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| #1016438 (Received by flhurricane at: 10:59 AM 13.Sep.2020) TCDAT4 Tropical Storm Sally Discussion Number 8 NWS National Hurricane Center Miami FL AL192020 1100 AM EDT Sun Sep 13 2020 Although the center of Sally remains near the northwestern edge of the deep convection, there is a large area of convection and some banding evident over the southeastern portion of the circulation. An Air Force Reserve Hurricane Hunter aircraft investigating the storm this morning reported a peak 850-mb flight-level wind of 62 kt, and believable SFMR winds of around 50 kt, and these data are the basis for the 50-kt initial wind speed. The plane reported minimum pressures in the 996-998 mb range, with the higher value being the most recent information available. Northwesterly shear continues over the cyclone, but this shear is expected to decrease later today and tonight as Sally moves beneath a narrow upper-level ridge axis. This more conducive upper-level pattern is expected to allow the tropical storm to strengthen while it moves over the north-central Gulf of Mexico tonight and Monday. Most of the intensity guidance calls for Sally to become a hurricane in about 24 hours and so does the official forecast. Additional strengthening is expected after that time and Sally could be slightly stronger at landfall than indicated below since it is forecast to reach the coast between the 36 h forecast point and 48 h when it is inland over southeast Louisiana. The NHC intensity forecast is close to the consensus aids through 24 hours and at or just above the SHIPS, LGEM and HFIP corrected consensus aids at 36 and 48 hours. Sally is moving west-northwestward at about 10 kt. The tropical storm should continue on that general heading and speed over the next 12 to 24 hours as it steered around the southern flank of a mid-level ridge. After 24 hours, Sally is expected to be near the western portion of the ridge which should cause the storm to slow down and turn northwestward. The global models have trended toward slightly more ridging over the northern Gulf during the next 24 hours, and the track guidance has edged westward. The NHC track has been adjusted slightly westward and lies near the lastest run of the GFS, but is not as far west as the ECMWF and the various consensus aids. As Sally rounds the ridge in 48 to 72 h, the steering flow is expected to be quite weak, and a slow northward motion is forecast at that time. Afterward, a north-northeastward to northeastward motion should commence as the cyclone moves in that direction ahead of a short-wave trough. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 36-48 h is 60-80 miles and the average intensity error is 10-15 mph. In addition, wind, storm surge, and rainfall hazards will extend far from the center. Unfortunately, confidence is increasing that Sally's expected slow forward speed near the Gulf Coast will exacerbate the storm surge and heavy rainfall threats. KEY MESSAGES: 1. An extremely dangerous and life-threatening storm surge is now expected, and a Storm Surge Warning is in effect for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from Port Fourchon, Louisiana to the Mississippi/Alabama border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are expected by late Monday from Grand Isle, Louisiana to Ocean Springs, Mississippi, including Metropolitan New Orleans, with tropical storm conditions likely by Monday. Preparations should be rushed to completion in those areas. 3. Sally is expected to produce flash flooding across southwest and central Florida and prolong existing minor river flooding across west-central Florida through Monday. Widespread significant flash flooding and minor to isolated major river flooding is likely across portions of the central Gulf Coast Monday through the middle of the week, with flooding impacts spreading farther into the Southeast in the middle to late parts of the week. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 27.5N 84.9W 50 KT 60 MPH 12H 14/0000Z 28.0N 86.6W 55 KT 65 MPH 24H 14/1200Z 28.5N 88.2W 65 KT 75 MPH 36H 15/0000Z 28.9N 89.4W 80 KT 90 MPH 48H 15/1200Z 29.7N 90.2W 65 KT 75 MPH...INLAND 60H 16/0000Z 30.3N 90.5W 50 KT 60 MPH...INLAND 72H 16/1200Z 31.5N 90.3W 35 KT 40 MPH...INLAND 96H 17/1200Z 33.6N 88.3W 25 KT 30 MPH...INLAND 120H 18/1200Z 34.8N 83.8W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown |
