ATL: SEAN - Advisories

[cycloneye]

BULLETIN
Tropical Depression Nineteen Advisory Number 1
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Tue Oct 10 2023

...TROPICAL DEPRESSION FORMS IN THE EASTERN TROPICAL ATLANTIC...


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...9.7N 32.0W
ABOUT 690 MI...1115 KM SW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...1008 MB...29.77 INCHES



Tropical Depression Nineteen Discussion Number 1
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Tue Oct 10 2023

The area of disturbed weather (AL92) that NHC has been monitoring
the past several days has acquired enough organization for the
system to be designated as a tropical depression. Although the
ASCAT instrument has missed the system during the past day or so,
geostationary satellite imagery and an earlier SSMIS microwave
overpass suggests that the circulation has become better defined.
Subjective satellite intensity estimates from TAFB and SAB were T2.5
(35 kt), but given the loose nature of the banding, the initial
wind speed is set at 30 kt for this advisory.

Given the large and sprawling current structure of the system,
strengthening is likely to be slow to occur during the next day or
so while it remains within low to moderate shear conditions.
After that time, the cyclone is forecast to approach an upper-level
trough over the central tropical Atlantic which will likely cause an
increase in southwesterly vertical wind shear. As a result, little
change in strength is shown during the middle portion of the
forecast period. Late in the period, decreasing mid-level humidity
is likely to cause additional weakening. The ECMWF and GFS models
show the system losing convection over the weekend, whereas the
UKMET and Canadian keep the system a tropical cyclone for longer.
The NHC forecast shows weakening by day 4 and degeneration into a
remnant low by day 5, but it is possible that the system loses
tropical cyclone status sooner.

The depression is moving westward at a somewhat uncertain 11 kt. A
mid-level ridge located to the northeast of the cyclone should
steer it west-northwestward to northwestward through Friday.
Very late in the forecast period, a bend back toward the
west-northwest could occur as the cyclone weakens and is steered by
the low-level trade wind flow. The models do not differ much on
the overall scenario, but the GFS takes a somewhat stronger system
more poleward within the first few days, while the ECMWF is faster
and farther west. This first NHC track on the depression lies
close to the various consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 11/0300Z 9.7N 32.0W 30 KT 35 MPH
12H 11/1200Z 10.1N 33.7W 35 KT 40 MPH
24H 12/0000Z 10.7N 35.5W 40 KT 45 MPH
36H 12/1200Z 11.4N 37.0W 40 KT 45 MPH
48H 13/0000Z 12.2N 38.7W 40 KT 45 MPH
60H 13/1200Z 13.1N 40.1W 40 KT 45 MPH
72H 14/0000Z 14.2N 41.3W 40 KT 45 MPH
96H 15/0000Z 16.2N 43.3W 35 KT 40 MPH
120H 16/0000Z 17.5N 45.1W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown

[cycloneye]

Tropical Storm Sean Discussion Number 2
NWS National Hurricane Center Miami FL AL192023
500 AM AST Wed Oct 11 2023

Satellite images indicate that deep convection has increased near
the center of the cyclone, with some curved banding features
located both north and south of the center. Subjective and
objective Dvorak estimates from TAFB/SAB/UW-CIMSS range from 30-47
kt, and a conservative blend of this data gives an initial wind
speed of 35 kt for this advisory. Thus Tropical Storm Sean has
formed, the 19th named storm in this busy 2023 Atlantic season.

Sean could intensify a bit more during the next day or so before
increasing southerly shear affects the core of the cyclone. Some
hints of this shear can already be seen in cirrus cloud motions from
convective activity northwest of the tropical cyclone. Thereafter,
moderate-to-strong shear is expected to generally cancel out the
enhancing effects of a relatively moist and unstable environment,
leading to little net change for a few days. By the end of the
week, while the shear could weaken, global models are generally
showing an influx of drier mid-level air along with upper-level
convergence, which would typically weaken any associated deep
convection. Many of the regional hurricane models, however,
actually show some strengthening at long range, but they have been
inconsistent at that time frame. The NHC forecast leans more on
the global models for now, similar to the previous forecast and the
model consensus, and continues to show Sean becoming a remnant low
on day 5.

The storm is moving west-northwestward at about 11 kt. A mid-level
ridge located to the northeast of the cyclone should steer Sean
west-northwestward to northwestward through Friday. Very late in the
forecast period, a bend back toward the west-northwest is
anticipated as the cyclone weakens and is steered by the low-level
trade wind flow. The new NHC track forecast is adjusted a bit west
of the last one at longer range, between the model-consensus and
corrected-consensus aids, which favors the weaker track aids on the
left side of the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 11/0900Z 10.3N 33.1W 35 KT 40 MPH
12H 11/1800Z 10.7N 34.8W 40 KT 45 MPH
24H 12/0600Z 11.2N 36.5W 40 KT 45 MPH
36H 12/1800Z 11.8N 38.0W 40 KT 45 MPH
48H 13/0600Z 12.6N 39.5W 40 KT 45 MPH
60H 13/1800Z 13.5N 40.8W 40 KT 45 MPH
72H 14/0600Z 14.5N 42.0W 35 KT 40 MPH
96H 15/0600Z 16.7N 44.6W 35 KT 40 MPH
120H 16/0600Z 18.0N 47.0W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Blake

