As of 11:00 AM Tuesday AST, the National Hurricane Center began to issue advisories on Tropical Storm Sebastien, 445 kilometers northeast of the Leeward Islands.
At 11:00 AM AST, the center of Tropical Storm Sebastien was located near latitude 20.1 North, longitude 58.7 West. Sebastien is moving toward the north-northwest near 13 KM/H. A turn to the north is expected on Wednesday followed by a turn to the northeast and an increase in forward speed Wednesday night.
Maximum sustained winds are near 75 KM/H with higher gusts. Some slight strengthening is possible over the next day or so. Sebastien is expected to become absorbed by a cold front in a couple of days.
Tropical-storm-force winds extend outward up to 165 kilometers from the center.
The estimated minimum central pressure is 1008 millibars.
Watches & Warnings
There are no coastal watches or warnings in effect nor are there any hazards affecting land.
This system is of no threat to the Windward Islands, including Trinidad and Tobago.
Tropical Storm Sebastien Forecast Discussion
Satellite imagery indicates that the overall cloud pattern of the disturbance has improved since yesterday and that the low has become well-defined. In addition, a late-arriving ASCAT-B scatterometer pass showed 35-38 kt winds extending 90 n mi from the center in the northeastern quadrant.
This pass also revealed that the low was nearly closed at the surface, and since the scatterometer may not have resolved the small scale of the low-level center it is likely that the surface low is indeed closed. Based on these data, advisories are being initiated on Tropical Storm Sebastian with an initial advisory intensity of 40 kt.
Sebastien will not be in an ideal environment for significant intensification, as it will be battling dry air to its west and about 20 kt of northwesterly shear for the next couple of days. However, due to the presence of an upper trough to the west of the system, a diffluent environment aloft may aid in some slight strengthening over the next day or so.
After that time, the storm will begin to interact with an approaching cold front, and some additional intensification may occur due to baroclinic processes. The cyclone is then expected to become absorbed by the front in about 48 hours. The various intensity guidance solutions are in decent agreement, and the official forecast is near the mean of these forecasts. There are some timing variations among the models on when the cyclone will become absorbed by the front, and it is possible that the storm could be absorbed sooner than indicated.
The initial motion is 330/07 kt. Sebastien will be steered to the northwest in the near term around a deep-layer ridge over the central Atlantic. By Wednesday, the cyclone should turn north and then should accelerate northeastward by Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of the approaching cold front. The model guidance is in good agreement on this scenario, and on the official forecast track, the cyclone will remain over open waters for the duration of its existence.
Forecast Discussion by Forecaster Latto from the NHC.