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Tropical Storm Florence Forecast Discussion Number 65
2018-09-15 16:47:36| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Sat Sep 15 2018 714 WTNT41 KNHC 151447 TCDAT1 Tropical Storm Florence Discussion Number 65 NWS National Hurricane Center Miami FL AL062018 1100 AM EDT Sat Sep 15 2018 Florence continues to creep slowly westward and weaken across eastern South Carolina. NOAA WSR-88D Doppler weather radar data indicate strong winds near 60 kt still exist between 3000-10000 ft within intense rainbands situated between the Cape Fear/Wilmington area and Bogue Inlet, North Carolina. Although those velocity values would typically correlate to 50-kt surface winds, those winds appear to be associated with small mesoscale circulations and possible supercell thunderstorms, and not the larger tangential wind field. In contrast, surface observations during the past couple of hours have only shown sustained winds of around 40 kt, so that is the intensity used for this advisory. The estimated central pressure of 995 mb is based on nearby surface observations across eastern South Carolina. Radar data and surface observations indicate that Florence has turned more westward, and has slowed down even more, and the initial motion estimate is now 270/02 kt. The models remain in very good agreement that a mid-level ridge currently centered over the upper Midwest will slide eastward across the lower Great Lakes to near the northeastern U.S. during the next 48 hours. This feature is expected to steer Florence and its remnants in a general westward motion for the next 24 hours or so, followed by a turn toward the northwest on Monday. The ridge will continue to shift eastward and weaken, allowing Florence's circulation to get caught up in the faster mid-latitude westerlies and accelerate toward the northeast on day 3 and toward the east-northeast on days 4 and 5 as an extratropical low pressure system. The new NHC track is similar to the previous advisory track, and lies near the consensus model TCVA/TVCN. Florence's inner-core convection and wind field will continue to weaken throughout the next 72 hours or so. However, the outer wind field and an associated band of deep convection in the eastern semicircle will likely produce sustained tropical-storm-force winds for another 12 hours or so, with some high gusts continuing until the band moves inland by late Sunday as per the latest the latest NOAA HRRR and other mesoscale model runs. More importantly, continued heavy rains will be produced by this band of convection, which will exacerbate the already catastrophic flooding that is occurring across much of southeastern North Carolina. The official intensity forecast closely follows the Decay-SHIPS model guidance and the intensity model IVCN through 72 hours, and then follows a blend of the IVCN, HCCA, and FSSE consensus models on days 4 and 5 when the post-tropical cyclone is forecast to strengthen due to baroclinic processes after moving over the relatively warm waters of the northwestern Atlantic Ocean. Although coastal storm surge flooding will gradually subside today, especially after the midday high-tide period ends, extremely heavy rainfall will continue to be a serious hazard associated with slow-moving Florence. More than two feet of rain has already fallen across portions of southeastern North Carolina, and more rain is still to come, which will cause disastrous flooding that will spread inland through the weekend. Key Messages: 1. Life-threatening storm surge will continue along portions of the North Carolina coast through today, and also along the Neuse and Pamlico Rivers in western Pamlico Sound, where rainfall and freshwater flooding will also contribute to high water levels. Dangerous storm surge could also affect portions of the northeast coast of South Carolina coast today. 2. Life-threatening, catastrophic flash floods and prolonged significant river flooding are likely over portions of the Carolinas and the southern to central Appalachians from western North Carolina into west-central Virginia and far eastern West Virginia through early next week, as Florence moves slowly inland. In addition to the flash flood and flooding threat, landslides are also possible in the higher terrain of the southern and central Appalachians across western North Carolina into southwest Virginia. 3. Tropical storm conditions will continue along the coast within the tropical storm warning area and also well inland across portions of South Carolina and North Carolina today. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 15/1500Z 33.6N 79.6W 40 KT 45 MPH...INLAND 12H 16/0000Z 33.6N 80.5W 35 KT 40 MPH...INLAND 24H 16/1200Z 34.4N 81.7W 25 KT 30 MPH...INLAND 36H 17/0000Z 36.0N 82.8W 25 KT 30 MPH...INLAND 48H 17/1200Z 38.0N 82.9W 20 KT 25 MPH...INLAND 72H 18/1200Z 41.3N 76.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 19/1200Z 44.3N 63.4W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 20/1200Z 48.5N 49.0W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart
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