[cycloneye]

BULLETIN
Tropical Storm Sean Advisory Number 3
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Wed Oct 11 2023

...SEAN CONTINUES MOVING WEST-NORTHWESTWARD...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...10.9N 34.4W
ABOUT 780 MI...1260 KM WSW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...1006 MB...29.71 INCHES



Tropical Storm Sean Discussion Number 3
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Wed Oct 11 2023

This morning, satellite imagery depicts Sean as a rather
disorganized tropical storm, with some convective bursting occurring
to the northwest and southeast of its elongated center. Subjective
satellite intensity estimates from SAB and TAFB are a consensus
T2.5/35 kt, and thus the initial intensity is held at 35 kt. An
ASCAT-C pass at around 12 UTC suggested that this wind speed value
could be a little generous.

The intensity is forecast to remain at 35 kt over the first 12 h as
the storm already appears embedded in an environment of moderate
wind shear. Thereafter, there may be an opportunity for Sean to
intensify slightly as it moves into a region of lower wind shear
before the current mid-level moisture begin to decrease. By the
weekend, the storm is forecast to gradually weaken due to dry air
entrainment, and is predicted to become a remnant low by the end of
the forecast period. This intensity forecast is in the middle of
the guidance envelope for the majority of the forecast period.

Sean continues to move west-northwestward at about 11 kt, and is
expected to continue moving in this general direction over the next
day or two. After 48 hours, the storm will begin to turn more to the
northwest and decrease in forward speed as it moves along the
southwestern portion of a mid-level ridge to its northeast. By this
weekend, a turn back to the west-northwest or west is anticipated as
a low- to mid-level ridge re-establishes itself to the north of Sean
and it becomes a more shallow cyclone. The NHC track forecast is
similar to the previous one and close to the HCCA track guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 11/1500Z 10.9N 34.4W 35 KT 40 MPH
12H 12/0000Z 11.2N 35.8W 35 KT 40 MPH
24H 12/1200Z 11.7N 37.4W 40 KT 45 MPH
36H 13/0000Z 12.4N 38.9W 40 KT 45 MPH
48H 13/1200Z 13.1N 40.5W 40 KT 45 MPH
60H 14/0000Z 14.1N 41.9W 40 KT 45 MPH
72H 14/1200Z 15.1N 43.0W 35 KT 40 MPH
96H 15/1200Z 16.8N 45.3W 35 KT 40 MPH
120H 16/1200Z 18.1N 48.9W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Camposano/Cangialosi

[cycloneye]

BULLETIN
Tropical Storm Sean Advisory Number 4
NWS National Hurricane Center Miami FL AL192023
500 PM AST Wed Oct 11 2023

...SEAN STRUGGLING TO ORGANIZE...


SUMMARY OF 500 PM AST...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...11.6N 35.3W
ABOUT 815 MI...1315 KM WSW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...WNW OR 295 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...1006 MB...29.71 INCHES



Tropical Storm Sean Discussion Number 4
NWS National Hurricane Center Miami FL AL192023
500 PM AST Wed Oct 11 2023

Sean remains poorly organized this afternoon. Satellite images show
a ragged convective pattern with the low-level center located on the
west side of a fragmented curved band, a bit more north than
previously estimated. The Dvorak estimates remain steady at T2.5/35
kt, so the initial intensity is held at 35 kt. However, as mentioned
this morning, this wind speed estimate could be a little generous
based on the earlier ASCAT pass.

The storm is currently experiencing moderate to strong southwesterly
wind shear, and may only have a brief opportunity to strengthen
over the next couple of days before it moves into an environment of
drier air by the end of the week. Over the weekend, another round of
shear should cause Sean to ultimately weaken and then degenerate
into a remnant low in 4 to 5 days. The NHC intensity forecast has
not changed much from the previous advisory and is within the
guidance envelope.

Sean is moving west-northwestward at about 11 kt, and is expected to
continue moving in this general direction over the next day or two.
After 48 hours, the storm will begin to turn to the northwest as it
moves along the southwestern portion of a mid-level ridge to its
northeast. Over the weekend, a turn back to the west-northwest or
west is anticipated as a low- to mid-level ridge re-establishes
itself to the north of Sean and it becomes a more shallow cyclone.
The NHC track forecast has been shifted northward of the previous
one over the entire forecast period based on the initial position
and subsequent model guidance.

FORECAST POSITIONS AND MAX WINDS

INIT 11/2100Z 11.6N 35.3W 35 KT 40 MPH
12H 12/0600Z 12.1N 36.5W 35 KT 40 MPH
24H 12/1800Z 12.7N 38.1W 40 KT 45 MPH
36H 13/0600Z 13.4N 39.7W 40 KT 45 MPH
48H 13/1800Z 14.3N 41.1W 40 KT 45 MPH
60H 14/0600Z 15.3N 42.3W 35 KT 40 MPH
72H 14/1800Z 16.4N 43.6W 35 KT 40 MPH
96H 15/1800Z 18.1N 46.1W 30 KT 35 MPH
120H 16/1800Z 18.7N 50.2W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Camposano/Cangialosi

[cycloneye]

BULLETIN
Tropical Depression Sean Advisory Number 5
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Wed Oct 11 2023

...SEAN A TROPICAL DEPRESSION ACCORDING TO SATELLITE-DERIVED WIND
DATA...
...UNLIKELY TO GET MUCH STRONGER IN THE COMING DAYS...


