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Tropical Storm Sally Forecast Discussion Number 6
2020-09-13 04:38:29| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Sat Sep 12 2020 000 WTNT44 KNHC 130238 TCDAT4 Tropical Storm Sally Discussion Number 6 NWS National Hurricane Center Miami FL AL192020 1100 PM EDT Sat Sep 12 2020 Sally's structure remains somewhat disorganized this evening. While there has been a recent increase in deep convection near the low- level center, there is a large CDO feature with -70 to -80C tops displaced 50 to 60 miles to the south and southeast. This structure could be due to the 10 to 15 kt of northwesterly shear analyzed by the SHIPS model and UW-CIMSS. The initial intensity remains 35 kt based on the latest Dvorak estimates from TAFB and SAB, and the central pressure of 1003 mb is based on surface observations in the Naples area as the center passed offshore before 00Z. An Air Force Reserve Hurricane Hunter aircraft will be investigating Sally overnight to provide more information on the storm's intensity and structure. Given the current structure, only gradual strengthening is expected in the short term. However, once the cyclone develops more of an inner core, it should be able to take advantage of warm SSTs and low shear over the Gulf of Mexico during the next couple of days. Steady intensification to hurricane strength is forecast through 48 hours, with a leveling off of the intensity in 60-72 hours as the shear is expected to increase. Overall, the intensity guidance is a little less bullish this cycle, with the HWRF more aggressive while the remainder of the guidance shows a slower rate of intensification. The NHC intensity forecast through landfall remains unchanged from the previous advisory, but now lies a little above HCCA and the intensity consensus. The initial motion estimate is a somewhat uncertain 285/07. Sally is currently being steered by a mid-level ridge centered over the Carolinas that will shift eastward and weaken as a longwave trough moves into the eastern United States over the next 24 to 36 hours. During this time, Sally should move generally west-northwestward or northwestward with a decrease in forward speed on Monday. By 48 hours, the trough lifts out and a narrow ridge builds in north and east of Sally, which should result in the tropical cyclone turning north-northwestward and then northward at less than 5 kt as it approaches the north-central Gulf Coast and moves inland over the Mississippi Valley. After that time, the next trough should induce a faster northeastward motion by day 5. There is a fair bit of spread in the track guidance after 24 hours, with the COAMPS-TC well to the right and the GEFS ensemble mean and UKMET to the left. However, the GFS and ECMWF are in good agreement through 60 hours near the middle of the guidance envelope. The NHC track is similar to the previous one and lies in the middle of the guidance and is near the TVCA multi-model consensus and HCCA. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 72 h is around 110 miles and the average intensity error is around 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. Life-threatening storm surge is possible along the Gulf Coast beginning on Monday, and a Storm Surge Watch is in effect for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from the Mouth of the Mississippi River to the Alabama/Florida border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are possible by early Tuesday from Grand Isle, Louisiana to the Alabama/Florida border, including Metropolitan New Orleans, with tropical storm conditions possible by Monday. 3. Sally is expected to produce flash flooding across portions of southern Florida and prolong existing minor river flooding across central Florida through Sunday. Flash and urban flooding and minor to moderate river flooding are likely across portions of the central Gulf Coast from Sunday through the middle of next week. 4. Tropical storm conditions are possible early next week in portions of the Florida Panhandle, where a Tropical Storm Watch is in effect. Wind gusts to tropical storm force could occur over portions of the lower Florida Keys overnight. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 26.3N 82.9W 35 KT 40 MPH 12H 13/1200Z 27.0N 84.4W 40 KT 45 MPH 24H 14/0000Z 27.7N 86.0W 50 KT 60 MPH 36H 14/1200Z 28.3N 87.5W 60 KT 70 MPH 48H 15/0000Z 28.8N 88.6W 70 KT 80 MPH 60H 15/1200Z 29.4N 89.2W 75 KT 85 MPH 72H 16/0000Z 30.1N 89.5W 75 KT 85 MPH 96H 17/0000Z 31.7N 89.1W 50 KT 60 MPH...INLAND 120H 18/0000Z 33.5N 86.5W 25 KT 30 MPH...INLAND $$ Forecaster Brennan
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