Post-Tropical Cyclone Sebastien Forecast Discussion


578 
WTNT45 KNHC 250232
TCDAT5

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:

High Seas Forecasts issued by Meteo France under WMO header FQNT50
LFPW and on the web at
www.meteofrance.com/previsions-meteo-marine/bulletin/grandlarge/
metarea2.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 41.0N  28.9W   50 KT  60 MPH...POST-TROP/EXTRATROP
 12H  25/1200Z 43.6N  22.7W   50 KT  60 MPH...POST-TROP/EXTRATROP
 24H  26/0000Z 48.1N  14.2W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  26/1200Z...MERGED WITH ANOTHER LOW

$$
Forecaster Latto


Tropical Storm Sebastien Forecast Discussion


000
WTNT45 KNHC 241441
TCDAT5

Tropical Storm Sebastien Discussion Number  21
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Sun Nov 24 2019

It is not clear if Sebastien is a closed cyclone at the surface. The
fast forward motion of the cyclone (28 kt) may have caused it to
open into a trough of low pressure. The ship MSC Beijing (DFDE2)
recently reported northwest winds about 70 n mi north-northeast of
the cyclone's estimated center, and it has been difficult to
identify easterly winds in visible imagery this morning. That said,
ASCAT data was inconclusive regarding the state of Sebastien's
circulation and the system is still producing some deep convection
near its center. AMSU data at 1135 UTC also indicated that Sebastien
still has a weak warm core. For those reasons, it appears prudent to
maintain advisories at this time.

As mentioned above, Sebastien is moving very quickly northeastward
and it should accelerate in that direction during the next 24 h.
This is shown by all of the dynamical models which are finally in
decent agreement. Based on simulated satellite imagery, it appears
that the cyclone will continue to produce central deep convection
for the next 12 h or so, and extratropical transition is forecast to
finish within 24 h. Although an extratropical point is shown at 36 h
for continuity purposes, in reality Sebastien will more likely open
into a trough of low pressure, given the forecast forward motion of
nearly 40 kt relative to its 50 kt forecast intensity at this time
tomorrow. This could happen at any time.

Regardless of whether it is a tropical cyclone, extratropical
cyclone, or a remnant trough, Sebastien or its remnants will likely
bring gusty winds and heavy rains to portions of the Azores later
today and tonight. Please see products issued by the Portuguese
Institute for the Sea and the Atmosphere (IPMA) for more
information.


FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 37.4N  37.0W   55 KT  65 MPH
 12H  25/0000Z 39.8N  31.0W   55 KT  65 MPH
 24H  25/1200Z 43.8N  22.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 36H  26/0000Z 47.7N  14.8W   45 KT  50 MPH...POST-TROP/EXTRATROP
 48H  26/1200Z...DISSIPATED

$$
Forecaster Zelinsky

Tropical Storm Sebastien Forecast Discussion


606 
WTNT45 KNHC 230833
TCDAT5

Tropical Storm Sebastien Discussion Number  16
NWS National Hurricane Center Miami FL       AL202019
500 AM AST Sat Nov 23 2019

The cloud pattern of Sebastien has changed significantly overnight.
Deep convection has decreased substantially during the past several
hours and the remaining thunderstorms are organized around a
mid-level eye feature seen in satellite images.  Model vertical
cross sections indicate that Sebastien is extremely titled with the
low-level center, although I can't see it, likely well displaced to
the southwest of the mid-level eye.  The initial intensity is held
at 55 kt based on the ASCAT data that came in a little after 00 Z,
but it looks like Sebastien is on a weakening trend.

Strong southwesterly vertically wind shear of about 40 kt is
responsible for the titled structure of the cyclone.  Since the
shear is not expected to let up and because Sebastien is headed for
steadily cooler waters, it seems reasonable to assume that weakening
is likely.  Although the models agree that Sebastien should slowly
lose its strength, it remains unclear if Sebastien will transition
to a post-tropical cyclone before it opens up into a trough during
the next few days.  The NHC forecast continues to show dissipation
by day 3, but it is certainty possible that it dissipates before
then.

The tropical storm is moving east-northeastward at 15 kt ahead of a
cold front and trough.  This motion with an increase in forward
speed is expected during the next few days as Sebastien becomes more
embedded in the faster mid-latitude flow.  The models are in
relatively good agreement, and only small changes were made to the
previous NHC forecast.

