Tropical Storm Sebastien Maintains Strength In The North Atlantic

The National Hurricane Center, as of 11:00 AM Saturday AST, continues to issue advisories on Tropical Storm Sebastien, 2075 KM WSW Of The Azores Islands.

At 11:00 AM AST, the center of Tropical Storm Sebastien was located near latitude 30.4 North, longitude 47.7 West. Sebastien is moving toward the northeast near 43 KM/H. A fast northeastward or east-northeastward motion is expected during the next day or two.

Maximum sustained winds are near 100 KM/H with higher gusts. Little change in the maximum wind speed is expected during the next couple of days. Sebastien is forecast to become post-tropical by Monday and it could become a remnant trough of low pressure sooner than that.

Tropical-storm-force winds extend outward up to 185 kilometers from the center. The estimated minimum central pressure is 997 millibars

Tropical Storm Sebastien Forecast Cone as of 11:00 AM Saturday 23rd November 2019. Credit: National Hurricane Center
Tropical Storm Sebastien Forecast Cone as of 11:00 AM Saturday 23rd November 2019. Credit: National Hurricane Center

Watches & Warnings

Tropical Storm Sebastien Watches and Warnings as of 11:00 AM Saturday 23rd November 2019. Credit: National Hurricane Center
Tropical Storm Sebastien Watches and Warnings as of 11:00 AM Saturday 23rd November 2019. Credit: National Hurricane Center

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

First-light visible imagery and a few late-arriving microwave overpasses revealed that Sebastien’s elongated center is located well northeast of previous estimates. It isn’t clear if the center reformed or if it simply accelerated more than expected overnight, but the initial motion estimate is now 055/23 kt. A significant change to the track forecast was made based on the updated initial position, and the NHC forecast now shows a much faster motion for Sebastien.

The cyclone’s cloud pattern is somewhat ragged and does not look entirely tropical. That said, Sebastien is still producing enough deep convection to be classified as a tropical storm. Unfortunately, no ASCAT data was available this morning to help assess the strength of the winds or how well-defined the circulation is at the surface. The initial intensity is conservatively maintained at 55 kt based primarily on continuity from previous advisories, but this could be generous since Dvorak intensity estimates are lower.

Every aspect of the forecast remains highly uncertain. The models have been extremely inconsistent in forecasting Sebastien, with the guidance varying drastically from run to run and with an unusually large spread in each forecast. The latest indications are that Sebastien will move quickly northeastward with little change in strength for the next few days before it eventually opens into a trough around the time it nears the Azores in a couple of days.

Simulated satellite imagery suggests that the cyclone will still produce enough deep convection to be classified as a tropical storm through Sunday, though it could become post-tropical before it reaches the Azores. Given the elongated nature of Sebastien’s circulation and its fast forward speed, it could also open into a trough and dissipate at any time.

It is not currently clear if Sebastien will reach the Azores as a tropical storm. Regardless of its status, Sebastien or its remnants could bring gusty winds and rain to the Azores beginning Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

Forecast Discussion by Forecaster Zelinsky from the NHC.

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