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Hurricane Florence Forecast Discussion Number 43
2018-09-10 04:48:51| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 09 2018 663 WTNT41 KNHC 100248 TCDAT1 Hurricane Florence Discussion Number 43 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Sun Sep 09 2018 Florence's satellite appearance has continued to improve quite markedly since the previous advisory. An eye was evident in GOES-16 high-resolution infrared imagery and other channels between 2300-0000 UTC, but it became cloud covered immediately thereafter due to a strong burst of deep convection in the southern and eastern eyewall where cloud tops colder than -80C and an abundance of lightning activity was observed. Since that time, the CDO has expanded and become more circular, outflow has increased and become more symmetrical, and an eye has begun to re-appear. The initial intensity has been increased to 80 kt for this advisory based on a subjective Dvorak intensity estimate of T4.5/77 kt from SAB and NHC objective intensity estimates ranging from 77 kt to 87 kt. The initial motion estimate is 280/06 kt. Once again, there is no significant change to the previous forecast track or synoptic reasoning. The models appear to be getting better dialed in on both the location and strength of the developing blocking ridge in the vicinity of Bermuda during the next 4 days as the mid-latitude flow amplifies across the CONUS and the northwestern Atlantic. In fact, the global models are now in very good agreement on forecasting Florence's upper-level mass outflow being deposited to the north and east of the hurricane, which will act to further strengthen the blocking ridge and help to drive the hurricane northwestward toward the southeastern U.S. coastal region. The new NHC model guidance is even more tightly packed about the previous forecast track, with less than a 90 n mi cross-track spread at 72 h and less than 120 n mi spread at 96 h, just prior to expected landfall. Therefore, the new official advisory track is essentially just an update and extension of the previous forecast, and lies between the nearly juxtaposed HCCA and FSSE corrected consensus track models. Now that Florence has developed an inner-core ring of deep convection, which has insulated the eye from intrusions of dry air, rapid intensification appears likely to begin soon and continue for the next 36 hours or so due to the expected very low vertical wind shear conditions, dual outflow jet pattern that will be developing, and very warm SSTs of 29-29.5 deg C beneath the hurricane. The most favorable combination of the aforementioned factors will occur in about 48 h, and that's when Florence is likely to achieve its maximum intensity. After 72 hours, the wind shear is expected to increase to around 10-15 kt from the south or southwest, and the dual outflow pattern is forecast to change to only a single poleward outflow pattern. This slight degradation in the upper-level environment, along with slightly cooler SSTs, is expected to result in a gradual weakening of the powerful cyclone. However, Florence is still forecast to be near category-4 strength when the dangerous hurricane makes landfall. The official intensity forecast is an average of the intensity forecast from the corrected consensus models HCCA and FSSE, with the latter explicitly forecasting a peak intensity of 134 kt in 72 hours. It is also 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 NOAA G-IV jet will conduct another synoptic surveillance mission early Monday morning in support of the 1200 UTC model cycle, and these flights 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 10/0300Z 24.6N 57.7W 80 KT 90 MPH 12H 10/1200Z 24.9N 59.1W 90 KT 105 MPH 24H 11/0000Z 25.6N 61.5W 105 KT 120 MPH 36H 11/1200Z 26.5N 64.4W 120 KT 140 MPH 48H 12/0000Z 27.8N 67.6W 130 KT 150 MPH 72H 13/0000Z 31.2N 73.6W 130 KT 150 MPH 96H 14/0000Z 34.0N 77.5W 120 KT 140 MPH 120H 15/0000Z 35.2N 79.0W 55 KT 65 MPH...INLAND $$ Forecaster Stewart
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