ATL: SAM - Advisories

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Re: ATL: SAM - Advisories

#41 Postby cycloneye » Sat Oct 02, 2021 9:53 am

BULLETIN
Hurricane Sam Advisory Number 40
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Sat Oct 02 2021

...STEADFAST SAM MAINTAINING CATEGORY 4 WINDS...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...33.9N 59.3W
ABOUT 335 MI...540 KM ENE OF BERMUDA
ABOUT 940 MI...1515 KM SSW OF CAPE RACE NEWFOUNDLAND
MAXIMUM SUSTAINED WINDS...130 MPH...215 KM/H
PRESENT MOVEMENT...NE OR 35 DEGREES AT 17 MPH...28 KM/H
MINIMUM CENTRAL PRESSURE...945 MB...27.91 INCHES


WATCHES AND WARNINGS
--------------------
There are no coastal watches or warnings in effect.


DISCUSSION AND OUTLOOK
----------------------
At 1100 AM AST (1500 UTC), the center of Hurricane Sam was located
near latitude 33.9 North, longitude 59.3 West. Sam is moving toward
the northeast near 17 mph (28 km/h). This general track with an
increase in forward speed is expected for the next few days. On
the forecast track, Sam will continue to move away from Bermuda.

Maximum sustained winds remain near 130 mph (215 km/h) with higher
gusts. Sam is a category 4 hurricane on the Saffir-Simpson
Hurricane Wind Scale. Some slow weakening is forecast during the
next 36 hours, followed by more significant weakening early next
week. Sam could become a powerful post-tropical cyclone on Tuesday.

Sam is becoming a large tropical cyclone. Hurricane-force winds
extend outward up to 65 miles (100 km) from the center and
tropical-storm-force winds extend outward up to 240 miles (390 km).

The estimated minimum central pressure is 945 mb (27.91 inches).


HAZARDS AFFECTING LAND
----------------------
Key messages for Sam can be found in the Tropical Cyclone Discussion
under AWIPS header MIATCDAT3 and WMO header WTNT43 KNHC, and on the
web at hurricanes.gov/graphics_at3.shtml?key_messages.

SURF: Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, the Bahamas, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


NEXT ADVISORY
-------------
Next complete advisory at 500 PM AST.

$$
Forecaster Blake




Hurricane Sam Discussion Number 40
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Sat Oct 02 2021

Sam has been an impressive hurricane during the past several days.
The latest satellite imagery shows it remains a powerful system,
with a well-defined eye and symmetric cloud pattern. In fact,
since the plane left overnight, the eye has actually warmed, with
little change in the eyewall convection. Thus the initial intensity
will stay 115 kt, above the latest satellite classifications, but
consistent with the earlier aircraft data and low bias of the
Dvorak technique for much of the this storm. Sam is now in the top
10 for consecutive days as a category 4 hurricane or greater in
the historical record, about the same as Matthew 2016.

The hurricane is forecast to gradually lose strength during the
next 36 hours or so while it remains over marginally warm waters,
but in fairly light shear. In fact, some of the guidance decrease
the shear overnight, which should allow Sam to keep much of its
strength, provided it doesn't undertake an eyewall cycle. The new
intensity forecast is higher in the first day or so, consistent
with the latest model solutions. In about 2 days, Sam will cross
the north wall of the Gulf Stream, and in 3 days is expected to
spectacularly transition into a large hurricane-force post-tropical
cyclone over the far north Atlantic. No changes were made to the
end of the intensity forecast.

Sam has turned northeastward at about 15 kt. The hurricane should
accelerate later this weekend due to increasing flow between a
deep-layer ridge to the east and a large mid- to upper-level low
over Atlantic Canada. By midweek the system is forecast to turn
more to the north-northeast as it becomes a spoke in a very large
extratropical low over the far North Atlantic. The new forecast has
shifted somewhat eastward during the first couple of days but ends
up very near the last advisory by day 5.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States coast and Atlantic Canada for the next couple of days.
These swells could cause life-threatening surf and rip current
conditions. Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 02/1500Z 33.9N 59.3W 115 KT 130 MPH
12H 03/0000Z 35.6N 57.8W 105 KT 120 MPH
24H 03/1200Z 37.4N 55.3W 100 KT 115 MPH
36H 04/0000Z 39.2N 52.0W 95 KT 110 MPH
48H 04/1200Z 41.8N 47.3W 85 KT 100 MPH
60H 05/0000Z 46.0N 42.0W 80 KT 90 MPH
72H 05/1200Z 50.0N 38.5W 75 KT 85 MPH...POST-TROP/EXTRATROP
96H 06/1200Z 53.0N 33.0W 55 KT 65 MPH...POST-TROP/EXTRATROP
120H 07/1200Z 59.0N 26.0W 45 KT 50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake
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Re: ATL: SAM - Advisories

