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Hurricane Ida Forecast Discussion Number 9
2021-08-28 16:57:13| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Sat Aug 28 2021 000 WTNT44 KNHC 281457 TCDAT4 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. FORECAST POSITIONS AND MAX WINDS 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
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