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Hurricane Florence Forecast Discussion Number 42
2018-09-09 22:53:18| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 09 2018 270 WTNT41 KNHC 092053 TCDAT1 Hurricane Florence Discussion Number 42 NWS National Hurricane Center Miami FL AL062018 500 PM AST Sun Sep 09 2018 The last reconnaissance fix indicated that the pressure had fallen to about 978 mb with Florence, a drop of about 6 mb in 4 hours, with uncontaminated SFMR winds of about 70 kt. Since then, the satellite presentation has continued to improve, with intensifying deep convection near the center. Thus the wind speed is set to 75 kt on this advisory. Radar data from the NOAA Hurricane Hunter indicated that there were still a few breaks in the eyewall, possibly allowing some dry air to mix into the core. This structure has potentially kept Florence from intensifying rapidly so far. It is likely, however, that the eyewall will close soon with low environmental shear present, resulting in a faster intensification rate. The guidance is remarkably well clustered, with all of the hurricane models showing category 4 strength within 48 hours, which is uncommon given the current intensity. The new forecast continues to show rapid intensification and is increased 5 or 10 kt at most time periods, but it is still not as high as the Florida State Superensemble or the NOAA corrected consensus models. It is worth noting that the model guidance is also significantly increasing the size of Florence's wind field over the next few days, and the official forecast reflects this trend. The fixes from the aircraft indicate that Florence hasn't gained much latitude yet, and continues moving westward at about 6 kt. There is no change to the synoptic reason as a very strong ridge is forecast to build over the northwestern Atlantic during the next few days. This pattern should steer Florence west-northwestward at a much faster forward speed by Tuesday. On Wednesday, the hurricane will likely turn northwestward and slow down somewhat due to another ridge forming over the Ohio Valley. The most notable change from the previous advisory is that most of the models are showing a faster motion in 3 or 4 days, which unfortunately increases the risk of a destructive hurricane landfall. The GFS and its ensemble mean are outliers from the main model envelope, showing the system near the Outer Banks, while the rest of the guidance is well to the southwest. While a near-miss isn't impossible given the spread of the ECMWF Ensemble, there is much more support for a track to the southwest, especially considering the poleward bias of the GFS thus far this season. Therefore, the official track forecast is shifted a little bit to the southwest, and is very close to the corrected-consensus models. The NOAA G-IV jet is conducting a synoptic surveillance mission this afternoon to gather data near and around Florence for assimilation into the numerical models, and these missions will continue through Tuesday. In addition, upper-air stations across portions of the central and eastern U.S. are conducting special 0600 UTC and 1800 UTC radiosonde launches to collect extra data for the numerical models. Hopefully these data will help improve the track and intensity forecasts. Key Messages: 1. There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland. While it is too soon to determine the exact timing, location, and magnitude of these impacts, interests at the coast and inland from South Carolina into the mid-Atlantic region should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. 2. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week. These swells will result in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 24.4N 57.0W 75 KT 85 MPH 12H 10/0600Z 24.6N 58.3W 90 KT 105 MPH 24H 10/1800Z 25.1N 60.4W 105 KT 120 MPH 36H 11/0600Z 25.9N 63.1W 115 KT 130 MPH 48H 11/1800Z 27.0N 66.2W 125 KT 145 MPH 72H 12/1800Z 30.3N 72.6W 130 KT 150 MPH 96H 13/1800Z 33.5N 77.5W 120 KT 140 MPH 120H 14/1800Z 35.0N 79.0W 60 KT 70 MPH...INLAND $$ Forecaster Blake
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