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Hurricane Ida Forecast Discussion Number 12
2021-08-29 10:59:59| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Sun Aug 29 2021 000 WTNT44 KNHC 290859 TCDAT4 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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