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Tropical Storm Ida Forecast Discussion Number 4
2021-08-27 10:57:12| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Fri Aug 27 2021 000 WTNT44 KNHC 270857 TCDAT4 Tropical Storm Ida Discussion Number 4 NWS National Hurricane Center Miami FL AL092021 500 AM EDT Fri Aug 27 2021 Radar data from Grand Cayman and satellite imagery indicate that Ida is becoming better organized just to the north of Grand Cayman. The convection has become more concentrated near the center, and the radar data is showing increasing curved banding. In addition, the pressure at Grand Cayman fell to 1003 mb, showing that the central pressure is lower than observed by the aircraft mission yesterday afternoon. Based on a combination of the Grand Cayman observations and satellite intensity estimates from TAFB and SAB, the initial intensity is increased to 40 kt. Air Force Reserve and NOAA Hurricane Hunter aircraft are scheduled to investigate Ida near 12Z. After the earlier wobbling, Ida seems to have resumed a northwestward motion of 320/13. A mid-level ridge centered off the southeast U.S. coast is expected to shift westward through the weekend. This feature should keep Ida moving on a general northwestward track for the next 2 or 3 days, taking the core of the system over western Cuba this afternoon or evening and then across the southern and central Gulf of Mexico this weekend. Although some model timing differences remain, they are in fairly good agreement that Ida will make landfall in Louisiana late Sunday or early Monday. The new forecast track is shifted a little to the east of the previous forecast through 48 h based on the initial location. However, the 60 and 72 h positions are similar to the previous forecast, and thus there is no significant change to the forecast landfall area in Louisiana at this time. Users are reminded to not focus on the exact forecast track as impacts will extend far from the center and the average 72-hour track error is around 120 miles. There continues to be some southwesterly shear affecting Ida due to the storm's location between an upper-level trough to the west and an upper-level anticyclone to the east. Some decrease in the shear is expected during the next 24 h, and that, combined with a moist environment and warm sea surface temperatures along the forecast track, should allow steady to rapid strengthening until the cyclone makes landfall along the northern Gulf coast. The new intensity forecast is slightly stronger than the previous forecast, and it now calls for Ida to reach a peak intensity of 100 kt before landfall. It should be noted that this intensity is lower than that forecast by the HMON model, the HCCA corrected consensus model, and the Florida State Superensemble. After landfall, Ida should weaken as it moves through the lower Mississippi and Tennessee River valleys. Based on the available guidance, there remains higher-than-normal confidence that a significant hurricane will approach and impact the Gulf coast during the weekend and early next week. Key Messages: 1. Tropical storm conditions are occurring in portions of the Cayman Islands, and they are expected in portions of western Cuba and the Isle of Youth today, where a dangerous storm surge is also possible in areas of onshore flow. Life-threatening heavy rains, flash flooding and mudslides are expected across Jamaica, the Cayman Islands, and western Cuba, including the Isle of Youth. 2. There is an increasing risk of life-threatening storm surge inundation along the coasts of Louisiana, Mississippi, and Alabama, where a Storm Surge Watch is in effect. Interests in these areas should follow any advice given by local officials. 3. There is an increasing risk of dangerous hurricane-force winds beginning Sunday along the portions of the coasts of Louisiana and Mississippi, including metropolitan New Orleans, where a Hurricane Watch is in effect. 4. Ida is also expected to produce heavy rains across the central Gulf Coast from southeast Louisiana to coastal Mississippi, Alabama, as well as the Lower Mississippi Valley starting Sunday into Monday, resulting in considerable flash, urban, small stream, and riverine flooding. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 20.0N 81.4W 40 KT 45 MPH 12H 27/1800Z 21.5N 82.9W 50 KT 60 MPH...OVER ISLE OF YOUTH 24H 28/0600Z 23.4N 84.9W 60 KT 70 MPH 36H 28/1800Z 25.3N 86.9W 70 KT 80 MPH 48H 29/0600Z 27.1N 88.8W 85 KT 100 MPH 60H 29/1800Z 28.7N 90.2W 100 KT 115 MPH 72H 30/0600Z 30.1N 91.1W 85 KT 100 MPH...INLAND 96H 31/0600Z 33.0N 90.5W 35 KT 40 MPH...INLAND 120H 01/0600Z 35.0N 88.0W 25 KT 30 MPH...INLAND $$ Forecaster Beven
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