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Tropical Storm Sally Forecast Discussion Number 10
2020-09-14 04:54:19| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sun Sep 13 2020 542 WTNT44 KNHC 140254 TCDAT4 Tropical Storm Sally Discussion Number 10 NWS National Hurricane Center Miami FL AL192020 1000 PM CDT Sun Sep 13 2020 Sally is gradually getting better organized. Satellite images show that deep convection has increased near the center, and the cyclone is now a little more symmetric and vertically aligned compared to earlier today. The Air Force Hurricane Hunters are currently investigating Sally, and so far they have found maximum 850-mb flight-level winds of 54 kt and maximum believable SFMR winds of 49 kt, which support the 50-kt initial intensity. Reports from the aircraft indicate that the center of Sally has jogged to the northeast, with the latest 12-hour motion estimated to be 305/7 kt. The global models show a trough exiting the northeast U.S. tomorrow and a ridge building to the north of Sally, which should cause the storm to resume a west-northwest motion at a relatively slow pace on Monday. By Monday night and Tuesday, the ridge is forecast to slide southeastward as another trough develops over the south-central U.S. This change in the pattern should cause Sally to slow down even more and gradually turn to the north and then the northeast. The new NHC tack forecast is slower and east of the previous one based on the initial position/motion and the latest models. However, the official forecast still lies west of the latest consensus aids, so further adjustments may be necessary overnight. While the current forecast shows landfall along the northern Gulf coast in 36 to 48 hours, the bottom line is that Sally is expected to be a slow- moving tropical cyclone near and over the northern Gulf Coast during the next few days. The upper-level low that was producing northwesterly shear over Sally is moving away, resulting in a more favorable upper-level wind pattern for strengthening. These more conducive winds aloft combined with the very warm Gulf of Mexico waters and a moist air mass should allow the cyclone to steadily strengthening until Sally crosses the coast in 36 to 48 hours. The NHC intensity forecast is largely an update of the previous one and lies near the high end of the model guidance. After landfall, rapid weakening is forecast, and Sally is expected to become a tropical depression by 72 hours and dissipate in about 5 days. The eastward shift in the track forecast necessitates the extension of the hurricane warning eastward to the Mississippi/Alabama border. KEY MESSAGES: 1. It is too early to determine where Sally's center will move onshore given the uncertainty in the timing and location of Sally's northward turn near the central Gulf Coast. Users should not focus on the details of the official forecast track, since NHC's average forecast error at 48 hours is around 80 miles, and dangerous storm surge, rainfall, and wind hazards will extend well away from the center. 2. An extremely dangerous and life-threatening storm surge is expected for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from Port Fourchon, Louisiana, to the Alabama/Florida border, where a Storm Surge Warning is in effect. Residents in these areas should follow any advice given by local officials. 3. Hurricane conditions are expected by late Monday within portions of the Hurricane Warning area from Morgan City, Louisiana, to the Mississippi/Alabama border, including Metropolitan New Orleans, with tropical storm conditions likely to begin Monday. Preparations should be rushed to completion in those areas. 4. Sally could continue to produce flash flooding across central and northern 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 southeastern Louisiana, Mississippi, and Alabama through the middle of the week. Flooding impacts are expected to spread farther across the Southeast U.S. through the week. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 28.2N 86.2W 50 KT 60 MPH 12H 14/1200Z 28.6N 87.3W 60 KT 70 MPH 24H 15/0000Z 29.0N 88.3W 70 KT 80 MPH 36H 15/1200Z 29.4N 89.0W 80 KT 90 MPH 48H 16/0000Z 30.3N 89.4W 70 KT 80 MPH...INLAND 60H 16/1200Z 31.2N 89.1W 45 KT 50 MPH...INLAND 72H 17/0000Z 32.1N 88.3W 30 KT 35 MPH...INLAND 96H 18/0000Z 33.6N 85.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 19/0000Z...DISSIPATED $$ Forecaster Cangialosi
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