ATL: SEBASTIEN - Post-Tropical

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Re: ATL: SEBASTIEN - Tropical Storm

#41 Postby wxman57 » Tue Nov 19, 2019 9:27 pm

Looks less organized this evening. Center is farther from the convection.
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Re: ATL: SEBASTIEN - Tropical Storm

#42 Postby storminabox » Tue Nov 19, 2019 9:36 pm

EquusStorm wrote:Also puts us as being the most active season since 2012. 18 storms does not happen often despite the recent couple decades' antics.


Generally speaking, this season has been much more about quantity and has had relatively few quality storms. This is clear through the low amount of hurricanes that have occurred this season in comparison to the number of named storms.
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Re: ATL: SEBASTIEN - Tropical Storm

#43 Postby Sciencerocks » Tue Nov 19, 2019 9:46 pm

103
WTNT45 KNHC 200237
TCDAT5

Tropical Storm Sebastien Discussion Number 3
NWS National Hurricane Center Miami FL AL202019
1100 PM AST Tue Nov 19 2019

There hasn't much of a change with Sebastien for most of tonight,
with the center still exposed to the northwest of a large area of
deep convection. Recently, the low-level center appears to be
slowing down and gaining some latitude as convection re-fires near
the center. The initial wind speed will stay 40 kt, pending receipt
of ASCAT data later this evening.

The initial motion has been more to the west during the past few
hours, but a longer-term motion is 290/8. Sebastien should move
generally northwestward on Wednesday, northward on Wednesday night
and much faster to the northeast on Thursday as the storm moves
along the southwestern periphery of the subtropical ridge and then
ahead of an approaching mid-latitude trough. The forecast has been
adjusted westward in the short-term due mostly to the initial
motion, and then eastward in the longer term due to models
suggesting the storm may stay a little more ahead of the cold front
than before. These forecast changes, however, are fairly typical
for a disorganized system.

Sebastien has some chance to intensify by late tomorrow as the shear
vector and the storm motion vector line up around the same time as
the upper-level divergence is forecast to increase. This should
cause the cyclone to strengthen, and the new forecast is closely
aligned with the previous one. Around 48 hours, Sebastien is
expected to become an extratropical cyclone as it is overtaken by a
cold front, and then become absorbed by the larger front on Friday.
The latest NHC wind speed forecast is close to the model consensus,
but below the regional hurricane models.


FORECAST POSITIONS AND MAX WINDS

INIT 20/0300Z 20.7N 60.6W 40 KT 45 MPH
12H 20/1200Z 21.5N 61.5W 40 KT 45 MPH
24H 21/0000Z 22.9N 61.5W 45 KT 50 MPH
36H 21/1200Z 24.4N 59.8W 50 KT 60 MPH
48H 22/0000Z 27.2N 56.5W 55 KT 65 MPH...POST-TROP/EXTRATROP
72H 23/0000Z...DISSIPATED

$$
Forecaster Blake
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Re: ATL: SEBASTIEN - Tropical Storm

#44 Postby HurricaneRyan » Tue Nov 19, 2019 11:40 pm

So fun facts:

1. Sebastien is the first original name that begins with the letter R or beyond to be used twice. Previously before this season, only the Letter P was used more than once with Philippe, and now with Pablo.
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Re: ATL: SEBASTIEN - Tropical Storm

#45 Postby Hypercane_Kyle » Wed Nov 20, 2019 12:53 am

storminabox wrote:
EquusStorm wrote:Also puts us as being the most active season since 2012. 18 storms does not happen often despite the recent couple decades' antics.


Generally speaking, this season has been much more about quantity and has had relatively few quality storms. This is clear through the low amount of hurricanes that have occurred this season in comparison to the number of named storms.


