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| #989946 (Received by flhurricane at: 3:42 AM 20.Nov.2019) TCDAT5 Tropical Storm Sebastien Discussion Number 4 NWS National Hurricane Center Miami FL AL202019 500 AM AST Wed Nov 20 2019 Deep convection developed closer to the center of Sebastien around the time of the previous advisory, but northwesterly shear and dry air have caused it to wane within the past hour or two. A late arriving ASCAT-C overpass revealed peak winds a little above 40 kt. Given the typical undersampling of that instrument, the initial intensity has been raised to 45 kt. The shear and dry air are expected to keep Sebastien`s intensity in check today, but by tonight the upper-level divergence is forecast to increase while the storm motion and shear vector line up. This is expected to allow for some strengthening and nearly all of the intensity guidance and global models predict deepening. The new intensity forecast is again similar to the previous advisory and lies between the lower statistical guidance and the more aggressive dynamical models. Sebastien appears to have turned northwestward with an initial motion of 305/7 kt. A turn to the north should occur later today, and then a northeastward motion is predicted to begin by Wednesday night as the storm is picked up by a deep layer trough moving eastward across the western Atlantic. The track guidance is in agreement on this overall scenario but there are some significant differences in how quickly Sebastien accelerates northeastward. The GFS, HWRF, and HMON show the storm recurving quickly ahead of the trough while the ECMWF is quite a bit slower. The NHC track is a little slower than the previous forecast, but remains close to the various consensus aids and the latest UKMET ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 21.0N 61.0W 45 KT 50 MPH 12H 20/1800Z 21.7N 61.5W 45 KT 50 MPH 24H 21/0600Z 23.2N 60.8W 50 KT 60 MPH 36H 21/1800Z 25.3N 58.6W 55 KT 65 MPH 48H 22/0600Z 28.7N 54.4W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 23/0600Z...DISSIPATED $$ Forecaster Brown |
