Tropical Storm Sebastien Moves Northeast, Begins to Weaken

The National Hurricane Center, as of 11:00 AM Friday AST, continues to issue advisories on Tropical Storm Sebastien, 1120 KM NE Of The Northern Leeward Islands.

At 11:00 AM AST, the center of Tropical Storm Sebastien was located near latitude 25.2 North, longitude 55.3 West. Sebastien is moving toward the east-northeast near 24 KM/H. An east-northeastward or northeastward motion at a similar forward speed is expected through the weekend.

Maximum sustained winds have decreased to near 85 KM/H with higher gusts. Continued gradual weakening is anticipated and Sebastien is forecast to dissipate by early next week.

Tropical-storm-force winds extend outward up to 260 kilometers from the center. The estimated minimum central pressure is 1000 millibars.

Tropical Storm Sebastien Forecast Cone as of 11:00 AM Friday 22nd November 2019. Credit: National Hurricane Center
Tropical Storm Sebastien Forecast Cone as of 11:00 AM Friday 22nd November 2019. Credit: National Hurricane Center

Watches & Warnings

Tropical Storm Sebastien Watches and Warnings as of 11:00 AM Friday 22nd November 2019. Credit: National Hurricane Center
Tropical Storm Sebastien Watches and Warnings as of 11:00 AM Friday 22nd 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

Sebastien is strongly sheared with deep convection limited to the northeast quadrant of the cyclone. The intensity has been set at 45 kt, based primarily on the latest TAFB Dvorak classification and recent ADT fixes. A partial ASCAT overpass at 1239 UTC may not have captured the strongest winds, but showed a peak value of only 40 kt.

Recent visible imagery and the ASCAT data also indicate that Sebastien’s circulation is not quite as well defined as it was yesterday, perhaps due to its close proximity to a nearby frontal boundary.

A significant change was made to the intensity forecast earlier this morning, and the latest forecast is in line with that new thinking. Strong shear is expected to prevent Sebastien from getting better organized, so gradual weakening is anticipated. The HWRF is once again a notable outlier, as the 06Z run stubbornly forecasts Sebastien to become a hurricane. While not impossible, that scenario appears unlikely and has been discounted.

Aside from the HWRF, the dynamical guidance otherwise dissipates Sebastien within about 3 days, and this is reflected in the official forecast. Given Sebastien’s shallow and disorganized structure, it is certainly possible that its wind field may become poorly-defined and the system could dissipate sooner than currently forecast.

The tropical storm continues to move at a slower pace and to the right of previous forecasts. The initial motion estimate is 075/13 kt. The latest NHC track forecast is slower and to the right of the previous advisory, closer to the global model consensus. The cyclone is expected to continue generally northeastward or east-northeastward near the southern end of an eastward-moving frontal boundary through the weekend or as long as it remains a tropical cyclone.

Forecast Discussion by Forecaster Zelinsky from the NHC.

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