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Hurricane IDA

Hurricane Ida Discussion Number   9
NWS National Hurricane Center Miami FL       AL092021
1000 AM CDT Sat Aug 28 2021
The satellite presentation of Ida has continued to improve this 
morning with the center embedded within a fairly symmetric Central 
Dense Overcast and hints of the eye in both visible in infrared 
satellite imagery. Earlier microwave imagery revealed a well- 
defined low- to mid-level eye and excellent spiral banding over the 
northern semicircle.  Both NOAA and Air Force Reserve hurricane 
hunter aircraft, somewhat surprisingly, have not found a 
significantly deepening tropical cyclone yet, but given the very 
recent increase in organization and structure, the winds are likely 
to catch up to the satellite appearance very soon.  Using the latest 
flight-level and SFMR wind data from the aircraft and satellite 
intensity estimates the initial wind speed remains 75 kt for this 
advisory.  The aircraft data does indicate that Ida's wind field has 
expanded, and the NHC wind radii have been adjusted accordingly.
Given the improved inner-core structure Ida appears poised to 
rapidly intensify during the next 12 to 24 hours as it remains 
within a favorable environment of low vertical wind shear and over 
warm water.  Although the interpolated intensity guidance has 
lowered somewhat due to the fact that Ida has not significantly 
strengthened yet, the explicit forecasts from the dynamical models 
continue to support significant deepening, and the latest runs of 
the HWRF and HMON models indicate Ida will reach Category 4 strength 
before landfall.  The new NHC intensity forecast calls for rapid 
intensification to Category 4 strength during the next 24 hours.  
After that time, fluctuations in intensity are possible due to 
eyewall replacement cycles as Ida approaches the northern Gulf 
coast.  After landfall, rapid weakening is expected and Ida is 
forecast to weaken to a tropical depression by Monday night, and 
become a post-tropical cyclone as it moves over the Tennessee Valley 
by mid-week.
Ida has been moving steadily northwestward or 315/14 kt.  A deep 
layer ridge near the southeastern United States coast is forecast to 
shift westward during the next day or so, and this should continue 
to steer Ida northwestward through landfall on Sunday. The track 
guidance remains in remarkably good agreement through 36 hours, and 
the new NHC track forecast is essentially unchanged during that 
time, and continues to indicate that Ida will reach the coast of 
Louisiana on Sunday. After landfall, Ida will be near the western 
extent of the ridge and should turn northward and then northeastward 
as it recurves into the southern portion of the mid-latitude 
westerlies.  The track guidance has shifted slightly westward beyond 
48 hours, and the track forecast over the southern U.S. and the 
Tennessee Valley has been adjusted accordingly.  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.  Wind and rainfall impacts will also penetrate inland 
through early next week after Ida makes landfall.

Key Messages:
1. There is a danger of life-threatening storm surge inundation 
Sunday along the coasts of Louisiana and Mississippi 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 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 today in the warning area. 

3. Ida is likely to produce heavy rainfall Sunday and Monday across 
the central Gulf Coast from southeast Louisiana to coastal 
Mississippi, resulting in life-threatening flash and urban flooding 
and significant river flooding impacts. As Ida moves inland, 
significant flooding impacts are likely across portions of the Lower 
Mississippi and Tennessee Valleys Monday and Tuesday.

INIT  28/1500Z 24.8N  86.1W   75 KT  85 MPH
 12H  29/0000Z 26.3N  87.9W  100 KT 115 MPH
 24H  29/1200Z 28.0N  89.8W  115 KT 130 MPH
 36H  30/0000Z 29.4N  91.0W  115 KT 130 MPH...INLAND
 48H  30/1200Z 31.0N  91.5W   60 KT  70 MPH...INLAND
 60H  31/0000Z 32.7N  91.0W   30 KT  35 MPH...INLAND
 72H  31/1200Z 34.4N  89.8W   25 KT  30 MPH...INLAND
 96H  01/1200Z 36.8N  85.3W   20 KT  25 MPH...POST-TROP/INLAND
120H  02/1200Z 37.9N  79.9W   20 KT  25 MPH...POST-TROP/INLAND
Forecaster Brown