Post-Tropical Cyclone Sebastien Discussion Number 23
NWS National Hurricane Center Miami FL AL202019
1100 PM AST Sun Nov 24 2019
Sebastien has lost its tropical characteristics this evening. The
deep convection has decoupled from the low-level circulation, and
these features are now separated by a distance of 100 n mi and
increasing. Two partial ASCAT passes revealed that the surface low
is becoming stretched out, and there is some evidence of a boundary,
possibly a front, extending northeast from the center. Furthermore,
recent observations from Corbo and Flores islands in the Azores
indicated a slight temperature decrease as the center passed by to
the north of those islands, suggesting that a weak cold front is
associated with the the cyclone. Based on all of these data, there
is high confidence that Sebastien has transitioned to an
extratropical cyclone. The initial advisory intensity is 50 kt which
is based on 47 kt winds sampled by the scatterometer and a
subjective intensity estimate from TAFB.
Post-tropical cyclone Sebastien is moving northeastward at 35 kt,
embedded in the strong flow to the southeast of a mid- to upper-
level trough. This motion, and the cyclone's current intensity, is
expected to persist for the next day or so, before the cyclone
merges with a higher-latitude low.
The cyclone will likely bring gusty winds and heavy rains to
portions of the Azores through Monday morning. Please see products
issued by the Portuguese Institute for the Sea and the Atmosphere
(IPMA) for more details.
Additional information on this system can be found in:
Tropical Storm Sebastien Discussion Number 1
NWS National Hurricane Center Miami FL AL202019
1100 AM AST Tue Nov 19 2019
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.
1. Showers and thunderstorms associated with a broad area of low
pressure centered a little more than 200 miles east-northeast of the
northern Leeward Islands continue to show signs of organization.
Although the surface circulation of the disturbance appears to be
gradually consolidating, recent satellite-derived wind data indicate
that it does not yet have a well-defined center. Additional
development is expected and a tropical or subtropical depression is
likely to form while the system moves northwestward and then
northward over the open Atlantic during the next day or two. The low
is forecast to interact with a frontal system by midweek and further
development is unlikely after that time. For more information, see
High Seas Forecasts issued by the National Weather Service.
* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...70 percent.
SUBJ/TROPICAL CYCLONE FORMATION ALERT//
WTNT22 KNGU 190500
RMKS/1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE
WITHIN 100 NM EITHER SIDE OF A LINE FROM 18.8N 57.9W TO 21.3N 59.7W
WITHIN THE NEXT 00 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY
ISSUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME.
WINDS IN THE AREA ARE ESTIMATED TO BE 25 TO 30 KNOTS. METSAT
IMAGERY AT 190000Z INDICATES THAT A CIRCULATION CENTER IS LOCATED
NEAR 18.9N 58.0W. THE SYSTEM IS MOVING NORTHWESTWARD AT 07 KNOTS.
2. REMARKS:THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT
TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS HIGH.A BROAD AREA OF
LOW PRESSURE HAS FORMED ABOUT 250 MILES EAST-NORTHEAST OF THE LESSER
ANTILLES. ENVIRONMENTAL CONDITIONS WILL BE CONDUCIVE FOR FURTHER
DEVELOPMENT INTO A TROPICAL OR SUBTROPICAL DEPRESSION OVER THE NEXT
24 HOURS. THE SYSTEM IS FORECAST TO MOVE NORTHWESTWARD 05-10 KTS
OVER THE NEXT SEVERAL HOURS.
3. THIS ALERT WILL BE REISSUED, UPGRADED TO WARNING OR CANCELLED
BY 200500Z.//
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