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Tropical Storm Ida Forecast Discussion Number 17
2021-08-30 10:59:29| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Mon Aug 30 2021 000 WTNT44 KNHC 300859 TCDAT4 Tropical Storm Ida Discussion Number 17 NWS National Hurricane Center Miami FL AL092021 400 AM CDT Mon Aug 30 2021 Ida is now located well inland over southwestern Mississippi and weakening rapidly. However, Doppler velocity data from the Slidell, Louisiana, WSR-88D radar before it went down was still indicating velocity values near 90 kt between 4,000-4,500 ft, while the latest velocity data from Jackson, Mississippi, have been in the 70-75 kt range between 7,500 and 8,000 ft well east of the center. Thus, Ida will still be capable of producing damaging wind gusts in some of the stronger showers and thunderstorms for the next few hours. The intensity at 0600 UTC was 65 kt, which was based on a decay rate of about 10 kt per hour. A slightly slower decay rate of about 5 kt per hour has been used since then, which is the basis for the 50-kt advisory intensity at 0900 UTC. The estimated pressure of 990 mb is based on surface observation data, especially from McComb, Mississippi (KMCB), which has been reporting pressures near 995 mb with 25-30 kt of wind the past couple of hours. The initial motion estimate is now 355/07 kt. Ida should continue moving northward today around the western periphery of a deep-layer ridge situated over the southeastern United States. A faster motion toward the northeast is expected by Tuesday as a mid- to upper-level trough approaches the cyclone, with that motion continuing through the remainder of the week. The new NHC track forecast is just a tad to the south of the previous advisory, and lies down the middle off the tightly packed consensus models. Rapid weakening will continue as Ida moves farther inland over Mississippi due to land interaction and southwesterly vertical wind shear of more than 20 kt. However, damaging winds, especially in gusts, are expected to continue over southeastern Louisiana and southwestern Mississippi through through this morning. To account for this, the gust factor in the Forecast/Advisory has been adjusted accordingly. Ida is likely to weaken to a tropical depression this evening. Some slight restrengthening as an extratropical storm is possible when Ida moves over the western Atlantic in the day 4-5 period. In addition, heavy rains will spread northward and then northeastward along the forecast track. Key Messages: 1. Dangerous storm surge inundation will continue through this morning along portions of the coast between Grand Isle, Louisiana, to Bay St. Louis, Mississippi. Overtopping of local levees outside of the Hurricane and Storm Damage Risk Reduction System is possible where local inundation values may be higher. 2. Damaging winds, especially in gusts, will continue to spread inland near the track of Ida's center into southwestern Mississippi through this morning and early afternoon. These winds will likely lead to widespread tree damage and power outages. 3. Ida will continue to produce heavy rainfall tonight through Tuesday morning across portions of southeast Louisiana, coastal Mississippi, and southwestern Alabama resulting in considerable to life-threatening flash and urban flooding and significant riverine flooding impacts. As Ida moves inland, considerable flooding impacts are possible across portions of the Lower Mississippi Valley, Tennessee Valley, Ohio Valley, Central and Southern Appalachians, and Mid-Atlantic through Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 31.0N 90.8W 50 KT 60 MPH...INLAND 12H 30/1800Z 32.2N 90.6W 30 KT 35 MPH...INLAND 24H 31/0600Z 33.7N 89.4W 25 KT 30 MPH...INLAND 36H 31/1800Z 35.1N 87.2W 25 KT 30 MPH...INLAND 48H 01/0600Z 36.6N 84.3W 20 KT 25 MPH...POST-TROP/INLAND 60H 01/1800Z 38.1N 80.8W 20 KT 25 MPH...POST-TROP/INLAND 72H 02/0600Z 39.0N 77.4W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 03/0600Z 40.1N 71.5W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 04/0600Z 40.5N 67.2W 30 KT 35 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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