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Hurricane Florence Forecast Discussion Number 46
2018-09-10 22:55:56| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Sep 10 2018 366 WTNT41 KNHC 102055 TCDAT1 Hurricane Florence Discussion Number 46 NWS National Hurricane Center Miami FL AL062018 500 PM AST Mon Sep 10 2018 Unfortunately, the models were right. Florence has rapidly intensified into an extremely dangerous hurricane, with 30-second GOES-16 visible imagery showing well-defined eyewall mesovortices rotating inside of the eye. A NOAA Hurricane Hunter aircraft found peak SFMR winds of about 120 kt, with flight-level winds and dropsonde measurements also supporting that value for the initial wind speed estimate. Notably, the aircraft data also show the size of the hurricane-force winds has doubled in the past 12 hours. None of the guidance suggest that Florence has peaked in intensity, and this is supported by a continuation of a low-shear environment, and even warmer waters over the next 36 hours. Thus, the intensity forecast is raised from the previous one, bringing Florence close to category 5 strength tomorrow. Near landfall, the vertical wind shear could increase, along with the increasing likelihood of eyewall cycles. While the intensity forecast shows some weakening of the maximum winds near landfall, the wind field is expected to grow with time, which increases the storm surge and inland wind threats. The bottom line is that there is high confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity. Florence has recently turned west-northwestward, still moving at 11 kt. The hurricane is expected to accelerate in that direction over the next day or two due to building mid-level ridge over the northwestern Atlantic Ocean. By late Wednesday, a turn toward the northwest is forecast due to the orientation of the Atlantic ridge, along with a decrease in forward speed due to a new ridge building over the Great Lakes. There is a new player in the forecast as well, with the disturbance over the northwestern Caribbean adding some uncertainty in the ridge strength over the southeastern United States. Perhaps it isn't surprising that the model spread has increased on this cycle, with a small eastward shift overall. The official forecast is nudged in the direction of the trend, but is west of the model consensus. It is important not to focus on the exact forecast track as average NHC errors at days 4 and 5 are about 140 and 180 n mi, respectively, and dangerous hazards will extend well away from the center. The NOAA G-IV jet will continue to conduct synoptic surveillance missions every 12 h through at least Wednesday. In addition, special 0600 UTC and 1800 UTC radiosonde launches have been expanded to additional upper-air stations across the U.S. are to collect extra data for the numerical models. 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 will likely be issued for some of these areas by Tuesday morning. 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 freshwater flooding is likely from a prolonged and exceptionally heavy rainfall event, which may extend inland over the Carolinas and Mid Atlantic for hundreds of miles 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 will likely be issued by Tuesday morning. 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 10/2100Z 25.4N 61.1W 120 KT 140 MPH 12H 11/0600Z 26.0N 63.2W 130 KT 150 MPH 24H 11/1800Z 27.0N 66.2W 135 KT 155 MPH 36H 12/0600Z 28.6N 69.3W 135 KT 155 MPH 48H 12/1800Z 30.4N 72.2W 130 KT 150 MPH 72H 13/1800Z 33.7N 77.0W 120 KT 140 MPH 96H 14/1800Z 35.6N 78.8W 50 KT 60 MPH...INLAND 120H 15/1800Z 36.5N 79.5W 25 KT 30 MPH...INLAND $$ Forecaster Blake
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