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Hurricane Florence Forecast Discussion Number 25
2018-09-05 16:40:08| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Wed Sep 05 2018 150 WTNT41 KNHC 051440 TCDAT1 Hurricane Florence Discussion Number 25 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Wed Sep 05 2018 Remarkably, Florence has continued to strengthen. The hurricane has a compact central dense overcast with cold cloud tops completely encircling a clear, well-defined eye. With the improved structure, subjective Dvorak estimates have increased to T5.5/102 kt from TAFB and T6.0/115 kt from SAB, while the latest objective UW-CIMSS ADT estimate is T5.8/110 kt. The initial intensity is therefore set at 110 kt as a blend of these numbers, with Florence having become a major hurricane earlier this morning. Given the estimated maximum winds, Florence has been rapidly intensifying since yesterday, an event that was not foreseen by any intensity models, nor forecasters. Diagnostics from the SHIPS model and UW-CIMSS shear analyses have been consistently showing southwesterly shear of 20-25 kt in the vicinity of Florence, but it is possible that those schemes are averaging over a larger area than might be reasonable given the hurricane's small size. Florence has apparently been able to find a small pocket of relatively low shear, and with waters becoming progressively warmer, the hurricane has strengthened significantly more than anticipated. This makes the intensity forecast incredibly uncertain. SHIPS actually shows the shear increasing over the next 24 hours, but global model fields suggest that the hurricane may still be able to continue within the protected pocket of lower shear for the next several days. As a result, the NHC official forecast keeps Florence's intensity well above the available guidance, which all show the hurricane weakening over the next day or two. The new NHC prediction follows this trend and also shows some weakening, but this is a low-confidence forecast. Either way, Florence is expected to remain a hurricane throughout the 5-day period. The track forecast also has its challenges. The initial motion estimate is northwestward, or 305/11 kt. It now appears that stronger upper-level ridging may take shape to the north of Florence over the next few days, forcing the hurricane to turn back toward the west-northwest from 36-72 hours. After 72 hours, a break in the ridge should allow Florence to turn back toward the northwest, but the bulk of the track models have trended westward since yesterday. In light of these trends, the NHC official track forecast has been shifted westward on days 4 and 5, but not quite as far as the various model consensus aids. It is worth stressing that there is still a significant amount of spread among the GFS and ECMWF ensemble members by the end of the forecast period, and just like the intensity forecast, the track forecast is of low confidence. FORECAST POSITIONS AND MAX WINDS INIT 05/1500Z 22.0N 45.7W 110 KT 125 MPH 12H 06/0000Z 22.7N 47.2W 110 KT 125 MPH 24H 06/1200Z 23.8N 49.0W 100 KT 115 MPH 36H 07/0000Z 24.7N 50.6W 95 KT 110 MPH 48H 07/1200Z 25.1N 52.1W 90 KT 105 MPH 72H 08/1200Z 25.7N 54.7W 90 KT 105 MPH 96H 09/1200Z 27.0N 57.0W 95 KT 110 MPH 120H 10/1200Z 28.5N 59.0W 95 KT 110 MPH $$ Forecaster Berg/Rhome
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