True, this season reminds me a lot of 2007 to be honest: excluding Dean and Felix most of the storms that season were relatively weak.
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Re: ATL: SEBASTIEN - Tropical Storm

#46 Postby Sciencerocks » Wed Nov 20, 2019 3:54 am

426
WTNT45 KNHC 200837
TCDAT5

Tropical Storm Sebastien Discussion Number 4
NWS National Hurricane Center Miami FL AL202019
500 AM AST Wed Nov 20 2019

Deep convection developed closer to the center of Sebastien
around the time of the previous advisory, but northwesterly shear
and dry air have caused it to wane within the past hour or two. A
late arriving ASCAT-C overpass revealed peak winds a little above
40 kt. Given the typical undersampling of that instrument, the
initial intensity has been raised to 45 kt. The shear and dry air
are expected to keep Sebastien's intensity in check today, but by
tonight the upper-level divergence is forecast to increase while the
storm motion and shear vector line up. This is expected to allow
for some strengthening and nearly all of the intensity guidance and
global models predict deepening. The new intensity forecast is
again similar to the previous advisory and lies between the lower
statistical guidance and the more aggressive dynamical models.

Sebastien appears to have turned northwestward with an initial
motion of 305/7 kt. A turn to the north should occur later today,
and then a northeastward motion is predicted to begin by Wednesday
night as the storm is picked up by a deep layer trough moving
eastward across the western Atlantic. The track guidance is in
agreement on this overall scenario but there are some significant
differences in how quickly Sebastien accelerates northeastward. The
GFS, HWRF, and HMON show the storm recurving quickly ahead of
the trough while the ECMWF is quite a bit slower. The NHC track is
a little slower than the previous forecast, but remains close to the
various consensus aids and the latest UKMET ensemble mean.

FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 21.0N 61.0W 45 KT 50 MPH
12H 20/1800Z 21.7N 61.5W 45 KT 50 MPH
24H 21/0600Z 23.2N 60.8W 50 KT 60 MPH
36H 21/1800Z 25.3N 58.6W 55 KT 65 MPH
48H 22/0600Z 28.7N 54.4W 55 KT 65 MPH...POST-TROP/EXTRATROP
72H 23/0600Z...DISSIPATED

$$
Forecaster Brown
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Re: ATL: SEBASTIEN - Tropical Storm

#47 Postby Nimbus » Wed Nov 20, 2019 8:06 am

The sheared low level center looks pretty healthy for a November storm.
How long will it stay classified tropical?
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Re: ATL: SEBASTIEN - Tropical Storm

#48 Postby TallyTracker » Wed Nov 20, 2019 10:09 am

NHC is now forecasting Sebastien to approach hurricane strength before becoming extratropical. That would be cool to see a November fish hurricane to end the season!
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Re: ATL: SEBASTIEN - Tropical Storm

#49 Postby al78 » Wed Nov 20, 2019 10:23 am

storminabox wrote:
EquusStorm wrote:Also puts us as being the most active season since 2012. 18 storms does not happen often despite the recent couple decades' antics.


Generally speaking, this season has been much more about quantity and has had relatively few quality storms. This is clear through the low amount of hurricanes that have occurred this season in comparison to the number of named storms.


It has been another season where the tropics have seen low activity and storms have either fizzled out or haven't got going until they moved north of 20N. A bit like last year which had an unusually high amount of subtropical storm activity which boosted the ACE index beyond what would be expected given the unfavourable Aug-Sep wind shear and SST anomalies (which is why seasonal forecasts busted).

Ideally to get quality (i.e. strong, pretty on satellite imagery) storms, you want favourable conditions south of 20N and steering winds that bring the storms west toward the Caribbean Sea/Gulf (although that is highly sub-optimal from the perspective of human impacts). It is only really Dorian, and briefly Lorenzo, that found a local area of very favourable conditions and was able to take advantage of them (to the detriment of the Bahamas in the former case).

Curious as to why we have had two years now where the subtropics have been more favourable for hurricanes than the deep tropics. Normally this happens in El Nino years.
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Re: ATL: SEBASTIEN - Tropical Storm

#50 Postby wxman57 » Wed Nov 20, 2019 12:05 pm

TallyTracker wrote:NHC is now forecasting Sebastien to approach hurricane strength before becoming extratropical. That would be cool to see a November fish hurricane to end the season!