#42 Postby cycloneye » Sat Oct 02, 2021 3:44 pm

Hurricane Sam Discussion Number 41
NWS National Hurricane Center Miami FL AL182021
500 PM AST Sat Oct 02 2021

The hurricane is gradually losing strength. Satellite images show
that the eye has become more cloud filled, with Sam's ring of deep
convection becoming less intense and two main towers in the
southeast and northwest quadrants. Still, the tropical cyclone is
quite a sight even on full-disk images with a large distinctive
cloud pattern. The latest intensity estimates have decreased, so
the initial wind speed is set to 110 kt, with higher uncertainty
than average in this figure due to recent recon/satellite
differences.

Sam wisely has avoided much shear during its 10-day journey across
the Atlantic, and it could remain a major hurricane for another
day or so. However, in 36 hours or so, the system should move
north of the Gulf Stream, which would normally cause a dramatic
weakening. But in this case, a favorable mid-latitude trough
interaction should cause Sam to transition into a large and
powerful extratropical low by 60 hours. After that, it loses its
baroclinic forcing and should gradually spin down over the far North
Atlantic southwest of Iceland. Guidance is in fairly solid
agreement on this scenario, and the new intensity forecast is
mostly just an update of the previous one, remaining close to the
consensus.

The system continues moving northeastward at about 15 kt. Sam
should move faster to the northeast by Monday as it encounters
stronger wind flow between a deep-layer ridge to the east and a
large mid- to upper-level low over Atlantic Canada. The track will
weave leftward a bit at longer ranges due to the upper trough
pulling the extratropical system north-northeastward at times, and
eventually another trough has a similar tug by day 5. There are no
significant changes to the previous forecast with guidance in close
agreement.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 02/2100Z 35.4N 58.2W 110 KT 125 MPH
12H 03/0600Z 36.8N 56.5W 100 KT 115 MPH
24H 03/1800Z 38.6N 53.6W 95 KT 110 MPH
36H 04/0600Z 40.8N 49.5W 90 KT 105 MPH
48H 04/1800Z 44.7N 44.0W 85 KT 100 MPH
60H 05/0600Z 49.3N 40.2W 75 KT 85 MPH...POST-TROP/EXTRATROP
72H 05/1800Z 51.6N 38.5W 65 KT 75 MPH...POST-TROP/EXTRATROP
96H 06/1800Z 53.0N 32.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 07/1800Z 60.0N 28.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake
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Re: ATL: SAM - Advisories

#43 Postby cycloneye » Sat Oct 02, 2021 9:40 pm

Hurricane Sam Discussion Number 42
NWS National Hurricane Center Miami FL AL182021
1100 PM AST Sat Oct 02 2021

Sam continues to gradually weaken. Conventional satellite imagery
indicates that the eye has disappeared and that the cloud tops near
the center are gradually warming. Various objective and subjective
satellite intensity estimates are in the 90-102 kt range, while
scatterometer and experimental synthetic aperture radar data
suggest lower winds than that. The initial intensity is reduced to
a possibly generous 100 kt.

The initial motion is northeastward or 040/14 kt. A faster motion
toward the northeast is expected during the next couple of days as
Sam is steered by southwesterly flow between a deep-layer ridge to
the east and a large mid- to upper-level low over Atlantic Canada.
After Sam completes extratropical transition in about 48 h, a
wobbly motion generally toward the northeast is expected as Sam
interacts with one or more mid-latitude shortwave troughs. The
new forecast track is similar to the previous track and lies close
to the various consensus models.

The hurricane is moving toward colder water, and it is approaching
a frontal zone over the north Atlantic. The cyclone should undergo
extratropical transition during the next couple of days, and due to
a favorable interaction with a mid-latitude trough it should weaken
only slowly as it becomes a large and powerful extratropical low by
48 h. After that time, the baroclinic forcing ends and the low
should gradually spin down over the far north Atlantic. The new
intensity forecast has only minor adjustments from the previous
forecast.


Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 03/0300Z 36.4N 57.1W 100 KT 115 MPH
12H 03/1200Z 37.7N 55.3W 95 KT 110 MPH
24H 04/0000Z 39.7N 51.8W 90 KT 105 MPH
36H 04/1200Z 42.7N 47.0W 85 KT 100 MPH
48H 05/0000Z 47.3N 41.9W 80 KT 90 MPH...POST-TROP/EXTRATROP
60H 05/1200Z 50.8N 39.5W 75 KT 85 MPH...POST-TROP/EXTRATROP
72H 06/0000Z 51.7N 37.9W 65 KT 75 MPH...POST-TROP/EXTRATROP
96H 07/0000Z 54.0N 30.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 08/0000Z 60.5N 28.5W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Beven
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Re: ATL: SAM - Advisories

#44 Postby cycloneye » Sun Oct 03, 2021 5:08 am

Hurricane Sam Discussion Number 43
NWS National Hurricane Center Miami FL AL182021
500 AM AST Sun Oct 03 2021

Sam has continued to weaken since the previous advisory, with cloud
tops in the central dense overcast (CDO) having warmed to -60 to -65
deg C near the center. However, the upper-level outflow pattern
remains impressive The initial intensity has been lowered to 90 kt
based on an average of the current data-T number of 4.5/77 kt and
current intensity (CI) estimate of 5.5/102 kt from TAFB, and a 0530
UTC UW-CIMSS SATCON intensity estimate of 89 kt. Further weakening
during the is expected as Sam moves over progressively cooler water
and southwesterly vertical wind shear of 20-25 kt continues. By
18-24 hours, Sam will be moving north of the north wall of the
Gulfstream, resulting in extratropical transition due to the cyclone
moving over sub-20-deg-C sea-surface temperatures. However,
favorable interaction with a strong mid-tropospheric trough will
result in Sam becoming a large and powerful extratropical low with
hurricane-force winds in 36-48 hours. Thereafter, Sam's wind field
is expected gradually spin down over the far north Atlantic when
baroclinic forcing will cease. he new intensity forecast is similar
to but slightly lower than the previous intensity forecast.

The initial motion is northeastward, or 045/15 kt. The latest NHC
model guidance is tightly clustered around the previous track
forecast, and thus, no significant changes were made to the previous
advisory track. A faster northeastward motion is expected during
the next couple of days, followed by a slow down in the forward
motion as Sam becomes entangled with an upper-level cutoff low. The
new track forecast is essentially down the middle of the tightly
packed simple and corrected-consensus models.

Recent ASCAT-A surface wind data indicate that no significant
adjustments to the 34- and 50-kt wind radii were needed, but that
the size of the 64-kt wind radii had to be decreased somewhat.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 03/0900Z 37.3N 55.9W 90 KT 105 MPH
12H 03/1800Z 38.6N 53.6W 85 KT 100 MPH
24H 04/0600Z 41.1N 49.4W 75 KT 85 MPH
36H 04/1800Z 44.9N 44.1W 70 KT 80 MPH...POST-TROP/EXTRATROP
48H 05/0600Z 49.5N 40.6W 65 KT 75 MPH...POST-TROP/EXTRATROP
60H 05/1800Z 51.4N 39.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
72H 06/0600Z 52.0N 36.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 07/0600Z 57.0N 26.9W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 08/0600Z 60.5N 29.5W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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Re: ATL: SAM - Advisories

#45 Postby cycloneye » Sun Oct 03, 2021 11:41 am

Hurricane Sam Discussion Number 44
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Sun Oct 03 2021

Sam's cloud pattern remains quite symmetric for a hurricane moving
into the higher latitudes. The eye has become more apparent in both
visible and infrared satellite imagery over the past few hours. A
very timely 1155 UTC GMI microwave image also reveals a well-defined
inner eye that was surrounded by an outer ring of convection at a
much larger radius. A blend of the T- and CI-numbers from the
lastest subjective Dvorak classifications yields an initial
intensity of 85 kt, this is also supported by earlier UW/CIMSS
SATCON estimates.