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...12.0N 36.2W
ABOUT 860 MI...1390 KM WSW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...WNW OR 295 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...1007 MB...29.74 INCHES




Tropical Depression Sean Discussion Number 5
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Wed Oct 11 2023

Sean is a highly sheared tropical cyclone, with the center located
well to the west of the associated deep convection. Similar to what
was observed this morning, two recent ASCAT passes showed maximum
winds a little over 25 kt and a poorly defined surface circulation.
Based on these data, Sean is being designated as a 30-kt tropical
depression.

The initial motion remains west-northwestward (295 degrees) at 11
kt. Weak mid-level ridging to the north is expected to keep Sean on
a west-northwestward or northwestward heading for the next several
days, with some decrease in forward speed. The NHC track forecast
is very close to the previous prediction through 48 hours, but then
deviates a little to the right on days 3 and 4. The GFS and ECMWF
are both on the far right side of the guidance envelope, and it's
possible that additional rightward adjustments could be possible in
subsequent forecasts.

Strong deep-layer shear out of the west or west-southwest is
expected to continue for the next 36 hours or so, with some
abatement possible after that. By then, however, drier air will
have infiltrated the circulation, and Sean will begin to struggle to
maintain organized deep convection. Due to the hostile atmosphere,
and based on the latest intensity guidance, Sean is held as a 30-kt
depression for the next 60 hours, with degeneration to a remnant low
by day 3. Dissipation to a trough is now forecast by day 5. Some
of the global models suggest that each of these transitions could
occur even sooner than what is shown in the official forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 12/0300Z 12.0N 36.2W 30 KT 35 MPH
12H 12/1200Z 12.4N 37.4W 30 KT 35 MPH
24H 13/0000Z 13.1N 39.0W 30 KT 35 MPH
36H 13/1200Z 14.0N 40.5W 30 KT 35 MPH
48H 14/0000Z 15.0N 42.0W 30 KT 35 MPH
60H 14/1200Z 16.1N 43.1W 30 KT 35 MPH
72H 15/0000Z 17.2N 44.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 16/0000Z 18.5N 46.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 17/0000Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Tropical Depression Sean Discussion Number 6
NWS National Hurricane Center Miami FL AL192023
500 AM AST Thu Oct 12 2023

Sean remains a sheared, poorly organized tropical cyclone. Recent
AMSR2 passive microwave images show the surface circulation lies
well to the west of the associated deep convection. The lackluster
satellite presentation of Sean has resulted in decreasing subjective
Dvorak data-T numbers from TAFB and SAB, which supports keeping the
system a tropical depression. The initial intensity is held at 30
kt, consistent with the ASCAT data from yesterday evening.

The depression is moving west-northwestward at 300/10 kt. A weak
low- to mid-level ridge to the north and northeast of Sean should
steer the cyclone to the west-northwest and northwest through early
next week. A slight rightward adjustment was made to the NHC track
forecast to reflect the latest multi-model consensus trends. But
otherwise, the updated forecast is similar to the previous
prediction.

Moderate to strong westerly shear is expected to persist over the
depression for the next day or two. By the time the shear relents,
simulated satellite images from the GFS and ECMWF models suggest
that Sean could struggle to sustain organized convection near its
center within a drier and more convergent upper-level environment.
While small intensity fluctuations cannot be ruled out during the
next couple of days, the NHC intensity forecast follows the HCCA and
IVCN aids and keeps Sean a depression through 60 h. Sean is forecast
to degenerate to a post-tropical remnant low soon thereafter and
dissipate to a trough by day 4, although this could occur even
sooner if the cyclone remains poorly organized.


FORECAST POSITIONS AND MAX WINDS

INIT 12/0900Z 12.5N 36.7W 30 KT 35 MPH
12H 12/1800Z 13.0N 38.0W 30 KT 35 MPH
24H 13/0600Z 13.8N 39.6W 30 KT 35 MPH
36H 13/1800Z 14.8N 41.2W 30 KT 35 MPH
48H 14/0600Z 16.0N 42.6W 30 KT 35 MPH
60H 14/1800Z 17.3N 43.9W 25 KT 30 MPH
72H 15/0600Z 18.2N 45.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 16/0600Z...DISSIPATED

$$
Forecaster Reinhart

[cycloneye]

Tropical Storm Sean Discussion Number 7
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Thu Oct 12 2023

Sean remains a poorly organized tropical cyclone on visible and
infrared satellite imagery this morning. The low-level center is
exposed on GOES-16 1-minute visible imagery, and displaced to the
west of the main convective band. The overall satellite appearance
has resulted in subjective Dvorak data-T numbers of T1.5/T2.0 from
TAFB and SAB, respectively. However, recent scatterometer passes
from both ASCAT-B and ASCAT-C depict a swath of
tropical-storm-force winds to the northeast of the center. Given
this scatterometer data, the initial intensity is raised back to 35
kt for this advisory, making Sean a tropical storm once again.

