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| #938834 (Received by flhurricane at: 4:51 PM 06.Sep.2018) TCDAT1 Hurricane Florence Discussion Number 30 NWS National Hurricane Center Miami FL AL062018 500 PM AST Thu Sep 06 2018 Vertical shear has taken its toll on Florence today as evidenced by a continued degradation of the overall cloud pattern. The circulation appears tilted, with the low-level center partially exposed to the southwest of the deep convection. Subjective and objective Dvorak Current Intensity (CI) numbers have fallen, and a blend of the various estimates supports an initial intensity of 70 kt. The intensity forecast is relatively straightforward in the short-term as shear is expected to remain strong, which should continue Florence`s weakening trend, potentially taking the system below hurricane strength. By 24 hours, vertical shear is forecast to decrease, and the SSTs gradually warm along the forecast track. Assuming the overall circulation remains intact, Florence shouldn`t have any problems restrengthening beginning in a day or so. In fact, guidance suggests that Florence could once again become a major hurricane in 4 or 5 days. The new NHC intensity forecast is essentially an update of the previous one, and is only adjusted to account for recent trends. Largely, the official forecast is close to the various consensus aids. Owing to the degraded structure and tilted nature of the system, Florence has wobbled a bit to the west, but the longer-term motion estimate is 305/09. Low- to mid-level ridging should cause Florence to turn toward the west-northwest and west between 12-48 hours while the cyclone recovers from the strong shear. Thereafter, Florence is anticipated to become a deep cyclone again, but an even stronger ridge should maintain the west-northwestward motion, at a faster speed, through day 5. The ridge is forecast to be sufficiently strong such that some track models show a motion just south of due west during the next 12-36 hours. Beyond day 3, the track forecast becomes increasingly uncertain due to differing evolutions of the steering pattern over the western Atlantic. While all of the global models show a progressive trough eroding the ridge, they differ in the strength of the trough and the ridge to the north of Florida. These differences result in a great deal of bifurcation in the track guidance, especially among global model ensemble members, at the end of the forecast period. In such situations, prudence suggests a reliance on continuity, and the the new official NHC track forecast is shifted only slightly south towards the TVCN multi-model consensus and HCCA. It is important to note that deterministic track models in these types of situations often display considerable run-to-run changes, and the uncertainty in this forecast remains larger than normal. Key Messages: 1. Regardless of Florence`s eventual track, large swells will begin to affect Bermuda on Friday and portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. There is still very large uncertainty in Florence`s track beyond day 5, and it is too soon to determine what, if any, other impacts Florence could have on the U.S. East Coast next week. 3. Since we are near the peak of hurricane season, this is a good time for everyone who lives in a hurricane-prone area to ensure they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 25.0N 49.6W 70 KT 80 MPH 12H 07/0600Z 25.4N 50.7W 60 KT 70 MPH 24H 07/1800Z 25.5N 52.1W 65 KT 75 MPH 36H 08/0600Z 25.5N 53.4W 70 KT 80 MPH 48H 08/1800Z 25.6N 54.6W 75 KT 85 MPH 72H 09/1800Z 26.4N 57.1W 90 KT 105 MPH 96H 10/1800Z 28.0N 61.0W 100 KT 115 MPH 120H 11/1800Z 30.0N 67.0W 105 KT 120 MPH $$ Forecaster Berg/Rhome |