The vertical shear over Sam is expected to remain low for the next
6-12 hours and only gradual weakening is anticipated while the
system moves over gradually decreasing SSTs. Later tonight, Sam is
expected to move over much cooler waters north of the north wall of
the Gulf Stream and into an area of increasing southwesterly shear,
which should result in continued weakening. However, a strong
mid-tropospheric trough approaching Sam from the west on Monday is
expected to result in a fairly quick transition of Sam to a large
and powerful extratropical low. The low is forecast to gradually
weaken over the far north Atlantic during the middle to latter
portions of the week as the baroclinic forcing decreases. The
latest NHC intensity forecast is similar to the previous advisory
and is a blend of the various intensity aids during the early
portion of the period, but leans toward the global model guidance
during the post-tropical phase.

Sam is moving northeastward or 050/14 kt. The cyclone should
continue to move northeastward in the flow between a strong
subtropical ridge over the eastern Atlantic and mid-latitude
trough over Atlantic Canada. As this trough approaches Sam from
the west, a much faster northeastward motion is expected by Monday
and Monday night. Around midweek, the post-tropical cyclone is
forecast to slow down as it rotates around a large cut-off low
over the north Atlantic. After that time, the low is forecast to
resume a northeastward motion before it slows once again to the
southwest of Iceland. Given the future complex interactions of the
post-tropical low with the cut-off low and trough over the north
Atlantic, the track guidance is in surprisingly good agreement, and
the NHC forecast is near a blend of the ECMWF and GFS global
models.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 03/1500Z 37.7N 54.6W 85 KT 100 MPH
12H 04/0000Z 39.3N 51.9W 80 KT 90 MPH
24H 04/1200Z 42.4N 47.0W 75 KT 85 MPH
36H 05/0000Z 47.0N 41.7W 70 KT 80 MPH...POST-TROP/EXTRATROP
48H 05/1200Z 50.9N 39.8W 65 KT 75 MPH...POST-TROP/EXTRATROP
60H 06/0000Z 50.9N 38.5W 60 KT 70 MPH...POST-TROP/EXTRATROP
72H 06/1200Z 51.5N 33.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 07/1200Z 59.2N 27.5W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 08/1200Z 60.5N 31.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown
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Re: ATL: SAM - Advisories

#46 Postby cycloneye » Sun Oct 03, 2021 3:38 pm

Hurricane Sam Discussion Number 45
NWS National Hurricane Center Miami FL AL182021
500 PM AST Sun Oct 03 2021

Sam is a resilient hurricane despite having just crossed the 26
degrees C isotherm. An eye remains readily apparent in satellite
imagery, while microwave images show a symmetrical concentric
eyewall structure. Recent scatterometer data revealed that the
tropical-storm-force wind field continues to expand, with these
winds now extending 200 n mi or greater in all quadrants. A blend
of the latest Dvorak intensity estimates from TAFB, SAB, and the
UW-CIMMS ADT supports maintaining an initial advisory intensity of
85 kt.

Gradual weakening is expected over the next 36 h as Sam moves over
progressively cooler waters. However, the wind field is forecast to
continue expanding. On Monday, the cyclone is expected to interact
with a strong mid-level trough moving off the coast of Newfoundland
which should cause Sam to transition into a powerful extratropical
cyclone. With a lack of baroclinic forcing once Sam becomes
vertically stacked with the trough in 48 h, the extratropical
cyclone is expected to steadily weaken over the northern Atlantic
through late this week. The latest NHC intensity forecast is little
changed from the previous one, and is near the various model
consensus solutions.

Sam has begun to accelerate and is now moving 060/14 kt. A
northwestward motion and continued acceleration is forecast through
Monday as the cyclone gets caught in southwesterly flow ahead of the
mid-level trough. Just before Sam merges with the trough in a couple
of days, the cyclone should slow down as it pivots around the
eastern flank of the trough. Thereafter, the merged system should
continue northeastward then northward in the flow around a larger
mid- to upper-level trough over the far northern Atlantic. The model
guidance is in very good agreement on this track scenario, and the
latest NHC track forecast remains near the previous one.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 03/2100Z 38.6N 52.8W 85 KT 100 MPH
12H 04/0600Z 40.3N 49.7W 80 KT 90 MPH
24H 04/1800Z 44.2N 44.3W 75 KT 85 MPH
36H 05/0600Z 49.0N 40.4W 70 KT 80 MPH...POST-TROP/EXTRATROP
48H 05/1800Z 50.8N 39.5W 65 KT 75 MPH...POST-TROP/EXTRATROP
60H 06/0600Z 51.5N 35.7W 55 KT 65 MPH...POST-TROP/EXTRATROP
72H 06/1800Z 53.2N 31.2W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 07/1800Z 59.4N 27.2W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 08/1800Z 61.2N 27.5W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Latto
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Re: ATL: SAM - Advisories

