ATL: NIGEL - Advisories

[cycloneye]

Tropical Storm Kenneth Discussion Number 3
NWS National Hurricane Center Miami FL EP132023
800 PM PDT Tue Sep 19 2023

Kenneth is a poorly organized tropical cyclone. The large
convective canopy from earlier this afternoon has mostly
collapsed, with only a couple new convective cells forming well to
the northwest of the center. The initial intensity remains 40 kt
based on the earlier ASCAT data, although satellite intensity
estimates are no higher than about 35 kt.

Microwave data suggests that Kenneth's center has jogged--or
re-formed--a bit to the southwest of the previous fixes. The
long-term motion is gradually slowing down and is now westward, or
270/11 kt. A mid-level high centered over west-central Mexico and a
deep-layer trough extending southwest of California are expected to
steer Kenneth generally toward the northwest through the end of the
week. The NHC track forecast is changed little from the previous
prediction, and lies to the right of the TVCE multi-model consensus,
although not as far to the right as the latest GFS, ECMWF, and HCCA
solutions.

Moderate easterly shear is affecting the storm, but SHIPS
diagnostics suggest the shear should decrease and be relatively
light during the next 36 hours. The NHC forecast shows some modest
strengthening during this period, and is at the upper end of the
guidance envelope to maintain continuity with the previous forecast.
By 48 hours, Kenneth is expected to reach cooler waters, and
deep-layer shear is forecast to increase out of the southwest.
Weakening is therefore expected, and GFS- and ECMWF-based simulated
satellite images indicate that Kenneth could lose organized deep
convection and degenerate into a remnant low by day 3. The remnant
low is forecast to dissipate by day 5.


FORECAST POSITIONS AND MAX WINDS

INIT 20/0300Z 14.6N 121.6W 40 KT 45 MPH
12H 20/1200Z 15.1N 123.0W 45 KT 50 MPH
24H 21/0000Z 15.8N 124.6W 50 KT 60 MPH
36H 21/1200Z 16.6N 125.9W 45 KT 50 MPH
48H 22/0000Z 17.7N 126.8W 40 KT 45 MPH
60H 22/1200Z 19.2N 127.2W 35 KT 40 MPH
72H 23/0000Z 20.4N 127.6W 30 KT 35 MPH...POST-TROP/REMNT LOW
96H 24/0000Z 21.4N 128.8W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 25/0000Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Hurricane Nigel Discussion Number 20
NWS National Hurricane Center Miami FL AL152023
500 AM AST Wed Sep 20 2023

Nigel's overall satellite appearance has not changed much from the
previous advisory. Deep convection remains on the southern portion
of a thick band around the center of the system. Infrared imagery
shows that the eyewall has been opened on the northern side, but has
tried to fill in recently. There have been no microwave passes over
the system overnight to get a better idea of the overall structure
of Nigel. Subjective intensity estimates have remained fairly
steady, with a CI of 4.5/5.0 from TAFB and SAB, respectively. Given
these estimates and similar satellite appearance, the initial
intensity is held at 85 kt.

Nigel has about 18-24 hours before it is expected to move over
cooler sea surface temperatures and into significantly stronger
vertical wind shear. As a result, the latest NHC intensity forecast
shows Nigel remaining steady to slightly weakening in the short
term, followed by steady weakening through the remainder of the
forecast period. Although weakening is forecast, Nigel is expected
to be a strong extratropical cyclone in about 48 hours.

The system is moving toward the north at 14 kt, as Nigel rounds the
edge of a mid-level ridge over the central subtropical Atlantic.
Later today, the system is expected to turn northeastward and
accelerate in the flow of a deep-layer trough located over Atlantic
Canada. The track guidance remains in fairly good agreement in the
short term with only slight along-track speed differences. The NHC
track forecast is slightly faster than the previous one, and lies
near the consensus aids, but still not as fast as HCCA. As Nigel
reaches high latitudes, the extratropical low is forecast to
interact with a broader cyclonic circulation over the north
Atlantic, and it is possible that Nigel gets absorbed by this
broader circulation towards the end of the period. There is higher
track uncertainty towards the end of the forecast period due to this
interaction.


FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 34.4N 54.9W 85 KT 100 MPH
12H 20/1800Z 36.7N 53.5W 85 KT 100 MPH
24H 21/0600Z 39.8N 49.5W 80 KT 90 MPH
36H 21/1800Z 42.8N 43.0W 70 KT 80 MPH
48H 22/0600Z 45.7N 35.5W 60 KT 70 MPH...POST-TROP/EXTRATROP
60H 22/1800Z 49.2N 28.4W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 23/0600Z 52.4N 24.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 24/0600Z 57.1N 23.7W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 25/0600Z 59.1N 20.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Kelly

[cycloneye]

Hurricane Nigel Discussion Number 21
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Wed Sep 20 2023

Nigel is maintaining a ragged-appearing eye on visible and
infrared satellite imagery. Bands of deep convection with tops to
around -70C continue to rotate around the center, and the system's
cloud pattern remains fairly symmetrical. Although microwave
imagery from around 0900 UTC showed a closed eyewall, recent
geostationary images suggest breaks in the eyewall over the
eastern portion of the circulation. Dvorak CI numbers from TAFB and
SAB are 4.5, corresponding to a 77-kt intensity, and an objective
ADT estimate from UW-CIMSS was 79 kt. Based on these values, the
advisory intensity estimate was reduced only slightly, to 80 kt.
NOAA Hurricane Hunter aircraft are scheduled to investigate Nigel
soon.

The hurricane is moving northward, or perhaps slightly east of
north, with an initial motion of 010/16 kt. The system has been
moving around the western periphery of a mid-level high pressure
area. A mid-latitude trough to the northwest of Nigel should cause
the cyclone to turn toward the northeast and the cyclone
should continue to move northeastward at a faster forward speed over
the next couple of days. In 48 to 72 hours, Nigel is expected to
rotate counter-clockwise around the eastern side of a large
extratropical cyclone over the north Atlantic. Although the
official forecast shows track points at 4 and 5 days, there is a
good chance that Nigel will merge with, or become absorbed by, the
other cyclone around that time. The official forecast is very
similar to the previous NHC track prediction.

Nigel has only about 12 hours remaining before moving over cooler
waters, and southwesterly vertical shear is forecast to increase
tonight. Gradual weakening is expected over the next 2-3 days, in
general agreement with the consensus intensity forecast guidance.
The intensity forecast in the latter part of the forecast period is
uncertain, given the distinct possibility that Nigel could merge
with the other extratropical cyclone.


FORECAST POSITIONS AND MAX WINDS

INIT 20/1500Z 36.1N 54.4W 80 KT 90 MPH
12H 21/0000Z 38.4N 51.8W 80 KT 90 MPH
24H 21/1200Z 41.6N 46.4W 75 KT 85 MPH
36H 22/0000Z 44.6N 39.0W 65 KT 75 MPH
48H 22/1200Z 47.6N 31.0W 55 KT 65 MPH...POST-TROP/EXTRATROP
60H 23/0000Z 51.3N 24.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 23/1200Z 55.0N 23.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 24/1200Z 57.0N 24.0W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 25/1200Z 58.0N 23.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 22
NWS National Hurricane Center Miami FL AL152023
500 PM AST Wed Sep 20 2023

Nigel has changed little in organization on satellite images since
earlier today. The ragged-looking eye remains quite large, reported
to be 60 n mi in diameter by the NOAA Hurricane Hunter aircraft.
There are still bands of deep convection with cloud tops to near -70
deg C, primarily over the northwestern and western portions of the
circulation, and the overall cloud pattern continues to have a
symmetrical appearance. The central pressure measured by the
aircraft has not changed much since earlier today and wind data from
the plane suggest that the intensity remains near 80 kt.