The initial wind radii were modified based on the aforementioned
ASCAT passes, and the 12-ft sea radii were adjusted based on ship
and altimeter data.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 27.8N  50.6W   55 KT  65 MPH
 12H  23/1800Z 29.4N  48.2W   50 KT  60 MPH
 24H  24/0600Z 31.6N  44.2W   45 KT  50 MPH
 36H  24/1800Z 34.0N  38.8W   40 KT  45 MPH
 48H  25/0600Z 37.0N  31.6W   35 KT  40 MPH
 72H  26/0600Z...DISSIPATED

$$
Forecaster Cangialosi


Tropical Storm Sebastien Forecast Discussion


000
WTNT45 KNHC 212032
TCDAT5

Tropical Storm Sebastien Discussion Number  10
NWS National Hurricane Center Miami FL       AL202019
500 PM AST Thu Nov 21 2019

Convection associated with Sebastien has become better organized
during the past several hours, as a band has formed near and over
the center in the eastern semicircle.  Overall, the cloud pattern
remains elongated from north-to-south as the storm is interacting
with a cold front not far to its northwest.  Scatterometer data
received just after the last advisory indicated 45-50 kt winds, and
based on this and current satellite intensity estimates the initial
intensity remains 50 kt.

The cyclone has turned northeastward with the initial motion now
050/8.  Southwesterly mid-latitude flow should steer Sebastien or
its remnants quickly northeastward, and the guidance is in good
agreement on the direction of movement during the next several
days.  However, there is a significant spread in the forward speed,
with the new ECMWF run being much slower than the other guidance.
The new track forecast will be only slightly slower than the
previous forecast, but adjustments to the forecast forward speed
may be necessary later.

Sebastien is in an apparently unfavorable environment for
development due to strong shear, decreasing sea surface temperatures
along the forecast track, and an expected extratropical transition
as it merges with the front.  The intensity guidance is rather
divergent during the first 24 h, with the SHIPS and LGEM models
showing weakening while the dynamical guidance continues to forecast
strengthening despite the environment.  After that time, all
guidance indicates that Sebastien should weaken.  The intensity
forecast continues the trend of the previous forecast in showing
some strengthening during the first 24 h in agreement with the
dynamical models, followed by weakening and extratropical transition
by 48 h. Given how close the cold front is to the storm at this
time, it is possible that extratropical transition could occur
earlier than currently forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 23.8N  59.7W   50 KT  60 MPH
 12H  22/0600Z 25.5N  57.1W   60 KT  70 MPH
 24H  22/1800Z 27.9N  53.2W   65 KT  75 MPH
 36H  23/0600Z 30.6N  48.4W   55 KT  65 MPH
 48H  23/1800Z 33.5N  43.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  24/1800Z 39.0N  31.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
 96H  25/1800Z...DISSIPATED

$$
Forecaster Beven

Tropical Storm Sebastien Forecast Discussion


000
WTNT45 KNHC 201447
TCDAT5

Tropical Storm Sebastien Discussion Number   5
NWS National Hurricane Center Miami FL       AL202019
1100 AM AST Wed Nov 20 2019

Sebastien continues to struggle with dry air and northwesterly
shear, with satellite imagery showing an exposed low-level
center to the west of the deep convection. The current structure of
the storm should prevent significant intensification during the next
12 hours. After that time, Sebastien will begin to interact with a
mid- to upper-level trough that will produce upper-level divergence
over the cyclone. This will provide a window for Sebastien to
intensify before making the transition to an extratropical cyclone
in about 48 hours. All available guidance intensifies the storm, and
the dynamical models continue to strengthen it more than the
statistical models. In fact, the mesoscale models unanimously make
Sebastian a hurricane within 36 hours. However, these models are
likely intensifying the cyclone too quickly over the next 12 hours,
and therefore may have a slight high bias. The official forecast
takes this into account and keeps the storm just below hurricane
strength before extratropical transition by 48 hours, which is a
blend of the ECMWF and GFS solutions.

Sebastien appears to be approaching its westernmost point and a turn
to the north should begin later today, followed by acceleration to
the northeast starting tonight due to the influence of the
approaching trough. The track guidance is in agreement on this
overall scenario. The NHC forecast track is just slightly slower
than the previous one in the first 24 hours, and is close to the
model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 21.1N  61.7W   45 KT  50 MPH
 12H  21/0000Z 22.0N  61.7W   45 KT  50 MPH
 24H  21/1200Z 23.7N  60.2W   55 KT  65 MPH
 36H  22/0000Z 26.3N  57.2W   60 KT  70 MPH
 48H  22/1200Z 29.8N  52.5W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  23/1200Z...DISSIPATED

$$
Forecaster Latto/Mello

Remnants of Fifteen Forecast Discussion


000
WTNT45 KNHC 160833
TCDAT5

Remnants Of Fifteen Discussion Number   7
NWS National Hurricane Center Miami FL       AL152019
500 AM AST Wed Oct 16 2019

Satellite images indicate that the surface circulation of the
system has become rather elongated north-to-south, with only a few
weak swirls in the circulation envelope, and has degenerated into a
trough of low pressure.  Thus it is no longer a tropical cyclone,
and this is the last advisory.  The winds are lowered to 25 kt,
matching the TAFB classification.

