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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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Tropical Depression Ten Forecast Discussion Number 9
2021-08-30 10:58:23| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Aug 30 2021 000 WTNT45 KNHC 300858 TCDAT5 Tropical Depression Ten Discussion Number 9 NWS National Hurricane Center Miami FL AL102021 500 AM AST Mon Aug 30 2021 The deep convection with Tropical Depression Ten is in a sheared bursting pattern this morning, associated with overshooting cloud top temperatures below -80 C. A 0518 UTC AMSR-2 microwave pass did show a bit of banding associated with this activity on the 37 GHz low-level channel. However, Proxy-Vis satellite imagery indicates this convective activity remains located downshear of the low-level center thanks to very strong 30-50 kt northwesterly flow associated with a subtropical jet at 200 mb. The latest round of subjective Dvorak estimates were 25 kt from SAB and 35 kt from TAFB. Taking a blend of these estimates and the earlier ASCAT wind data supports keeping the intensity at 30 kt for this advisory. The depression has resumed a northward motion this morning, with the latest estimate at 360 degrees at 7 kt. A deep-layer trough passing by well to the north is continuing to provide a weakness in the subtropical ridge, allowing the cyclone to escape northward. The system could even move a bit east of due north over the next 24-36 hours if down-shear convective bursts help to drag the low-level center a bit right of the steering flow. After 36 hours, the deep-layer trough moves eastward, allowing the subtropical ridge to build back in. The net result is that the depression should turn leftward and begin a more northwestward motion by the latter part of this week. The latest track guidance has once again made another westward shift this cycle after 36 hours, and the NHC track forecast has been nudged in that direction as well. However, the latest track is still not as far west as the GFS & ECMWF models, and further westward adjustments may be needed in subsequent forecasts. Strong upper-level flow is the primary hindrance for the depression currently. In fact, both GFS & ECMWF-based SHIPS guidance shows the vertical wind shear remaining above 30 kt for the next 24 hours as the cyclone moves through the core of a subtropical jet streak. Interestingly, this shearing flow seems to be mostly based in the upper-levels, with much lower mid-level shear diagnosed by UW-CIMSS. This lower mid-level shear may help explain why deep-convection has not yet been completely stripped away from the low-level center. After 36 hours, most of the guidance agrees that an upper-level low will cut off to the southwest of the depression, providing a more favorable upper-level environment over the system. However, it remains unclear what will be left of the depression by that time, and the latest 00z ECMWF, HWRF, and HMON runs suggest the vortex will be too weak and diffuse to take advantage of the more favorable conditions. For now, the latest NHC intensity forecast will maintain the current intensity through 48 hours, with only modest intensification beginning after that time assuming the circulation is coherent enough to take advantage of the more favorable environment. The latest intensity forecast is just a bit lower than the previous forecast, and is also lower than the HCCA and IVCN consensus aids. It remains distinctly possible that the depression could become a remnant low if its convection is completely stripped away. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 20.8N 50.6W 30 KT 35 MPH 12H 30/1800Z 21.7N 50.6W 30 KT 35 MPH 24H 31/0600Z 22.6N 50.3W 30 KT 35 MPH 36H 31/1800Z 23.6N 50.1W 30 KT 35 MPH 48H 01/0600Z 24.8N 50.7W 30 KT 35 MPH 60H 01/1800Z 26.1N 51.7W 35 KT 40 MPH 72H 02/0600Z 27.6N 53.1W 40 KT 45 MPH 96H 03/0600Z 30.8N 55.3W 45 KT 50 MPH 120H 04/0600Z 35.1N 53.6W 50 KT 60 MPH $$ Forecaster Papin
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Tropical Depression Ten Graphics
2021-08-30 10:56:45| National Hurricane Center (Atlantic)
5-Day Uncertainty Track last updated Mon, 30 Aug 2021 08:56:45 GMT Wind Speed Probabilities last updated Mon, 30 Aug 2021 09:28:46 GMT
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Tropical Storm Ida Storm Surge Watch/Warning Map
2021-08-30 10:56:24| National Hurricane Center (Atlantic)
Storm Surge Watch/Warning Map last updated Mon, 30 Aug 2021 08:56:24 GMT
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Summary for Tropical Depression Ten (AT5/AL102021)
2021-08-30 10:54:53| National Hurricane Center (Atlantic)
...TROPICAL DEPRESSION CONTINUES WITH LITTLE CHANGE IN INTENSITY UNDER STRONG UPPER-LEVEL WINDS... As of 5:00 AM AST Mon Aug 30 the center of Ten was located near 20.8, -50.6 with movement N at 8 mph. The minimum central pressure was 1008 mb with maximum sustained winds of about 35 mph.
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