#47 Postby cycloneye » Sun Oct 03, 2021 9:36 pm

Hurricane Sam Discussion Number 46
NWS National Hurricane Center Miami FL AL182021
1100 PM AST Sun Oct 03 2021

Sam still had a surprise up its sleeve tonight, with its eye
becoming warmer on satellite images along with a stronger eyewall.
Microwave data from earlier today indicated that Sam has been
undergoing a concentric eyewall cycle, and it appears that the inner
eyewall has become better defined. Regardless, it is not every day
you see a hurricane with that clear of an eye near 40N, and the
intensity estimates the evening range from 90-100 kt. Given the
concentric eyewalls, the initial wind speed is conservatively
raised to 90 kt, but it could be higher.

High-resolution NOAA OISST data indicate that Sam is moving near a
warm eddy along the north wall of the Gulf Stream. This favorable
factor, in addition to low shear, should keep Sam's weakening to a
minimum in the near term. Later on, although the shear increases
rapidly and SSTs fall quickly, Sam is expected to transition into a
powerful extratropical cyclone late Monday or early Tuesday due to a
mid-latitude trough interaction well east of Newfoundland. This
should cause extratropical Sam to maintain hurricane-force winds
until early Wednesday. A slow weakening is expected thereafter as
it slowly spins down as an occluded low. No significant changes
were made to the previous forecast.

Sam is moving faster, now 050/17 kt. A northeastward motion and
continued acceleration is forecast through Monday as the cyclone
gets caught in southwesterly flow ahead of the mid-latitude trough.
The tropical cyclone is expected to be the main surface low center
as it merges with the trough in a couple days, hooking briefly to
the left. Thereafter, the system should resume a northeastward
motion and gradually turn northward and even westward at long range
as it moves around another trough. Model guidance is close to the
previous cycle, even with the loopy track, and the new NHC forecast
is basically just an update of the last advisory.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 04/0300Z 39.3N 51.2W 90 KT 105 MPH
12H 04/1200Z 41.6N 47.3W 85 KT 100 MPH
24H 05/0000Z 46.5N 42.1W 75 KT 85 MPH
36H 05/1200Z 50.4N 40.3W 70 KT 80 MPH...POST-TROP/EXTRATROP
48H 06/0000Z 50.8N 38.3W 65 KT 75 MPH...POST-TROP/EXTRATROP
60H 06/1200Z 51.6N 33.3W 55 KT 65 MPH...POST-TROP/EXTRATROP
72H 07/0000Z 54.5N 28.2W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 08/0000Z 61.0N 28.0W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 09/0000Z 61.5N 32.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake
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Re: ATL: SAM - Advisories

#48 Postby cycloneye » Mon Oct 04, 2021 4:25 am

Hurricane Sam Discussion Number 47
NWS National Hurricane Center Miami FL AL182021
500 AM AST Mon Oct 04 2021

A 0255Z GPM microwave pass indicated that Sam had continued to go
through an eyewall replacement cycle, consisting of a 20-nmi-wide
inner eye and an 85-nmi-wide outer eye. The inner eye was rapidly
eroding on the northwest side compared to microwave data from just a
few hours earlier. Additionally, late-arriving ASCAT surface wind
data from 0000-0100Z indicated that the inner-core wind field had
contracted or weakened, while the outer wind field (34-kt radii) had
continued to expand in all quadrants. Satellite intensity estimates
have decreased to around 80 kt. However, the advisory intensity has
been maintained at 90 kt based on the 10-kt increase in forward
speed, which should offset any weakening of the tangential winds.
Gradual weakening is expected now that Sam has passed north of the
warm Gulf Stream and is moving over sub-25-deg-C sea-surface
temperatures (SST), with much colder SSTs lying ahead of the
hurricane. Vertical wind shear is forecast to increase to more than
30 kt by 18 h, which should cause further weakening. By 24 h and
beyond, interaction with a strong upper-level trough/low should
cause Sam to transition into a powerful extratropical cyclone by
early Tuesday to the east of Newfoundland. This baroclinic
interaction is expected to cause extratropical-Sam to maintain
hurricane-force winds until early Wednesday. Slow weakening is
forecast thereafter as the very large cyclone slowly spins down as
an occluded low pressure system. No significant changes were made
to the previous intensity forecast.