Center fixes from the Hurricane Hunters and satellite images show
that the hurricane's motion is beginning to bend to the right and is
now north-northeastward, or around 030/16 kt. Nigel should move
along the northwestern periphery of a subtropical high pressure cell
tonight. In 12 to about 60 hours, the flow on the southeastern
side of a strong mid-latitude 500 mb trough should carry the system
northeastward at a fast pace toward, and over, the northern
Atlantic. According to the global models, in 72 to 96 hours, Nigel
should undergo a binary interaction with a large extratropical low
over the North Atlantic. By 120 hours or sooner, the system is
forecast to merge with this low.

Over the next few days, Nigel will be moving over progressively
cooler ocean waters and into an environment of stronger
southwesterly vertical wind shear. Gradual weakening is forecast,
and the system is expected to become embedded in a baroclinic zone
on Friday. Therefore, extratropical transition is indicated in the
48 hour forecast. Since Nigel is expected to merge with the
aforementioned North Atlantic low in about 5 days, dissipation is
shown at the end of the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT 20/2100Z 37.4N 53.0W 80 KT 90 MPH
12H 21/0600Z 39.8N 49.6W 80 KT 90 MPH
24H 21/1800Z 42.8N 43.2W 75 KT 85 MPH
36H 22/0600Z 45.7N 35.2W 60 KT 70 MPH
48H 22/1800Z 49.0N 27.6W 55 KT 65 MPH...POST-TROP/EXTRATROP
60H 23/0600Z 53.0N 23.4W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 23/1800Z 56.0N 23.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 24/1800Z 57.0N 23.5W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 25/1800Z...DISSIPATED

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 23
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Wed Sep 20 2023

Geostationary satellite imagery shows Nigel's outflow being
impinged upon by an upstream trough. Still, the hurricane has
maintained its large, ragged eye surrounded by deep convection
with cloud top temperatures ranging from -60 to -70 degrees C.
Subjective Dvorak estimates from TAFB and SAB were both T4.5/77 kt,
and the initial intensity is held at 80 kt for this advisory.

The hurricane has begun to accelerate northeastward at an estimated
045/22 kt. Nigel's forward speed is expected to increase through
Friday in the flow ahead of the mid-latitude trough exiting the
northeast US coast. By the weekend, global models show Nigel
interacting with a large extratropical system over the North
Atlantic. Little changes have been made to the latest NHC forecast,
which is largely an update of the previous prediction.

Nigel is nearing the 26 degree isotherm and should cross over it
in a few hours. Deep-layer vertical wind shear is also expected to
increase significantly over the next 24 hours. Nigel should begin
to transition into an extratropical cyclone soon, and this process
is expected to be complete in about 48 hours. The official
intensity prediction shows gradual weakening through day 2, and
Nigel is forecast to become a powerful extratropical cyclone on
Friday. By the end of the forecast period, Nigel should be absorbed
by the larger mid-latitude cyclone mentioned previously.


FORECAST POSITIONS AND MAX WINDS

INIT 21/0300Z 39.0N 50.4W 80 KT 90 MPH
12H 21/1200Z 41.2N 46.4W 75 KT 85 MPH
24H 22/0000Z 44.2N 39.1W 70 KT 80 MPH
36H 22/1200Z 47.3N 31.2W 60 KT 70 MPH
48H 23/0000Z 50.9N 25.4W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 23/1200Z 54.9N 22.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 24/0000Z 56.6N 22.8W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 25/0000Z 58.0N 23.0W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 26/0000Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Hurricane Nigel Discussion Number 24
NWS National Hurricane Center Miami FL AL152023
500 AM AST Thu Sep 21 2023

Nigel is starting to feel the effects of increased vertical wind
shear this morning. Proxy-vis and infrared satellite imagery show
that the large, ragged eye from the past few days has become
obscured. An AMSR microwave pass at 0513 UTC confirmed that the
eyewall structure has collapsed, and a thick curved band on the
northern side is all that remains. Vertical wind shear has the deep
convection displaced to the northern side of the system, within the
main banding feature, with cold cloud tops in the northwest
semicircle near -70 Celsius. Subjective satellite current intensity
(CI) values were 4.5 from both TAFB and SAB. Given the degradation
in the satellite appearance and these estimates, the initial
intensity is set to 75 kt for this advisory.

