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Hurricane Florence Forecast Discussion Number 50
2018-09-11 22:57:03| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Sep 11 2018 545 WTNT41 KNHC 112056 TCDAT1 Hurricane Florence Discussion Number 50 NWS National Hurricane Center Miami FL AL062018 500 PM AST Tue Sep 11 2018 Microwave satellite data indicate that Florence completed a full eyewall replacement cycle (ERC) a few hours ago, and recent visible and infrared imagery suggests that the eye has contracted slightly. Outflow continues to expand in all quadrants, and the outflow jets to the northwest and east have become better defined. Satellite intensity estimates from TAFB, SAB, and UW-CIMSS ADT are all T6.0/115 kt, and recent NHC objective intensity estimates are T6.2/120 kt. Given that the eye has mostly cleared out and has also warmed to near 19 deg C, the initial intensity has been bumped upward to 120 kt, which could be conservative. All of the wind radii had to be expanded/increased based on a blend of the earlier reconnaissance data and a 1430 UTC ASCAT scatterometer pass. The initial motion estimate is now 300/15 kt. There remains no significant to the previous track forecast or reasoning. Overall, the global and regional models have done a good job capturing the evolving synoptic- scale flow pattern across CONUS, with an amplifying trough moving onshore the the northwestern U.S. coast, which is inducing downstream ridging across the northeastern U.S. and the northwestern Atlantic Ocean. Water vapor imagery indicates that the blocking high pressure northwest of Bermuda is continuing to build and shift slowly eastward. The 12Z GFS model made a significant shift to the west, the UKMET made a shift to the east, and the ECMWF track has remained basically unchanged through 72 hours. As a result the consensus models have made only minor track shifts to the west. What is noticeable is that all of the global and regional models are indicating that the steering currents will collapse by 72 h when Florence is approaching the southeast U.S. coast. The weak steering currents are expected to continue through the weekend, which makes the forecast track on days 3-5 quite uncertain. The latest NHC forecast track is very similar to the previous two advisory tracks, and lies the middle of the guidance envelope between the consensus models TVCA to the north and the HCCA and FSSE models to the south. During the next 24 hours or so, Florence is expected to remain in a very favorable environment consisting of low shear near 5 kt, an expanding upper-level outflow pattern, and above-average SSTs of 29.0-29.5 deg C, which should result in additional strengthening. By 48 h, the decreasing forward speed along with the large inner-core wind field should induce some upwelling and gradual weakening. Although the GFS- and ECMWF-based SHIPS intensity models are indicating an increase in the southwesterly shear to near 20 kt, this could be due to the SHIPS model capturing Florence's own strong outflow from the GFS and ECMWF model fields. Despite the weakening shown at 72 hours, Florence is still expected to remain a dangerous hurricane through landfall. After Florence moves inland, the slow forward speed of less than 5 kt should cause a rapid spin down and weakening of the hurricane's circulation. The new NHC intensity forecast is a little above the highest 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 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 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 likely 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 Warning has been issued 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 11/2100Z 27.5N 67.1W 120 KT 140 MPH 12H 12/0600Z 28.7N 69.3W 130 KT 150 MPH 24H 12/1800Z 30.4N 72.1W 135 KT 155 MPH 36H 13/0600Z 32.1N 74.5W 130 KT 150 MPH 48H 13/1800Z 33.4N 76.2W 120 KT 140 MPH 72H 14/1800Z 34.5N 77.7W 100 KT 115 MPH...NEAR THE COAST 96H 15/1800Z 35.0N 78.8W 40 KT 45 MPH...INLAND 120H 16/1800Z 35.7N 81.7W 25 KT 30 MPH...INLAND $$ Forecaster Stewart
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