The tropical storm is moving northwestward at 305/8 kt. A weak low-
to mid-level ridge to the north and northeast of Sean should steer
the cyclone west-northwestward or northwestward throughout the
forecast period. There was a nudge to the right with the latest
model track guidance, and the NHC track has been adjusted slightly
right accordingly, and lies near the simple and corrected consensus
aids.

Westerly vertical wind shear is expected to persist over the system
for the next day or so. This shear is forecast to then decrease
along Sean's forecast track. However, the system will also be moving
into a drier airmass and less favorable upper-level environment.
Simulated satellite imagery from the GFS and ECMWF models suggest
that Sean could struggle to produce organized convection in the
coming days. While small intensity fluctuations are possible, as
seen this morning, the NHC intensity forecast remains near the HCCA
and IVCN consensus aids, keeping Sean a tropical storm the next day
or so, then weakening back into a depression once again. Afterwards,
the system is forecast to degenerate into a post-tropical remnant
low and dissipate into a trough by day 4. Some of the global models
suggest that each of these transitions could occur even sooner than
what is shown in the official forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 12/1500Z 13.1N 37.2W 35 KT 40 MPH
12H 13/0000Z 13.5N 38.4W 35 KT 40 MPH
24H 13/1200Z 14.4N 40.1W 35 KT 40 MPH
36H 14/0000Z 15.6N 41.6W 30 KT 35 MPH
48H 14/1200Z 16.9N 42.9W 30 KT 35 MPH
60H 15/0000Z 18.2N 44.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 15/1200Z 19.0N 45.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 16/1200Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

BULLETIN
Tropical Storm Sean Advisory Number 8
NWS National Hurricane Center Miami FL AL192023
500 PM AST Thu Oct 12 2023

...SEAN MOVING NORTHWESTWARD...


SUMMARY OF 500 PM AST...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...13.8N 38.2W
ABOUT 960 MI...1545 KM W OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...NW OR 310 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1006 MB...29.71 INCHES





Tropical Storm Sean Discussion Number 8
NWS National Hurricane Center Miami FL AL192023
500 PM AST Thu Oct 12 2023

Sean remains fairly disorganized on satellite imagery this
afternoon. The low-level center is still exposed, however there has
been a recent burst of convection closer to the center. Subjective
Dvorak intensity estimates have increased since the previous cycle,
with final-T numbers of T2.5/T2.0 from TAFB and SAB, respectively.
These estimates are in fairly good agreement with the scatterometer
data from earlier. Therefore, the initial intensity for this
advisory is held at 35 kt.

The tropical storm is moving at a slightly faster northwestward
motion at 310/10 kt, gaining a little more latitude since this
morning. A weak ridge to the northeast of the system will continue
to steer Sean northwestward to west-northwestward throughout the
forecast period. Model guidance remains in fairly good agreement
with this scenario. There was a slight shift to the right in the
short-term center positions to account for the slightly faster
northwestward motion. Otherwise, the track forecast is similar to
the previous and lies near the consensus aids.

Westerly vertical wind shear is expected to persist over the system
for the next day or so, before decreasing along Sean's forecast
track. However, the system will be moving into an area with drier
mid-level RH values, a more stable environment, and a less favorable
upper-level wind pattern. Simulated satellite imagery from the GFS
and ECMWF models suggest that Sean could struggle to produce
organized convection in the coming days. While small intensity
fluctuations are possible, the NHC intensity forecast remains near
the HCCA and IVCN consensus aids, keeping Sean a tropical storm for
the next day or so, then weakening it back into a depression.
Afterwards, the system is forecast to degenerate into a
post-tropical remnant low as it becomes devoid of convection, and
dissipate into a trough by day 4. Although, some global models
depict that these transitions could occur even sooner than forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 12/2100Z 13.8N 38.2W 35 KT 40 MPH
12H 13/0600Z 14.5N 39.5W 35 KT 40 MPH
24H 13/1800Z 15.5N 41.2W 35 KT 40 MPH
36H 14/0600Z 16.8N 42.6W 30 KT 35 MPH
48H 14/1800Z 18.0N 43.8W 30 KT 35 MPH
60H 15/0600Z 19.0N 44.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 15/1800Z 19.9N 45.9W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 16/1800Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Tropical Storm Sean Discussion Number 9
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Thu Oct 12 2023

Sean continues to be a sheared tropical cyclone. The exposed
low-level center is gradually becoming more displaced from the
deepest convection in the northeast quadrant. Surprisingly,
satellite wind data that partially captured the center of the storm
revealed winds of 38-39 kt. Therefore, despite Sean's ragged
appearance, the initial intensity is raised to 40 kt for this
advisory.

The cyclone is accelerating west-northwestward at an estimated
300/12 kt. Sean is being steered by a weak ridge centered over the
eastern tropical Atlantic. This ridge is expected to build westward
and keep Sean on a west-northwestward to northwestward track with a
decrease in forward speed through the forecast period. The
guidance envelope is decently clustered, and the latest advisory
lies close to the various consensus aids and just south of the
previous track forecast.

