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Hurricane Florence Forecast Discussion Number 55
2018-09-13 04:49:14| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Wed Sep 12 2018 874 WTNT41 KNHC 130249 TCDAT1 Hurricane Florence Discussion Number 55 NWS National Hurricane Center Miami FL AL062018 1100 PM EDT Wed Sep 12 2018 Satellite data and reports from Hurricane Hunter aircraft indicate significant changes in the structure of Florence and the environment near the storm since the last advisory. Microwave satellite imagery shows that the convection on the southern side of the storm has been disrupted, and reports from an Air Force Reserve Hurricane Hunter aircraft indicate the eyewall now wraps less than 50 percent of the way around the center. The aircraft also reports that the hurricane again has concentric wind maxima, the inner at a radius 20-30 n mi and the outer at 50-60 n mi. The convection seems to have been affected by 20-25 kt of southerly vertical wind shear, most of which appears to be due to strong winds between 200-250 mb seen in dropsonde data from the NOAA G-IV jet. The central pressure has risen to 957 mb, and the maximum 700-mb flight-level winds reported so far are 103 kt. Based on the latter data, the initial intensity reduced to a probably generous 95 kt. The initial motion is 315/15. During the next 12-36 hours, the hurricane is expected to turn toward the west-northwest and west with a decrease in forward speed as it moves into an area of weakening steering currents near and over the southeastern United States. The new forecast track now brings the center onshore in southern North Carolina near the 36 h point. After landfall, the cyclone should move slowly westward to west-southwestward through the 72 h point, then it should turn northwestward to northward by the end of the forecast period as it moves through the Appalachian Mountains. The new forecast track lies between the HCCA corrected consensus model and the other consensus aids, and it is nudged just a little to the north of the previous track. The dynamical models forecast the current shear to subside after 6-12 h as Florence moves farther from an upper-level low currently near northeastern Florida. This, combined with sea surface temperatures near 29C, would allow a last chance for strengthening before landfall. However, the storm structure, particularly the large outer wind maxima, would likely be slow to respond to the more favorable environment. The pre-landfall part of the intensity forecast thus calls for little change in strength, but given the uncertainties the confidence in this is low. After landfall, Florence should gradually weaken during the 36-48 h period while the center is near the coast, then weaken more quickly when the center moves farther inland. While Florence has weakened below major hurricane intensity, the wind field of the hurricane continues to grow in size. This evolution will produce storm surges similar to that of a more intense, but smaller, hurricane, and thus the storm surge values seen in the previous advisory are still valid. The threat of rainfall has also not diminished, and these impacts will cover a large area regardless of exactly where the center of Florence moves. Key Messages: 1. A life-threatening storm surge is now highly likely along portions of the coastlines of South Carolina and North Carolina, and a Storm Surge Warning is in effect for a portion of this area. All interests in these areas should complete preparations and follow any advice given by local officials. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians 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 Warning is in effect. Strong winds could also spread inland into portions of the Carolinas. 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 13/0300Z 32.0N 73.7W 95 KT 110 MPH 12H 13/1200Z 33.1N 75.3W 95 KT 110 MPH 24H 14/0000Z 33.9N 76.7W 95 KT 110 MPH 36H 14/1200Z 34.2N 78.0W 80 KT 90 MPH...INLAND 48H 15/0000Z 34.1N 78.7W 60 KT 70 MPH...INLAND 72H 16/0000Z 33.5N 80.5W 40 KT 45 MPH...INLAND 96H 17/0000Z 34.0N 83.0W 25 KT 30 MPH...INLAND 120H 18/0000Z 37.5N 83.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven
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