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Hurricane Sally Forecast Discussion Number 15
2020-09-15 04:46:15| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Mon Sep 14 2020 000 WTNT44 KNHC 150246 TCDAT4 Hurricane Sally Discussion Number 15 NWS National Hurricane Center Miami FL AL192020 1000 PM CDT Mon Sep 14 2020 After rapidly strengthening earlier today, Sally's intensity has plateaued for now. Both the NOAA and Air Force Hurricane Hunters have been investigating Sally this evening and they have found that the minimum pressure and winds have leveled off, and support perhaps a generous initial intensity of 85 kt. Doppler radar images and reports from both aircraft indicate the inner core of the hurricane is quite small and that the eyewall is open on the south side, likely due to some dry air that has wrapped into that portion of the circulation. Aircraft and Doppler radar fixes indicate that Sally is moving very slowly to the west-northwest, with the latest initial motion estimated to be 300/3 kt. Weak high pressure ridging to the north and east of Sally is expected to cause the hurricane to continue to move slowly west-northwestward to northwestward for another 12 hours, bringing the center of the storm very near the northern Gulf coast. By Tuesday afternoon, when the hurricane will likely be just offshore, the models show the steering currents collapsing and Sally is likely to drift northward before finally turning northeastward ahead of a developing mid-level trough over the central U.S. by late Wednesday. There continues to be a significant amount of uncertainty on exactly where and when Sally turns northward and makes landfall, with model solutions ranging from a landfall on the Florida panhandle to a landfall in extreme southeastern Louisiana. It should be emphasized that it is always challenging to forecast the track of hurricanes in weak steering currents, and in Sally's case the weak steering is occurring very near land. The new NHC track forecast is a little to the east of the previous one, trending toward the latest consensus aids. Sally is still in generally favorable environmental conditions consisting of very warm SSTs and low wind shear. Since the hurricane will likely remain in those conditions through Tuesday morning, some strengthening seems likely in the short term. In 12 to 24 hours, when Sally is forecast to be very near the coast, a combination of an increase in westerly shear and cooler upwelled shelf waters should limit additional intensification. After the hurricane makes landfall, rapid weakening is forecast and Sally should become post-tropical in 3 to 4 days over the southeast U.S. The NHC intensity forecast lies at the high end of the model guidance and is quite similar to the previous one. Users are reminded to not focus on the exact forecast track or the specific timing and location of landfall. Hurricane-force winds, dangerous storm surge, and flooding rainfall will affect a large portion of the north-central Gulf Coast during the next few days. KEY MESSAGES: 1. It is still 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 36 hours is around 60 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 Okaloosa/Walton County Line in the Florida Panhandle, 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 early Tuesday within the Hurricane Warning area in southeastern Louisiana and are expected by late Tuesday and Tuesday night within the Hurricane Warning area along the Mississippi and Alabama coastlines and the western Florida Panhandle. Tropical storm conditions are already occurring in some of these areas. 4. Life-threatening flash flooding is likely with Sally, as well as widespread minor to isolated major flooding on area rivers, along and just inland of the Central Gulf Coast. Significant flash and urban flooding, as well as widespread minor to moderate river flooding is likely across inland portions of Mississippi and Alabama and into northern Georgia, southeastern Tennessee and the western Carolinas through the week. Sally may continue to produce flash flooding across the Florida peninsula and prolong existing minor river flooding across west-central Florida through tonight. FORECAST POSITIONS AND MAX WINDS INIT 15/0300Z 28.9N 87.6W 85 KT 100 MPH 12H 15/1200Z 29.1N 88.1W 95 KT 110 MPH 24H 16/0000Z 29.6N 88.4W 90 KT 105 MPH 36H 16/1200Z 30.4N 88.3W 85 KT 100 MPH...INLAND 48H 17/0000Z 31.3N 87.4W 50 KT 60 MPH...INLAND 60H 17/1200Z 32.0N 86.3W 30 KT 35 MPH...INLAND 72H 18/0000Z 32.7N 84.9W 25 KT 30 MPH...INLAND 96H 19/0000Z 33.1N 82.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 20/0000Z...DISSIPATED $$ Forecaster Cangialosi
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