Environmental conditions are generally not conducive for additional
strengthening. Persistent moderate-to-strong deep-layer vertical
wind shear and an increasingly dry airmass should inhibit further
development of the storm. The model guidance envelope shows Sean
gradually weakening for the next couple of days. By the end of the
weekend, the storm is expected to lose its convective organization
and become a remnant low. The latest NHC intensity prediction has
been slightly raised in the near-term forecast hours based on
Sean's initial intensity. Dissipation is still anticipated within
4 days.


FORECAST POSITIONS AND MAX WINDS

INIT 13/0300Z 14.1N 39.5W 40 KT 45 MPH
12H 13/1200Z 14.9N 40.8W 40 KT 45 MPH
24H 14/0000Z 16.1N 42.3W 35 KT 40 MPH
36H 14/1200Z 17.3N 43.6W 35 KT 40 MPH
48H 15/0000Z 18.4N 44.7W 30 KT 35 MPH
60H 15/1200Z 19.2N 45.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 16/0000Z 20.1N 46.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 17/0000Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Tropical Storm Sean Discussion Number 10
NWS National Hurricane Center Miami FL AL192023
500 AM AST Fri Oct 13 2023

Moderate westerly deep-layer shear continues over Sean this morning,
and most of the associated deep convection is displaced to the east
of Sean's center. Although the low-level circulation remains at
least partially exposed in proxy-visible satellite images, new
bursts of deep convection have recently developed closer to the
center of Sean. It remains to be seen whether this activity will be
persistent enough to support any additional strengthening. The
latest objective and subjective satellite intensity estimates range
from 30-45 kt, and a blend of these data support keeping the initial
intensity at 40 kt.

Sean is moving west-northwestward (285 degrees) at 11 kt. The
general track forecast reasoning has not changed. Sean should move
west-northwestward to northwestward over the next few days while
being steered by a weak subtropical ridge over the eastern and
central Atlantic. There are no significant changes noted in the
latest track guidance. The updated NHC forecast lies slightly to the
left of the previous prediction, primarily the result of a more
west-northwestward initial motion based on recent center fixes.

The various global and regional models suggest Sean is likely near
or at its peak intensity. Although the shear could weaken some
during the next couple of days, the convective structure of Sean is
unlikely to improve much while the storm gains latitude and moves
into a drier, more stable environment. The NHC forecast continues to
show gradual weakening during the next few days, in good agreement
with the latest multi-model consensus aids. The global models show
an even faster rate of weakening, as the system becomes devoid of
organized convection by early next week in both GFS and ECMWF
model-simulated satellite imagery. Sean is still forecast to open
into a trough and dissipate between 72-96 h.


FORECAST POSITIONS AND MAX WINDS

INIT 13/0900Z 14.3N 40.5W 40 KT 45 MPH
12H 13/1800Z 15.1N 41.6W 40 KT 45 MPH
24H 14/0600Z 16.4N 42.9W 35 KT 40 MPH
36H 14/1800Z 17.6N 44.2W 35 KT 40 MPH
48H 15/0600Z 18.4N 45.3W 30 KT 35 MPH
60H 15/1800Z 19.2N 46.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 16/0600Z 19.7N 47.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 17/0600Z...DISSIPATED

$$
Forecaster Reinhart

[cycloneye]

Tropical Storm Sean Discussion Number 11
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Fri Oct 13 2023

Sean's structure and overall satellite appearance have begun to
degrade. Deep convection associated with the tropical storm is
limited in both strength and coverage as a result of dry surrounding
air and continued westerly wind shear, and has decreased noticeably
in the last few hours. Recent intensity estimates range from about
25 to 45 kt, but are generally lower than they were for the previous
advisory. The initial intensity is therefore lowered to 35 kt for
this advisory.

Continued gradual weakening is expected for the next several days as
Sean moves through a stable environment. Simulated satellite output
from several models suggests that Sean may continue to produce
occasional bursts of convection for another day or two, but these
should become less organized in time. This should cause Sean to
become a post-tropical remnant low by Sunday, but recent trends
suggest this could happen much sooner. Sean's wind's should
gradually decrease as its convection wanes, and it is forecast to
open into a trough by Monday.

The tropical storm has continued to move west-northwestward for the
last few hours. A turn northwestward is forecasted by every
available dynamical model and is expected to begin at any time, but
since Sean hasn't turned yet, the NHC track forecast had to be
adjusted westward again with this advisory. Otherwise, no
significant changes were made to the forecast reasoning, with the
models in good agreement on Sean's track for the next few days until
it dissipates.