Sam has continued to accelerate and is now moving at 045/26 kt. A
northeastward motion along with continued acceleration is forecast
through today as Sam moves into stronger southwesterly flow ahead of
an approaching mid-latitude trough. The large cyclone is expected to
remain the primary surface low center as it merges with the trough
in a couple days. A gradual turn toward the north and northwest is
expected on days 4 and 5 when extratropical-Sam moves around another
high-latitude trough. The new NHC forecast track is essentially
just an update of the previous advisory track, and lies close to
the tightly packed consensus track models.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward Islands,
the Greater Antilles, the Bahamas, Bermuda, the eastern United
States and Atlantic Canada for the next couple of days. These
swells could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 04/0900Z 40.8N 48.7W 90 KT 105 MPH
12H 04/1800Z 43.7N 45.0W 80 KT 90 MPH
24H 05/0600Z 47.8N 41.5W 75 KT 85 MPH...POST-TROP/EXTRATROP
36H 05/1800Z 50.1N 39.3W 70 KT 80 MPH...POST-TROP/EXTRATROP
48H 06/0600Z 51.0N 35.8W 60 KT 70 MPH...POST-TROP/EXTRATROP
60H 06/1800Z 52.8N 31.2W 55 KT 65 MPH...POST-TROP/EXTRATROP
72H 07/0600Z 56.0N 28.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 08/0600Z 60.6N 29.3W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 09/0600Z 61.5N 30.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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Re: ATL: SAM - Advisories

#49 Postby cycloneye » Mon Oct 04, 2021 9:46 am

Hurricane Sam Discussion Number 48
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Mon Oct 04 2021

Sam remains a formidable high-latitude hurricane this morning. An
earlier 1102 UTC GMI microwave overpass continued to depict
concentric eyewalls with a fairly well-defined inner-eye that was
open over the southwestern quadrant. Although there has been a
generally warming of the cloud tops in infrared satellite imagery,
the eye remains apparent and the system is still fairly symmetric.
A blend of the subjective Dvorak T- and CI-numbers from both SAB
and TAFB yield an intensity of 85 kt, which is used as the initial
intensity for this advisory.

Now that Sam has moved north of the north wall of the Gulf Stream
it will be moving over progressively colder waters and into much
higher vertical wind shear conditions by this evening, which should
cause some gradual weakening. However, the interaction of the
cyclone with a strong upper-level trough is likely to lead to the
rapid transition of Sam into a powerful extratropical cyclone later
this evening. The post-tropical cyclone is expected to maintain
hurricane-force winds until late Tuesday. Gradual weakening should
occur after that time as the baroclinic forcing decreases, and the
system is forecast to spin down slowly over the north Atlantic later
this week.

Sam continues to accelerate northeastward and is now moving at
045/28 kt. A faster northeastward motion is forecast through
tonight as Sam moves within the strong southwesterly flow ahead of
a mid-latitude trough. The post-tropical cyclone is forecast to
slow down on Tuesday as it interacts with the trough/cut-off low.
After that time, the system is forecast to turn east-northeastward,
and then rotate cyclonically around the eastern flank of yet
another cut-off low later in the week. The dynamical model
guidance remains in generally good agreement on the overall track
forecast scenario, albeit for some forward speed variations. The
latest official forecast is similar to the previous advisory and
lies near the TVCA multi-model consensus in deference to the
aforementioned along-track differences.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland for the next day or so. These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 04/1500Z 42.6N 45.7W 85 KT 100 MPH
12H 05/0000Z 46.5N 41.9W 75 KT 85 MPH
24H 05/1200Z 50.0N 40.5W 70 KT 80 MPH...POST-TROP/EXTRATROP
36H 06/0000Z 50.3N 38.3W 60 KT 70 MPH...POST-TROP/EXTRATROP
48H 06/1200Z 51.4N 33.2W 55 KT 65 MPH...POST-TROP/EXTRATROP
60H 07/0000Z 54.3N 27.7W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 07/1200Z 58.3N 24.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 08/1200Z 61.0N 29.7W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 09/1200Z 60.0N 29.8W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown
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Re: ATL: SAM - Advisories

