as storm surge, wind, and rainfall impacts will
extend far from the center.  Rainfall impacts will also spread
inland across the Lower Mississippi and Tennessee Valleys through
early next week.
 
Key Messages:
 
1. There is a danger of life-threatening storm surge inundation
Sunday along the coasts of Louisiana, Mississippi, and Alabama
within the Storm Surge Warning area. Extremely life-threatening
inundation of 9 feet or greater above ground level is possible
somewhere within the area from Morgan City, Louisiana, to the coast
of Mississippi. Overtopping of local levees outside of the Hurricane
and Storm Damage Risk Reduction System is possible where local
inundation values may be higher. Interests throughout the warning
area should follow any advice given by local officials.
 
2. Ida is expected to be an extremely dangerous major hurricane when 
it reaches the coast of southeastern Louisiana. Hurricane-force 
winds are expected Sunday in portions of the Hurricane Warning area 
along the Louisiana coast, including metropolitan New Orleans, with 
potentially catastrophic wind damage possible where the core of Ida 
moves onshore. Actions to protect life and property should be rushed 
to completion in the warning area.
 
3. Damaging winds, especially in gusts, will spread inland near the
track of the center of Ida across portions of southeastern Louisiana
and southwestern Mississippi Sunday night and early Monday. These
winds will likely lead to widespread tree damage and power outages.
 
4. Ida will produce heavy rainfall today through Monday across the 
central Gulf Coast from southeastern Louisiana, coastal Mississippi, 
to far southwestern Alabama resulting in considerable to 
life-threatening flash and urban flooding and significant riverine 
flooding impacts. As Ida moves inland, significant flooding impacts 
are possible across portions of the Lower Mississippi, Tennessee, 
and Ohio Valleys through Wednesday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  29/0900Z 28.0N  89.1W  120 KT 140 MPH
 12H  29/1800Z 29.1N  90.3W  125 KT 145 MPH...NEAR SERN LOUISIANA
 24H  30/0600Z 30.6N  91.1W   70 KT  80 MPH...INLAND
 36H  30/1800Z 32.2N  91.0W   45 KT  50 MPH...INLAND
 48H  31/0600Z 33.8N  90.0W   30 KT  35 MPH...INLAND
 60H  31/1800Z 35.4N  87.9W   25 KT  30 MPH...INLAND
 72H  01/0600Z 36.7N  85.2W   20 KT  25 MPH...INLAND
 96H  02/0600Z 38.9N  78.9W   20 KT  25 MPH...POST-TROP/INLAND
120H  03/0600Z...DISSIPATED
 
$$
Forecaster Stewart
 
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