FORECAST POSITIONS AND MAX WINDS

INIT 13/1500Z 14.6N 41.7W 35 KT 40 MPH
12H 14/0000Z 15.3N 42.6W 35 KT 40 MPH
24H 14/1200Z 16.6N 44.0W 30 KT 35 MPH
36H 15/0000Z 17.5N 45.0W 30 KT 35 MPH
48H 15/1200Z 18.1N 46.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 16/0000Z 18.6N 47.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 16/1200Z...DISSIPATED

$$
Forecaster D. Zelinsky

[cycloneye]

Tropical Storm Sean Discussion Number 12
NWS National Hurricane Center Miami FL AL192023
500 PM AST Fri Oct 13 2023

Sean's satellite appearance continues to deteriorate this afternoon.
Convection has waned with warming cloud tops to the east of the
exposed low-level center. Sean is noticeably struggling with drier
air impacting the system and the continued westerly wind shear. Any
convection that tries to develop near the center is sheared off and
dissipates. Satellite intensity estimates this afternoon range from
25 to 40 kt. The initial intensity is held at 35 kt for this
advisory, but this could be generous given the waning convection the
last several hours.

As Sean moves through the drier and more stable airmass, further
weakening is anticipated. Simulated satellite from the ECMWF and GFS
depict that Sean may have occasional burst of convection the next
day or so, but will continue to become less organized. This should
cause Sean to become a post-tropical remnant low this weekend.
However, recent trends suggest this could occur as early as
tomorrow. The NHC intensity forecast is similar to the previous
advisory, near the consensus intensity aids.

The tropical storm has continued to move west-northwestward today,
around 290 at 10 kt. A turn northwestward has been forecast to occur
today, but the center has continued west-northwestward, which has
resulted in a westward shift of the NHC forecast track. The forecast
still calls for the northwestward motion to begin later this evening
and continue over the next day or so. As Sean becomes a remnant low
and further weakens, the system will turn more west-northwestward
again and dissipate into an open trough in a few days.


FORECAST POSITIONS AND MAX WINDS

INIT 13/2100Z 14.9N 42.5W 35 KT 40 MPH
12H 14/0600Z 15.8N 43.6W 30 KT 35 MPH
24H 14/1800Z 16.9N 44.7W 30 KT 35 MPH
36H 15/0600Z 17.7N 45.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 15/1800Z 18.4N 47.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 16/0600Z 18.8N 48.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 16/1800Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Tropical Storm Sean Discussion Number 13
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Fri Oct 13 2023

Sean is still showing signs of life with a recent burst of
convection in the northeast quadrant obscuring the surface
circulation. Unfortunately, no new satellite surface wind data is
available this evening. Objective and subjective Dvorak estimates
range between 25-45 kt. The initial intensity is held at a possibly
generous 35 kt, which is closest to the TAFB final T-number of T2.5.

The storm has noticeably slowed since the last advisory, with an
estimated motion of 290/7 kt. The model guidance continues to
insist Sean will turn northwestward on the southwest side of a weak
ridge over the eastern Atlantic. However, this has yet to occur.
As the cyclone weakens and becomes more shallow, it is expected to
bend back to west-northwest. Once again, the latest NHC track
forecast was shifted south of the previous prediction, and favors
the southern side of the guidance envelope due to the recent
northward bias of the models.

There have been no changes to the official intensity forecast.
Sean is expected to continue to weaken in the relatively hostile
atmospheric conditions and become a remnant low later this weekend.
Most model guidance shows the storm opening into a trough early
next week, and the NHC forecast shows dissipation on Monday.


FORECAST POSITIONS AND MAX WINDS

INIT 14/0300Z 15.1N 42.9W 35 KT 40 MPH
12H 14/1200Z 15.8N 43.8W 30 KT 35 MPH
24H 15/0000Z 16.7N 44.9W 30 KT 35 MPH
36H 15/1200Z 17.5N 46.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 16/0000Z 18.0N 47.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 16/1200Z 18.4N 49.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 17/0000Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Tropical Storm Sean Discussion Number 14
NWS National Hurricane Center Miami FL AL192023
500 AM AST Sat Oct 14 2023

Sean has been producing bursts of deep convection overnight that
have obscured the center of the cyclone. The bursts do not appear
to have much organization, and the latest subjective and objective
satellite intensity estimates range from about 25 to 40 kt. The
initial intensity for this advisory is kept at 35 kt, which is in
agreement with the latest TAFB Dvorak CI number of T2.5 (35 kt).
However, this could be generous.

Sean is moving west-northwestward or 300 degrees at about 7 kt.
The cyclone should turn northwestward today around the southwestern
periphery of a narrow ridge over the eastern-central Atlantic. A
turn back toward the west-northwest is anticipated on Sunday when
Sean weakens and becomes more vertically shallow. There is a fair
amount of spread in the track guidance that is related to how
quickly Sean weakens. The ECMWF which is the weaker of the
dynamical models takes the cyclone on a more southerly track. The
NHC forecast is a little south of the previous advisory, especially
at 48 and 60 hours, and lies near the middle of the guidance
envelope.

Sean is forecast to encounter an increasingly dry mid-level
environment during the next couple of days, which should cause
gradual weakening during that time. The global model guidance
indicates that Sean will struggle to produce deep convection by
later today and Sunday, and it seems likely that the system will
degenerate into a remnant low by 36 hours, if not sooner. The
circulation is expected to open up into a trough in 60 to 72 hours,
and dissipation is indicated in the official forecast by day 3.