#50 Postby cycloneye » Mon Oct 04, 2021 3:33 pm

Hurricane Sam Discussion Number 49
NWS National Hurricane Center Miami FL AL182021
900 PM GMT Mon Oct 04 2021

Sam's cloud pattern has finally begun to degrade as it moves over
the cooler waters of the north Atlantic. The cloud tops have
warmed and the eye is no longer discernible in infrared imagery,
but banding features still remain well defined in all quadrants.
Dvorak T- and CI-numbers from both SAB and TAFB have continued to
decline and a blend of the latest subjective and objective
estimates yields an initial intensity estimate of 80 kt for this
advisory. Scatterometer data that arrived just after the release
of the previous advisory showed that Sam's wind field has continued
to expand and the latest radii estimates are based on those data.

Sam should gradually weaken this evening as it moves over
progressively colder waters and into an area of strong southwesterly
upper-level winds. However, an approaching mid-latitude trough
should result in the system's quick transition into a powerful
extratropical cyclone overnight. The post-tropical cyclone is
forecast to maintain hurricane-force winds for until late Tuesday,
but gradual weakening should occur after that time as the system
occludes and baroclinic forcing decreases. It is not entirely
clear as to whether Post-Tropical Sam will remain the dominant low
(GFS solution) or if it will be absorbed by another extratropical
low (ECMWF solution) later in the week. As a compromise, the new
official forecast calls for absorbed by day 5 which is between
the solutions from those typically reliable global models.

Sam is moving briskly northeastward or 035/31 kt. The cyclone is
currently caught in strong southwesterly flow ahead of the
aforementioned mid-latitude trough and a rapid northeastward motion
is expected overnight. After that time, the post-tropical cyclone
is forecast to slow down as it merges with the cut-off low. By
late Tuesday, the post-tropical cyclone should turn east-
northeastward before it rotates cyclonically around the eastern
portion of another trough/cut off low over the north Atlantic. The
latest NHC forecast is a blend of the GFS and ECMWF models and
their respective ensemble means.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through early Tuesday. These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 04/2100Z 45.1N 42.5W 80 KT 90 MPH
12H 05/0600Z 49.0N 39.8W 70 KT 80 MPH...POST-TROP/EXTRATROP
24H 05/1800Z 50.9N 39.9W 65 KT 75 MPH...POST-TROP/EXTRATROP
36H 06/0600Z 50.5N 36.0W 55 KT 65 MPH...POST-TROP/EXTRATROP
48H 06/1800Z 52.5N 30.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 07/0600Z 56.5N 25.0W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 07/1800Z 60.0N 25.5W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 08/1800Z 60.0N 32.0W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 09/1800Z...DISSIPATED

$$
Forecaster Brown
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Re: ATL: SAM - Advisories

#51 Postby cycloneye » Mon Oct 04, 2021 9:52 pm

Hurricane Sam Discussion Number 50
NWS National Hurricane Center Miami FL AL182021
300 AM GMT Tue Oct 05 2021

Sam's extratropical transition is well underway. The central core
convection has decreased over the past several hours, with the
telltale elongated cloud pattern forming on the northwestern side
of the circulation. Still, AMSU-microwave data just before 00Z was
very clear it is still has a healthy warm core, so advisories will
continue. The initial wind speed is decreased to 75 kt, a bit
above the satellite estimates.

An approaching mid-latitude trough should cause Sam to quickly
transition into a powerful extratropical cyclone within the next
6-12 hours while maintaining hurricane-force winds for about the
next day. A slow weakening is expected after that time as Sam loses
its baroclinic forcing. The only significant change from the
last advisory is that models are in better agreement that Sam will
become absorbed by another extratropical low in about 3 days,
rather than be the dominant center, so the 96 h forecast is now
dissipated.

