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Hurricane Florence Forecast Discussion Number 29
2018-09-06 16:39:47| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 06 2018 231 WTNT41 KNHC 061439 TCDAT1 Hurricane Florence Discussion Number 29 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Thu Sep 06 2018 Vertical shear has increased since yesterday, which has caused a degradation of Florence's structure and a decrease in its maximum winds. The cloud-filled eye has been eroded over the past hour or so, and the deep convection is no longer symmetric, with the low-level circulation peeking out from under the higher clouds. Dvorak CI numbers from TAFB and SAB have fallen to T5.0-5.5 (90-100 kt), while the objective numbers from UW-CIMSS support 70-90 kt. The initial intensity is set at 90 kt, near the middle of this wide range of estimates. The intensity forecast has been somewhat of a self-defeating prophecy due to the nuances of the environmental shear. Even though Florence was able to rapidly intensify yesterday in an area just south of a zone of strong shear, the hurricane's stronger-than- expected intensity caused it to move more poleward, into that stronger shear. Right now, shear analyses range anywhere from 25-30 kt, and the latest available guidance suggests that this level of shear should continue for another 12-24 hours. As a result, continued weakening is forecast over the next day or so. After 36 hours, Florence is likely to encounter an upper-level environment that is more conducive for reintensification. The NHC forecast is adjusted downward toward the newest consensus aids, especially during the first 48 hours, but it still shows Florence reaching major hurricane strength again by days 4 and 5. Florence's forward motion has slowed just a little to 9 kt toward the northwest (315 degrees). A mid-level ridge is building to the north, which is likely to cause the hurricane to turn westward by 36 hours, with that motion continuing through about day 3. After that time, there is still considerable uncertainty in the evolution of the steering pattern over the western Atlantic, especially on day 4. On one hand, the GFS and HWRF dig a strong shortwave trough over Atlantic Canada by Monday, creating a break in the ridge which would allow Florence to turn northwestward. On the other hand, the ECMWF and UKMET both have weaker troughs and maintain stronger ridging over the northwestern Atlantic, allowing Florence to maintain a westward or west-northwestward course. All the models show a mid-level high over the western Atlantic by Tuesday. Due to typical biases among these models in the part of the Atlantic, we prefer to be between the GFS and ECMWF solutions at this time, which places the official NHC track forecast close to the TVCN multi-model consensus and just north of HCCA. There is still considerable model ensemble spread for Florence's track beyond day 5. Given the large uncertainty at these time ranges, it is far too soon to speculate what, if any, impacts Florence may have on the U.S. East Coast next week. Regardless of Florence's eventual track, large swells emanating from the hurricane will reach Bermuda beginning on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 06/1500Z 24.6N 48.6W 90 KT 105 MPH 12H 07/0000Z 25.2N 49.8W 80 KT 90 MPH 24H 07/1200Z 25.6N 51.3W 75 KT 85 MPH 36H 08/0000Z 25.6N 52.6W 75 KT 85 MPH 48H 08/1200Z 25.7N 54.0W 80 KT 90 MPH 72H 09/1200Z 26.3N 56.4W 90 KT 105 MPH 96H 10/1200Z 28.0N 59.5W 100 KT 115 MPH 120H 11/1200Z 29.5N 65.0W 105 KT 120 MPH $$ Forecaster Berg
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