FORECAST POSITIONS AND MAX WINDS

INIT 14/0900Z 15.4N 43.4W 35 KT 40 MPH
12H 14/1800Z 16.1N 44.2W 30 KT 35 MPH
24H 15/0600Z 16.9N 45.4W 30 KT 35 MPH
36H 15/1800Z 17.5N 47.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 16/0600Z 17.9N 48.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 16/1800Z 18.1N 50.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 17/0600Z...DISSIPATED

$$
Forecaster Brown

[cycloneye]

Tropical Depression Sean Discussion Number 15
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Sat Oct 14 2023

Sean continues to have a lackluster satellite appearance, with
intermittent burst of convection near the center this morning.
Earlier noted convective bursts were fairly short lived, and didn't
have much organization. Latest subjective and objective satellite
intensity estimates range from 25 to 35 kt. With final-T numbers
from SAB and TAFB both T1.5, with CI numbers of T1.5/T2.5,
respectively. Given a blend of these values and the current
satellite imagery, the initial intensity for this advisory is
lowered to 30 kt.

Sean is moving northwestward or 305 degrees at about 7 kt. The
cyclone should continue to move northwestward today around the
southwestern edge of a mid-level ridge. On Sunday, a turn back
toward the west-northwest is anticipated as Sean continues to
weaken into a more shallow vortex and thus will be steered by the
low-level wind flow. How quickly Sean weakens will depict when the
system turns back west-northwestwards. The NHC forecast is similar
to the previous advisory and lies near the middle of the guidance
envelope.

Sean is moving into an increasingly dry mid-level airmass, which is
well depicted on GOES-16 water mid-level water vapor imagery. This
more stable and dry environment should cause additional weakening
during the next couple of days. Global models depict that Sean will
struggle to produce deep convection by this evening and into Sunday.
The time of the system degenerating into a remnant low has been
moved up to 24 hours, but that could occur even sooner. The
circulation is expected to open up into a trough in 60 hours, and
dissipation is now indicated in the official forecast by that time.


FORECAST POSITIONS AND MAX WINDS

INIT 14/1500Z 16.0N 44.2W 30 KT 35 MPH
12H 15/0000Z 16.6N 45.0W 30 KT 35 MPH
24H 15/1200Z 17.2N 46.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 16/0000Z 17.7N 48.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 16/1200Z 17.9N 50.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 17/0000Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Tropical Depression Sean Discussion Number 16
NWS National Hurricane Center Miami FL AL192023
500 PM AST Sat Oct 14 2023

Sean has had another deep convective burst near the center of
circulation this afternoon. The system continues to fend off dry air
with these convective burst from time to time. Latest subjective
Dvorak intensity satellite estimates are T2.0 from both TAFB and
SAB. Using these estimates and the recent burst of convection
the initial intensity for this advisory remains 30 kt.

Sean is moving northwestward or 310 degrees at about 8 kt. The
system will continue to move northwestward today and tonight around
the southwestern periphery of a mid-level ridge. On Sunday, a turn
toward the west-northwest is anticipated as Sean is forecast to
weaken into a more shallow vortex, and thus will be steered by the
low-level flow. This turn is dependent on just how quickly Sean
weakens. The NHC forecast is an update of the previous one, with no
real changes to the forecast track.

Although Sean is moving into a drier mid-level airmass, low wind
shear and warm sea surface temperatures have allowed the system to
maintain convective bursts the past day or so. Along the forecast
track, the environment is forecast to become increasingly drier and
more stable. This should cause Sean to further weaken the next
couple of days. Global models continue to depict that Sean will
struggle to produce deep convection tonight and into Sunday. The
system is forecast to be devoid of convection and become a remnant
low in about 24 hours, and then the circulation is expected to open
up into a trough and dissipate in about 60 hours.


FORECAST POSITIONS AND MAX WINDS

INIT 14/2100Z 16.5N 44.8W 30 KT 35 MPH
12H 15/0600Z 17.0N 45.8W 30 KT 35 MPH
24H 15/1800Z 17.5N 47.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 16/0600Z 18.0N 49.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 16/1800Z 18.4N 51.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 17/0600Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Tropical Depression Sean Discussion Number 17
NWS National Hurricane Center Miami FL AL192023
1100 PM AST Sat Oct 14 2023

Deep convection continues to burst near Sean's center, and as a
result, Dvorak estimates from TAFB and SAB remain T2.0/30 kt. In
addition, a recent ASCAT-B pass showed winds a little over 25 kt,
and all these estimates support maintaining Sean as a 30-kt
tropical depression. With relatively low vertical shear, warm sea
surface temperatures of 28-29 degrees Celsius, and an unstable
atmosphere, the environment is just conducive enough for additional
bursts of convection. The biggest limiting factor is a lack of
mid-level moisture, with relative humidities running about 50
percent. Consequently, the convection is likely to become less
persistent and less organized, and global models indicate that
Sean's small circulation should open up into a trough over the next
day or two. The NHC forecast shows Sean degenerating into a remnant
low in 24 hours and then dissipating by 48 hours, but it's also
possible that the system remains a tropical depression right up
until it opens up into a trough.

