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| #990172 (Received by flhurricane at: 3:48 AM 22.Nov.2019) TCDAT5 Tropical Storm Sebastien Discussion Number 12 NWS National Hurricane Center Miami FL AL202019 500 AM AST Fri Nov 22 2019 Sebastien has become less organized this morning. Strong southwesterly vertical wind shear of about 40 kt and dry air have caused the low-level center of the storm to become exposed to the southwest of the main area of deep convection. The initial intensity remains 50 kt, based on the earlier ASCAT data, but this is a little above the latest Dvorak estimates and could be generous. The center of Sebastien has turned more to the right than expected, with the initial motion estimated to be 065/13 kt. The models have changed significantly this cycle, and now show a much slower forward motion to the northeast during the next few days. This change appears to be connected to a more vertically shallow system that moves in the lower-level flow. The NHC track forecast has been adjusted considerably slower and a little to the right of the previous one, but it still is faster than nearly all of the typically reliable models. Future adjustments will likely be made if the model trend continues. It no longer appears that Sebastien will strengthen. In fact, all of the intensity guidance now suggests that the storm will gradually weaken during the next few days as Sebastien remains in strong wind shear conditions and moves over progressively cooler waters. In addition, the model guidance has finally come into agreement that Sebastien will not merge with the nearby cold front. Instead, the cyclone is expected to move ahead of the weakening front and become a non-convective post-tropical cyclone in about 2 days, or less, when it moves over SSTs below 22 C. As mentioned in the previous discussion, NHC and the models have had a challenging time figuring out when Sebastien would lose its tropical characteristics, and we have held the transition timing steady at 48 hours for a while. Although there is still uncertainty, the models are in better agreement in the timing of this transition. This intensity forecast is a little lower than the previous one to come into better agreement with the latest models. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 24.8N 57.0W 50 KT 60 MPH 12H 22/1800Z 26.0N 54.3W 50 KT 60 MPH 24H 23/0600Z 27.2N 51.7W 45 KT 50 MPH 36H 23/1800Z 28.7N 49.1W 40 KT 45 MPH 48H 24/0600Z 30.0N 46.2W 35 KT 40 MPH...POST-TROPICAL 72H 25/0600Z 33.3N 38.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 26/0600Z...DISSIPATED $$ Forecaster Cangialosi |