00Z EC is less bullish on strengthening than 12Z yesterday, about 8-9 mb higher with the min pressure. I have my doubts it'll get anywhere near 60 kts. I don't think it has 45 kt winds now. ASCAT perfectly missed it a couple hours ago, so nothing to estimate winds, except for Dvorak, which is 2.0 from NOAA (depression) and 2.4 from CIMSS (35kt TS). I'd say it may have a small pocket of 35-40 kt winds in the squalls well east of the low-level swirl. Maybe closer to 35 kts than 40 kts. Pretty poorly-organized. I am measuring a track to about 12 degrees at 7.2 kts in the past hour. The northward turn has occurred and it will be accelerating NNE-NE today/tonight. That will result in the relative shear decreasing, which should allow for some strengthening. I'm thinking 45-50 kts peak, though.
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Re: ATL: SEBASTIEN - Tropical Storm

#51 Postby wxman57 » Wed Nov 20, 2019 12:13 pm

Looks like we did get a 14Z ASCAT-C pass right over the heavy convection. I don't see anything over 35 kts, but there could be a small area of 40 kts. Don't see anything to indicate 45 kts.

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Re: ATL: SEBASTIEN - Tropical Storm

#52 Postby 1900hurricane » Wed Nov 20, 2019 12:33 pm

SMAP from 1027Z was about 40-45 kt. I think 40 kt is a reasonable estimate right now. NHC initially bumped Sebastien up to 45 kt based on Met-Op C ASCAT data from ~12 hours ago. Given that the same instrument passed over showing weaker winds in the 35-40 kt range at maximum, I think it is justifiable to bump it back down a bit.

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Re: ATL: SEBASTIEN - Tropical Storm

#53 Postby CyclonicFury » Wed Nov 20, 2019 4:15 pm

A new hot tower is firing up close to the center. Will be interesting to see if it leads to more significant intensification tonight.
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Re: ATL: SEBASTIEN - Tropical Storm

#54 Postby Sciencerocks » Wed Nov 20, 2019 4:32 pm

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Re: ATL: SEBASTIEN - Tropical Storm

#55 Postby DioBrando » Wed Nov 20, 2019 6:12 pm

al78 wrote:
storminabox wrote:
EquusStorm wrote:Also puts us as being the most active season since 2012. 18 storms does not happen often despite the recent couple decades' antics.


Generally speaking, this season has been much more about quantity and has had relatively few quality storms. This is clear through the low amount of hurricanes that have occurred this season in comparison to the number of named storms.


It has been another season where the tropics have seen low activity and storms have either fizzled out or haven't got going until they moved north of 20N. A bit like last year which had an unusually high amount of subtropical storm activity which boosted the ACE index beyond what would be expected given the unfavourable Aug-Sep wind shear and SST anomalies (which is why seasonal forecasts busted).

Ideally to get quality (i.e. strong, pretty on satellite imagery) storms, you want favourable conditions south of 20N and steering winds that bring the storms west toward the Caribbean Sea/Gulf (although that is highly sub-optimal from the perspective of human impacts). It is only really Dorian, and briefly Lorenzo, that found a local area of very favourable conditions and was able to take advantage of them (to the detriment of the Bahamas in the former case).

Curious as to why we have had two years now where the subtropics have been more favourable for hurricanes than the deep tropics. Normally this happens in El Nino years.

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Re: ATL: SEBASTIEN - Tropical Storm

#56 Postby TropicalAnalystwx13 » Wed Nov 20, 2019 6:18 pm

Sebastien is beginning to intensify as the storm's forward trajectory becomes better aligned with the shear vector, and as the approaching upper-level trough promotes enhanced upper-level divergence. The system is likely to near or exceed hurricane threshold before extratropical transition about 48 hours from now.

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Re: ATL: SEBASTIEN - Tropical Storm

#57 Postby Hammy » Wed Nov 20, 2019 6:19 pm

al78 wrote:Curious as to why we have had two years now where the subtropics have been more favourable for hurricanes than the deep tropics. Normally this happens in El Nino years.