The remnants of the cyclone are expected to move northwestward
during the next few days.  There is a slight chance of
regeneration, as suggested by the ECMWF model and some of its
ensemble members due to a trough interaction, but that's considered
unlikely at this point.


FORECAST POSITIONS AND MAX WINDS

INIT  16/0900Z 17.3N  24.1W   25 KT  30 MPH
 12H  16/1800Z...DISSIPATED

$$
Forecaster Blake

Tropical Depression Fifteen Forecast Discussion


000
WTNT45 KNHC 160235
TCDAT5

Tropical Depression Fifteen Discussion Number   6
NWS National Hurricane Center Miami FL       AL152019
1100 PM AST Tue Oct 15 2019

The tropical depression located near the Cabo Verde Islands has
become less organized over the past 6 hours. An ASCAT-A overpass at
2154 UTC showed that the depression still had a closed and
well-defined center, but the strongest winds were only 20-25 kt. A
subsequent ASCAT-B pass about 1 hour later showed slightly higher
winds but an elongated circulation. Furthermore, deep convection is
limited to a small area of showers located nearly 100 n mi to the
east of the depression's center. If its convection continues to
decrease, the system could become a remnant low later today.
Alternatively, if its circulation continues to become elongated, it
could soon open into a trough and dissipate. Either way, it seems
unlikely that the depression will remain a tropical cyclone for
long, and this shown by all of the models and the official forecast.

The depression has made a westward jog since the last advisory, but
a longer-term motion estimate is 300/09 kt. The westward jog
resulted in a substantial westward shift in the official forecast
track, but the system is still expected to move generally
west-northwestward to northwestward for the next day or two until it
dissipates. The track guidance is in good agreement on this
scenario, and the main source of uncertainty is just how quickly the
system will open into a trough.

The latest ECMWF deterministic forecast and few of its ensemble
members indicate that the cyclone could regenerate later this week.
However, most other models do not forecast regeneration and the
uncertainty is too high to justify explicitly showing this in the
official forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 16.8N  23.7W   30 KT  35 MPH
 12H  16/1200Z 17.7N  24.6W   30 KT  35 MPH
 24H  17/0000Z 18.6N  25.7W   25 KT  30 MPH...POST-TROP/REMNT LOW
 36H  17/1200Z 19.6N  27.2W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  18/0000Z 20.8N  29.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  19/0000Z...DISSIPATED

$$
Forecaster Zelinsky

Tropical Depression Fifteen Forecast Discussion


000
WTNT45 KNHC 142040
TCDAT5

Tropical Depression Fifteen Discussion Number   1
NWS National Hurricane Center Miami FL       AL152019
500 PM AST Mon Oct 14 2019

The large low pressure system that moved off the coast of Africa
yesterday has gradually become better defined based on earlier ASCAT
scatterometer wind data and recent visible satellite imagery.
Several fragmented curved bands have been developing during the past
several hours, especially in the northern semicircle, and the
aforementioned ASCAT passes indicated that winds of 30-32 kt were
present northwest of the center. The initial intensity is set at 30
kt based on a blend of the scatterometer wind data and a satellite
intensity estimate of T1.5/25 kt from SAB, and the rather broad
nature of the depression.

A surge of mid-level dry air has recently penetrated into the
inner-core region, causing some erosion of the central deep
convection. However, this is expected to be a temporary condition
with convection returning later tonight and early Tuesday during the
normal nocturnal convective maximum period. However, the large size
of the cyclone -- more than 1000 nmi wide -- should prevent any
rapid or significant intensification. With very low vertical shear
conditions forecast by the GFS and ECMWF models to continue for the
next 24 to 36 hours, at least some modest strengthening seems likely
given the unusually warm SSTs of near 28.5 deg C that the cyclone
will be traversing during that time. By 48 hours, the shear is
forecast to increase from the southwest in excess of 25 kt,
resulting in weakening into a remnant low shortly thereafter. By 72
hours and beyond, the shear is expected to increase to more than 40
kt, which will cause rapid weakening and dissipation by the 120-h
period, if not sooner. The official intensity forecast is similar to
a blend of the NOAA-HCCA and IVCN consensus models, and the GFS-
and ECMWF-based Decay-SHIPS statistical intensity models.

The initial motion estimate is northwestward or 310/07 kt. The
latest NHC model guidance is in very good agreement on the tropical
cyclone moving generally northwestward around the southwestern
periphery of a deep-layer ridge for the next 48 hours. Thereafter,
the models diverge significantly based on how soon the cyclone
weakens and turns westward within the low-level easterly trade wind
flow. The ECMWF holds on to a vertically deeper system a little
longer than the GFS, UKMET, HWRF, and HMON models, resulting in a
longer northwestward track. However, since the cyclone will likely
have become a vertically shallow remnant low by 72 hours, the NHC
official forecast track is a little to the left of ECMWF solution,
closer to the TVCN and HCCA consensus model tracks at 72 and 96 h.

FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 13.2N  20.2W   30 KT  35 MPH
 12H  15/0600Z 14.3N  21.1W   35 KT  40 MPH
 24H  15/1800Z 15.5N  22.4W   40 KT  45 MPH
 36H  16/0600Z 16.7N  23.4W   40 KT  45 MPH
 48H  16/1800Z 18.1N  24.6W   35 KT  40 MPH
 72H  17/1800Z 20.4N  26.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  18/1800Z 21.4N  29.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  19/1800Z...DISSIPATED

$$
Forecaster Stewart

Post-Tropical Cyclone Jerry Forecast Discussion


716 
WTNT45 KNHC 252038
TCDAT5

Post-Tropical Cyclone Jerry Discussion Number  34
NWS National Hurricane Center Miami FL       AL102019
500 PM AST Wed Sep 25 2019

Multiple scatterometer passes over Jerry indicate that the cyclone
no longer has sustained tropical-storm-force winds.  Therefore
Jerry is now a remnant low, and the Bermuda Weather Service has
discontinued the Tropical Storm Warning for Bermuda.  However, wind
gusts to tropical storm force are still possible on the island
during the next few hours, especially at elevated observing sites.
Based on the scatterometer data, the current intensity is estimated
to be 30 kt. The low-cloud swirl is becoming less well-defined, and
since the system will continue moving through a hostile environment
of strong shear and dry mid-level air, steady weakening is likely.
The cyclone should dissipate in 2-3 days.  The NHC intensity
forecast is close to the latest intensity model consensus.

The system is moving east-northeastward at a slightly faster clip,
or 070/10 kt.  A gradual turn to the east and east-southeast is
forecast as the cyclone moves along the southern edge of the band of
mid-latitude westerlies, and then turns to the right along the
northeastern periphery of a subtropical anticyclone before
dissipating.

This is the last advisory on Jerry.

FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 32.2N  65.6W   30 KT  35 MPH...POST-TROP/REMNT LOW
 12H  26/0600Z 32.8N  63.8W   30 KT  35 MPH...POST-TROP/REMNT LOW
 24H  26/1800Z 33.7N  61.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 36H  27/0600Z 34.7N  59.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  27/1800Z 35.0N  56.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  28/1800Z...DISSIPATED

$$
Forecaster Pasch


Tropical Storm Jerry Forecast Discussion


000
WTNT45 KNHC 241445
TCDAT5

Tropical Storm Jerry Discussion Number  29
NWS National Hurricane Center Miami FL       AL102019
1100 AM AST Tue Sep 24 2019

Jerry's interaction with an upper-level trough has resulted in a
system that is less tropical in appearance, with a lack of
central convection and most of the heavier showers displaced
to the west and well southeast of the center.  In fact, the cyclone
looks somewhat subtropical at this time.  For simplicity, however,
we will continue to carry Jerry as a tropical storm.  Flight-level
winds from an Air Force Reserve Unit Hurricane Hunter plane still
support an intensity of 50 kt.  Jerry will be encountering an
increasingly unfavorable environment during the next few days, with
the ambient air mass becoming drier and vertical shear becoming
stronger.  The official intensity forecast calls for continued
weakening and degeneration to a remnant low later in the forecast
period.  This is similar to the consensus of the model guidance.

Based on fixes from the Hurricane Hunters, the center has moved a
little to the left of previous estimates, and the initial motion
estimate is 350/6 kt.  There are no significant changes to the
track forecast philosophy, however.  Over the next couple of days
Jerry should turn northeastward and then east-northeastward in the
flow to the south of a broad mid-latitude trough.  Later in the
period, the weakening cyclone should move eastward to
east-southeastward on the northeast side of a subtropical
anticyclone.  The official track forecast is close to the previous
one and in general agreement with the corrected multi-model
consensus guidance.


Key Messages:

1. Tropical-storm-force winds are expected on Bermuda by tonight or
early Wednesday.  Large swells are also expected to affect the coast
of Bermuda during the next few days.  These swells could cause
life-threatening rip currents.  See products issued by the Bermuda
Weather Service for more information.


FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 30.8N  69.3W   50 KT  60 MPH
 12H  25/0000Z 31.7N  68.5W   45 KT  50 MPH
 24H  25/1200Z 32.6N  66.7W   40 KT  45 MPH
 36H  26/0000Z 33.5N  64.2W   35 KT  40 MPH
 48H  26/1200Z 34.2N  61.5W   30 KT  35 MPH
 72H  27/1200Z 35.0N  57.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  28/1200Z 34.5N  54.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  29/1200Z...DISSIPATED

$$
Forecaster Pasch

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