Sam is moving north-northeastward or 030/25 kt. The system is
forecast to slow down as it merges with the mid-latitude trough. By
late Tuesday, the post-tropical cyclone should turn east-
northeastward before it moves northward and northwestward around the
eastern portion of another trough/cut-off low over the north
Atlantic. The latest NHC forecast is very close to the last one and
the model consensus.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through midweek. These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 05/0300Z 47.7N 40.2W 75 KT 85 MPH
12H 05/1200Z 50.6N 39.3W 70 KT 80 MPH...POST-TROP/EXTRATROP
24H 06/0000Z 51.0N 38.2W 65 KT 75 MPH...POST-TROP/EXTRATROP
36H 06/1200Z 51.5N 33.3W 55 KT 65 MPH...POST-TROP/EXTRATROP
48H 07/0000Z 54.0N 28.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 07/1200Z 58.0N 22.9W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 08/0000Z 61.5N 25.0W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 09/0000Z...DISSIPATED

$$
Forecaster Blake
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Re: ATL: SAM - Advisories

#52 Postby cycloneye » Tue Oct 05, 2021 4:24 am

Post-Tropical Cyclone Sam Discussion Number 51
NWS National Hurricane Center Miami FL AL182021
900 AM GMT Tue Oct 05 2021

Sam's low-level circulation center has become exposed on the south
side of an east-to-west oriented cloud band containing cloud tops
that have warmed to around -50 deg C. The cyclone's overall cloud
pattern in satellite imagery resembles that of an occluded
extratropical low pressure system, and surface analyses from the
NOAA Ocean Prediction Center indicate that Sam's inner-core is now
interacting with a frontal system. Furthermore, the system is
located over sea-surface temperatures of around 14 deg C. Although
the system likely still has a warm-core thermal structure in the
mid- and upper-levels of the troposphere based on earlier 04/2321Z
AMSU-A/-B data, the low-level circulation now is comprised of an
extensive field of cold-air stratocumulus clouds. Based on these
data, Sam has made the transition to an extratropical cyclone over
the far north Atlantic. The initial wind speed has been lowered to
70 kt based on a standard decay rate.

The initial motion estimate is northward or 010/20 kt.
Extratropical-Sam has been moving north-northeastward at 35 kt
since the previous advisory, but the latest global and regional
models are all in very good agreement that the system will slow
down significantly during the next 6-12 hours as it interacts with
an approaching baroclinic mid- to upper-level trough/low. By late
Tuesday, the post-tropical cyclone should turn east-northeastward,
and then make a counter-clockwise loop southwest of Iceland on
Wednesday. By Thursday, the large cyclone is expected to move
east-northeastward or eastward and pass very near the southern coast
of Iceland on days 4 and 5 as a weakening extratropical low pressure
system. The new NHC track forecast is very similar to the previous
advisory track, and lies close to the tightly packed simple- and
corrected-consensus track models.

Extratropical-Sam is expected to maintain hurricane-force winds for
the next 12-18 hours, followed by a slow weakening trend as the
cyclone loses its baroclinic forcing. However, Post-Tropical-Sam
is still expected to pass close to Iceland in the 96-120-h period as
an expansive low pressure system producing gale-force winds over a
very large area.

This is the last advisory issued by the National Hurricane Center on
Sam. Additional information on this system can be found in High
Seas Forecasts issued by the National Weather Service, under AWIPS
header NFDHSFAT1, WMO header FZNT01 KWBC, and online at
ocean.weather.gov/shtml/NFDHSFAT1.php.

Key Messages:

1. Swells generated by Sam will impact the northern Leeward
Islands, the Greater Antilles, portions of the Bahamas, and
southeastern Newfoundland through midweek. These swells
could cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.


FORECAST POSITIONS AND MAX WINDS

INIT 05/0900Z 51.0N 39.4W 70 KT 80 MPH...POST-TROP/EXTRATROP
12H 05/1800Z 52.1N 38.9W 65 KT 75 MPH...POST-TROP/EXTRATROP
24H 06/0600Z 52.8N 35.1W 60 KT 70 MPH...POST-TROP/EXTRATROP
36H 06/1800Z 54.3N 30.0W 55 KT 65 MPH...POST-TROP/EXTRATROP
48H 07/0600Z 57.9N 24.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 07/1800Z 61.8N 23.9W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 08/0600Z 62.7N 27.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 09/0600Z 61.7N 26.2W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 10/0600Z 63.6N 15.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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