That's really been the norm since 2011, it's just that 2017 was a huge exception as far as non-Nino years.
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Re: ATL: SEBASTIEN - Tropical Storm

#58 Postby EquusStorm » Wed Nov 20, 2019 6:29 pm

Interested to see if this burst holds. Probably the best it's looked, but it keeps getting exposed.
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Re: ATL: SEBASTIEN - Tropical Storm

#59 Postby Florida1118 » Wed Nov 20, 2019 10:01 pm

wxman57 wrote:
TallyTracker wrote:NHC is now forecasting Sebastien to approach hurricane strength before becoming extratropical. That would be cool to see a November fish hurricane to end the season!


00Z EC is less bullish on strengthening than 12Z yesterday, about 8-9 mb higher with the min pressure. I have my doubts it'll get anywhere near 60 kts. I don't think it has 45 kt winds now. ASCAT perfectly missed it a couple hours ago, so nothing to estimate winds, except for Dvorak, which is 2.0 from NOAA (depression) and 2.4 from CIMSS (35kt TS). I'd say it may have a small pocket of 35-40 kt winds in the squalls well east of the low-level swirl. Maybe closer to 35 kts than 40 kts. Pretty poorly-organized. I am measuring a track to about 12 degrees at 7.2 kts in the past hour. The northward turn has occurred and it will be accelerating NNE-NE today/tonight. That will result in the relative shear decreasing, which should allow for some strengthening. I'm thinking 45-50 kts peak, though.


...SEBASTIEN COULD BECOME A HURRICANE ON THURSDAY OR FRIDAY...
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Re: ATL: SEBASTIEN - Tropical Storm

#60 Postby Sciencerocks » Wed Nov 20, 2019 10:10 pm

378
WTNT45 KNHC 210253
TCDAT5

Tropical Storm Sebastien Discussion Number 7
NWS National Hurricane Center Miami FL AL202019
1100 PM AST Wed Nov 20 2019

Deep convection has intensified near and to the southeast of the
center of Sebastien this evening, and a recent partial ASCAT
pass suggests the center is still on the edge of the thunderstorms.
Satellite intensity estimates have risen slightly, so the initial
wind speed is set to 50 kt, on the high side of the recent values.

The storm continues to move slowly to the north tonight.
Increasing deep-layer flow ahead of a mid-latitude trough should
turn Sebastien northeastward overnight and cause the cyclone to
further accelerate by late Thursday and Friday. The models aren't
in great agreement on how quickly the storm will move to the
northeast, however, with the model guidance having a stronger storm
generally moving faster to the northeast. Since Sebastien
isn't expected to get that intense, it makes physical sense to
avoid the fastest solutions. Thus, the new forecast is slower than
the last one, closer to the model consensus than the GFS-based
guidance.

The environment is forecast to become more conducive for
strengthening during the next 24 hours as significant upper-level
divergence increases near Sebastien. Combined with low or
moderate shear and warm waters, these conditions should support
further intensification, and the new NHC forecast now shows
Sebastien as a hurricane for a short period of time. Notably, this
forecast is still on the conservative side of the guidance, with all
of the regional hurricane models showing Sebastien becoming a fairly
potent hurricane in a day or two. This doesn't seem likely after
examining the model initial structure of the HWRF/HMON models, which
show a much more vertically aligned cyclone than Sebastien appears
to be now, so the NHC forecast is only a bit higher than the last
one. In a couple of days, the storm will be moving over cooler
waters, with increasing shear and baroclinicity. That should
facilitate Sebastien's transition to a non-tropical cyclone by the
end of the work week, but note that the extratropical transition
has been shifted back about a day due to the stronger-than-forecast
cyclone likely staying a little more separate from an incoming cold
front.


FORECAST POSITIONS AND MAX WINDS

INIT 21/0300Z 22.2N 61.5W 50 KT 60 MPH
12H 21/1200Z 23.3N 60.5W 55 KT 65 MPH
24H 22/0000Z 25.5N 57.7W 65 KT 75 MPH
36H 22/1200Z 28.0N 53.9W 65 KT 75 MPH
48H 23/0000Z 31.0N 49.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
72H 24/0000Z 37.0N 38.0W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 25/0000Z...DISSIPATED

$$
Forecaster Blake
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