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Hurricane Florence Forecast Discussion Number 49
2018-09-11 16:59:00| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Tue Sep 11 2018 257 WTNT41 KNHC 111458 TCDAT1 Hurricane Florence Discussion Number 49 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Tue Sep 11 2018 Data from satellites and an Air Force Reserve reconnaissance aircraft mission indicate that Florence has completed an overnight eyewall replacement cycle (ERC). The recon data indicate that the eye has now expanded to a diameter of 30-32 n mi, and this was confirmed by an 1103Z SSMI/S microwave satellite image. The aircraft provided various intensity estimates with a peak SFMR surface wind of 113 kt noted in the northwest quadrant, a peak 700-mb flight-level wind of 143 kt in the northeast quadrant, and a central pressure of 950 mb. The 143-kt flight-level wind would normally correlate to an equivalent surface wind of about 129 kt. However, coincident SFMR surface winds were only 108 kt, indicating that the weak convection that region of the hurricane was not vigorous enough to bring down the strongest winds to the surface. The 950-mb central pressures corespondents to about 113 kt. Based on a blend of all these data, the initial intensity has been set to 115 kt. The initial motion estimate is now 295/14 kt based on the recent recon fix data. The broken record continues -- there is no significant to the previous track forecast or reasoning. Although the global and regional models continue to make minor shifts northward and southward, the consensus models have changed little. GOES-16 high-resolution water vapor imagery indicates that the amplifying large-scale flow pattern across CONUS is inducing a downstream ridge over the western Atlantic, with a high pressure cell centered northwest of Bermuda. This blocking ridge pattern is expected to keep Florence moving west-northwestward to northwest at around 15 kt for the next 48 hours or so. However, embedded within the large-scale flow is a weak shortwave trough over the central and southern Plains that is expected to eject out northeastward and weaken the ridging across the mid-Atlantic and southeastern U.S., causing Florence to slow down significantly in 72 hours as the powerful hurricane approaches the Carolinas. On days 4 and 5, an even slower motion or drift to the west and northwest is forecast, which will exacerbate the heavy rainfall threat. The new NHC forecast track is just an update of the previous one, and basically lies the middle of the guidance envelope between the consensus models TVCA to the north and HCCA and FSSE to the south. Water vapor imagery indicates that Florence has finally developed the much anticipated dual outflow pattern, with outflow jets noted in the northwestern and eastern quadrants. The latter outflow jet is flowing into an upper-level low, which is acting as an impressive mass sink near 25N/49W. These two outflow channels are producing significant deformation stretching across Florence's inner-core, which should aid in the re-strengthening process. Now that the eye has become stable with a diameter of about 30 n mi and since Florence is expected to remain in a low-shear environment of around 5 kt and over above-average SSTs of 29.0-29.5 deg C, slow but steady strengthening is expected for the next 24-36 hours. By 48 h and beyond, Florence's slow forward speed, coupled with the large eye and relatively shallow depth of the warm water should induce some upwelling beneath the cyclone that will initiate a slow weakening trend. By 72 hours the vertical wind shear is expected to increase to near 20 kt from the southwest, which will cause more significant weakening to occur. However, given the large overall wind field of Florence along with the large eye, only gradual weakening is expected. Once Florence moves inland, the slow forward speed of 3-5 kt will result in rapid spin down and weakening of the wind field. The new official intensity forecast is above of all of the intensity guidance based on the aforementioned very favorable synoptic outflow pattern, and to maintain continuity with the previous forecast. Key Messages: 1. A life-threatening storm surge is likely along portions of the coastlines of South Carolina, North Carolina, and Virginia, and a Storm Surge Watch is in effect for a portion of this area. All interests from South Carolina into the mid-Atlantic region should ensure they have their hurricane plan in place and follow any advice given by local officials. 2. Life-threatening, catastrophic flash flooding and significant river flooding is possible over portions of the Carolinas and Mid-Atlantic states from late this week into early next week, as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina, and a Hurricane Watch is in effect for a part of this area. Damaging winds could also spread well inland into portions of the Carolinas and Virginia. 4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 26.7N 65.3W 115 KT 130 MPH 12H 12/0000Z 27.7N 67.6W 120 KT 140 MPH 24H 12/1200Z 29.4N 70.6W 130 KT 150 MPH 36H 13/0000Z 31.1N 73.1W 130 KT 150 MPH 48H 13/1200Z 32.6N 75.2W 120 KT 140 MPH 72H 14/1200Z 34.2N 77.1W 105 KT 120 MPH 96H 15/1200Z 35.0N 78.1W 40 KT 45 MPH...INLAND 120H 16/1200Z 35.5N 79.5W 25 KT 30 MPH...INLAND $$ Forecaster Stewart
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