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Hurricane Sally Forecast Discussion Number 12
2020-09-14 16:51:58| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Mon Sep 14 2020 000 WTNT44 KNHC 141451 TCDAT4 Tropical Storm Sally Discussion Number 12 NWS National Hurricane Center Miami FL AL192020 1000 AM CDT Mon Sep 14 2020 An intense burst of deep convection with cloud tops colder than -80 degrees Celsius has developed over and the to east of the center this morning. A recent fix from an Air Force Reserve Hurricane Hunter aircraft indicates that the center has reformed to the east of the previous estimated location, beneath the burst of deep convection. NWS WSR-88D radar imagery shows an increase in banding around the eastern and southeastern portion of new center found by the aircraft and it appears that an eye is in its formative stage. The aircraft has reported believable SFMR winds of 55 kt, and that is the basis for the initial intensity. The most recent minimum pressure estimated from the aircraft data is 991 mb, down several millibars from the first fix on this flight. Sally is located within a conducive environment of low wind shear, warm waters, and a moist atmosphere. These conditions are likely to lead to steady strengthening over the next 24 hours or so. With the recent increase in organization of the inner core, there is more confidence that Sally will strengthen to a hurricane later today or tonight. Additional strengthening is possible on Tuesday while the storm moves slowly northwestward near the coast of southeast Louisiana. Increasing westerly wind shear and land interaction will probably slow the intensification rate by late tomorrow. The new NHC intensity forecast is similar to the previous advisory and is near the upper-end of the guidance envelope. Since Sally is forecast to be moving very slowly around the time of landfall a slower rate of weakening is indicated since a large portion of the circulation will remain over water for some time. Given the recent re-formation of the center, the initial motion is a somewhat uncertain west-northwestward at 5 kt. Weak ridging over the southeastern United State should steer Sally slowly west- northwestward through tonight. After that time, a northwestward to northward turn is anticipated but the exact timing and location of the turn remains uncertain. The general trend in the guidance has been eastward for the past few cycles, and the NHC forecast has been nudged in that direction and lies between the GFS and ECMWF models but a little west of the various consensus aids. Regardless of the exact forecast track and intensity of Sally, the slow-moving storm is expected to cause a life-threatening storm surge and freshwater flooding event. 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 tonight within the Hurricane Warning area in southeastern Louisiana and are expected by late Tuesday within the Hurricane Warning area along the Mississippi and Alabama coastline. Tropical storm conditions are likely to begin later today and this evening in these areas and preparations should be rushed to completion. 4. Life-threatening flash flooding is likely, 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 Mississippi and Alabama through the middle of the week. Flooding impacts are expected to spread farther across the Southeast through the week. Sally could continue to produce flash flooding across the Florida peninsula and prolong existing minor river flooding across west-central Florida through today. FORECAST POSITIONS AND MAX WINDS INIT 14/1500Z 28.4N 86.9W 55 KT 65 MPH 12H 15/0000Z 28.7N 88.0W 65 KT 75 MPH 24H 15/1200Z 29.2N 88.8W 75 KT 85 MPH 36H 16/0000Z 29.8N 89.1W 80 KT 90 MPH 48H 16/1200Z 30.8N 88.7W 60 KT 70 MPH...INLAND 60H 17/0000Z 31.8N 87.7W 30 KT 35 MPH...INLAND 72H 17/1200Z 32.6N 86.5W 25 KT 30 MPH...INLAND 96H 18/1200Z 33.1N 84.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 19/1200Z...DISSIPATED $$ Forecaster Brown
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