Sean continues to move northwestward, or 305/7 kt. As it becomes a
weaker system, the depression is expected to become increasingly
steered by lower-level ridging. As a result, Sean is expected to
turn west-northwestward overnight and then westward by early Monday,
just before or as it degenerates into a trough. The NHC track
forecast is very close to the previous prediction and the latest
TVCA multi-model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT 15/0300Z 16.9N 45.5W 30 KT 35 MPH
12H 15/1200Z 17.3N 46.6W 30 KT 35 MPH
24H 16/0000Z 17.7N 48.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 16/1200Z 18.1N 50.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 17/0000Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Tropical Depression Sean Discussion Number 18
NWS National Hurricane Center Miami FL AL192023
500 AM AST Sun Oct 15 2023

Deep convection associated with Sean has dissipated overnight, and
within the past hour or so the circulation center has become easier
to pinpoint as the mid- and high-level clouds that were obscuring
it have thinned. The latest Dvorak Current Intensity (CI) numbers
from both TAFB and SAB are T2.0/30 kt but the Data-T numbers have
fallen even more. Based on the Dvorak CI numbers Sean's initial
intensity is held at 30 kt, but this could be generous.

Although Sean is located within an area of relatively low vertical
wind shear and over warm waters, a surrounding dry mid-level
environment is expected to cause weakening during the next day or
so. The system may continue to produce intermittent bursts of
convection, but these are likely to become less persistent and even
less organized than they have been over the past day or so. As a
result, the NHC forecast now calls for Sean to degenerate into a
remnant low as soon as today. An alternate scenario is that Sean
continues to produce enough organized convection to remain a
tropical depression until the small circulation opens up into a
trough in a day or so.

Sean appears to have turned west-northwestward. As the cyclone
weakens and becomes increasingly vertically shallow, its motion is
forecast to bend westward within the low-level steering flow. The
NHC track is similar to the previous advisory and is near the
middle of the latest guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 15/0900Z 17.4N 46.2W 30 KT 35 MPH
12H 15/1800Z 17.7N 47.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 16/0600Z 18.0N 49.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 16/1800Z 18.1N 51.7W 20 KT 25 MPH...POST-TROP/REMNT LOW
48H 17/0600Z...DISSIPATED

$$
Forecaster Brown

[cycloneye]

Tropical Depression Sean Discussion Number 19
NWS National Hurricane Center Miami FL AL192023
1100 AM AST Sun Oct 15 2023

Satellite imagery depicts that Sean has regained convection once
again this morning. Visible imagery shows that the low-level center
is displaced on the northern side of the recent convective burst.
The latest Dvorak Current Intensity (CI) from TAFB is T2.0/30 kt,
with the trend of the data-T numbers continuing to fall. A recent
scatterometer pass indicated peak winds around 20-25 kt. Given the
satellite-derived winds and lowering Data-T Dvorak intensity
estimates, the initial intensity is lowered to 25 kt for this
advisory.

Sean continues to produce intermittent convective bursts as it
remains in a low vertical wind shear environment over warm
sea surface temperatures. However, dry mid-level air will
eventually cause the convection to become less persistent and lose
organization. As the system decays, Sean should become a remnant
low, which could occur tonight. The NHC official forecast is
similar to the previous, although it is also possible, that Sean
remains a depression until the circulation opens up into a trough.

The depression has turned west-northwest with an estimated motion of
290/9 kt. As the system weakens and becomes vertically shallow, a
turn more westward is forecast within the low-level steering flow.
The NHC track is similar to the previous advisory and is near the
corrected and simple consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 15/1500Z 17.7N 47.1W 25 KT 30 MPH
12H 16/0000Z 18.0N 48.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 16/1200Z 18.2N 50.7W 20 KT 25 MPH...POST-TROP/REMNT LOW
36H 17/0000Z 18.5N 53.1W 20 KT 25 MPH...POST-TROP/REMNT LOW
48H 17/1200Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Tropical Depression Sean Discussion Number 20
NWS National Hurricane Center Miami FL AL192023
500 PM AST Sun Oct 15 2023

Sean continues to produce bursts of convection this afternoon to
the south of the low-level center. An earlier scatterometer pass
depicted that winds had weakened to 20-25 kt. Dvorak satellite
intensity estimates continue to decrease as well with a T1.0/25 kt
from TAFB this cycle. Using the previous scatterometer data and the
current intensity estimate, the initial intensity remains 25 kt for
this advisory.

Sean continues to be resilient and produce convective bursts from
time to time. The dry mid-level air has not been able to fully
suppress the convection, even though the convection is becoming less
organized. The convective pattern could become unorganized enough to
no longer declare Sean a tropical cyclone soon. Model simulated
satellite imagery continues to insist that the system will gradually
decay and become devoid of convection. The NHC forecast is similar
to the previous, with Sean forecast to become a post-tropical
remnant low tonight, although the system may hold on until the
circulation opens up into a trough in about 36 hours.

The depression is moving west-northwestward with an estimated
motion of 285/10 kt. As the system weakens and becomes vertically
shallow, a turn more westward is forecast within the low-level
steering flow. The NHC track is similar to the previous advisory and
is near the corrected and simple consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 15/2100Z 18.0N 48.2W 25 KT 30 MPH
12H 16/0600Z 18.2N 49.9W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 16/1800Z 18.4N 52.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
36H 17/0600Z...DISSIPATED

$$
Forecaster Kelly