Nigel is moving swiftly to the northeast at an estimated motion of
50/ 26 kt. The system is currently moving within the flow ahead of a
mid-latitude trough off the northeast US Coast and Atlantic
Canada, with this quick northeast motion expected to continue
during the next day or two. Late Friday and into the weekend,
models show Nigel interacting with a larger polar low over the
North Atlantic. The NHC track forecast is very similar to the
previous advisory and remains near the consensus aids.

The hurricane is moving into a more hostile environment with
vertical deep layer shear forecast to further increase, and sea
surface temperatures along the forecast track continue to cool over
the northern Atlantic. Nigel is also entering a drier mid-level air
mass with relative humidities below 40 percent. Nigel should begin
to transition into an extratropical cyclone very soon, and this
process is expected to be complete in about 24 hours. The ECMWF and
GFS SHIPS guidance show extratropical transition occuring even
sooner than what is forecast. The official intensity forecast shows
gradual weakening, with Nigel becoming a powerful extratropical
cyclone on Friday. By the end of the forecast period, Nigel will
become absorbed in the aforementioned larger polar low.


FORECAST POSITIONS AND MAX WINDS

INIT 21/0900Z 40.6N 47.8W 75 KT 85 MPH
12H 21/1800Z 42.9N 42.7W 70 KT 80 MPH
24H 22/0600Z 45.9N 35.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
36H 22/1800Z 49.5N 27.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 23/0600Z 53.1N 23.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 23/1800Z 56.8N 23.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 24/0600Z 58.3N 24.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 25/0600Z...DISSIPATED

$$
Forecaster Kelly

[cycloneye]

Hurricane Nigel Discussion Number 25
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Thu Sep 21 2023

Nigel is still maintaining some deep convection, with cloud tops
in bands to near -70 deg C. However, increasing southwesterly
vertical wind shear is beginning to cause an elongation of the cloud
pattern toward the northeast. Subjective Dvorak satellite
classifications from TAFB and SAB continue to give Current
Intensity numbers corresponding to 77 kt, and objective ADT
estimates from UW-CIMSS are a little lower than that value. The
advisory intensity is held at 75 kt.

The current motion is quickly northeastward, or about 050/26 kt.
Nigel is embedded in the flow on the southeastern side of a strong
deep-layer mid-latitude trough. This steering pattern should
continue to carry the cyclone rapidly toward the northeast through
tonight. On Friday, Nigel should rotate around the eastern side of
a large extratropical cyclone over the North Atlantic.

Vertical wind shear over Nigel should become very high within the
next 24 hours, and the cyclone will be moving over significantly
cooler waters later today. Therefore weakening should occur, and
global model guidance indicates that the system will become
embedded in a frontal zone by early Friday. In 2 to 3 days, Nigel
is expected to merge with the large cyclone over the North
Atlantic, and dissipate.


FORECAST POSITIONS AND MAX WINDS

INIT 21/1500Z 42.2N 45.0W 75 KT 85 MPH
12H 22/0000Z 44.2N 38.8W 65 KT 75 MPH
24H 22/1200Z 47.3N 30.8W 55 KT 65 MPH...POST-TROP/EXTRATROP
36H 23/0000Z 51.0N 24.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 23/1200Z 55.2N 22.9W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 24/0000Z 57.4N 24.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 24/1200Z...DISSIPATED

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 26
NWS National Hurricane Center Miami FL AL152023
900 PM GMT Thu Sep 21 2023

Nigel's satellite presentation has begun to degrade as increasing
southwesterly vertical wind shear has caused the deep convection to
become displaced to the northeast of the low-level center. A blend
of the latest Dvorak Current Intensity (CI) numbers and the
objective estimates yields an initial intensity estimate of 70 kt.

A continued increase in vertical wind shear and cooler waters along
the track of Nigel should cause additional weakening. Nigel is
expected to quickly complete its extratropical transition by 12
hours, and only gradual weakening is forecast after that time. The
storm has grown in size, and a continue expansion of the 34-kt wind
field is expected during the next couple of days.

Nigel is moving east-northeastward or 060/32 kt. An east-
northeastward to northeastward motion along the southeastern
side of a strong deep-layer mid-latitude trough is expected during
the next 12 to 24 hours. After that time, Nigel should rotate
around the eastern side of a large extratropical cyclone over the
North Atlantic. The dynamical model guidance is tightly clustered,
and the NHC forecast is near a blend of the GFS and ECMWF models.

FORECAST POSITIONS AND MAX WINDS

INIT 21/2100Z 43.7N 40.8W 70 KT 80 MPH
12H 22/0600Z 46.0N 34.5W 60 KT 70 MPH...POST-TROP/EXTRATROP
24H 22/1800Z 49.4N 27.1W 55 KT 65 MPH...POST-TROP/EXTRATROP
36H 23/0600Z 53.6N 23.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 23/1800Z 57.0N 24.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 24/0600Z 57.5N 25.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 24/1800Z...DISSIPATED

$$
Forecaster Brown

[cycloneye]

Hurricane Nigel Discussion Number 27
NWS National Hurricane Center Miami FL AL152023
300 AM GMT Fri Sep 22 2023

Nigel has not quite completed its transition to a post-tropical
cyclone. A burst of deep convection formed near the center a few
hours ago, and it is therefore still considered a tropical system.
Based on a partial ASCAT pass and the latest satellite intensity
estimates, the initial intensity is lowered to 65 kt for this
advisory.

Cold ocean waters and significant vertical wind shear should cause
Nigel to weaken further. The hurricane should become a powerful
extratropical cyclone within about 12 hours. As noted previously,
Nigel continues to grow in size and an expansion of the 34-kt wind
field is expected during the next couple of days.

The hurricane is moving east-northeastward at 32 kt. An east-
northeastward to northeastward motion along the southeastern side of
a deep-layer mid-latitude trough is expected during the next day or
so. Beyond a day, Nigel should rotate around the eastern side of a
large extratropical cyclone over the North Atlantic. The two
features should merge in about 60 h and the official forecast now
shows dissipation at the time. The model guidance remains tightly
clustered, and the NHC forecast lies near the center of the
envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 22/0300Z 45.1N 36.7W 65 KT 75 MPH
12H 22/1200Z 47.5N 30.7W 55 KT 65 MPH...POST-TROP/EXTRATROP
24H 23/0000Z 51.3N 25.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 23/1200Z 55.0N 23.2W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 24/0000Z 56.9N 25.2W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 24/1200Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Post-Tropical Cyclone Nigel Discussion Number 28
NWS National Hurricane Center Miami FL AL152023
900 AM GMT Fri Sep 22 2023

Nigel has completed its transition to an extra-tropical cyclone this
morning. Convection from earlier has waned, and is well displaced
from the exposed low-level center. Based on a blend of an earlier
partial ASCAT pass and subjective Dvorak satellite intensity
estimates, the initial intensity is lowered to 60 kt for this
advisory.

The system is encountering very strong vertical wind shear and ocean
temperatures below 20 degrees Celsius. The system is forecast to
continue to gradually weaken over the next couple of days. However,
the system will remain a strong extra-tropical cyclone with an
expanding wind field. The NHC intensity forecast is similar to the
previous advisory and lies near the model consensus.

Nigel continues to race northeastward at 32 kt. A northeastward
motion is forecast to continue the next day or so, as it remains in
the flow along the southern edge of a deep mid-latitude trough.
Afterwards, Nigel should rotate around the eastern side of a large
extratropical cyclone over the North Atlantic, with the two
extratropical systems merging in about 48 h. The model guidance
remains in fairly good agreement, and the NHC forecast lies in the
center of the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 22/0900Z 46.3N 32.6W 60 KT 70 MPH...POST-TROPICAL
12H 22/1800Z 48.8N 27.2W 50 KT 60 MPH...POST-TROP/EXTRATROP
24H 23/0600Z 53.0N 23.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 23/1800Z 56.4N 23.3W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 24/0600Z...DISSIPATED

$$
Forecaster Kelly