ATL: NIGEL - Advisories

[cycloneye]

Another major cane for the 2023 season.



TROPICAL DEPRESSION FIFTEEN FORECAST/ADVISORY NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL152023
ISSUED BY THE NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1500 UTC FRI SEP 15 2023

NOTICE... LAND-BASED TROPICAL CYCLONE WATCHES AND WARNINGS ARE NO
LONGER INCLUDED IN THE TROPICAL CYCLONE FORECAST/ADVISORY...(TCM).
CURRENT LAND-BASED COASTAL WATCHES AND WARNINGS CAN BE FOUND IN THE
MOST RECENTLY ISSUED TROPICAL CYCLONE PUBLIC ADVISORY...(TCP).

TROPICAL DEPRESSION CENTER LOCATED NEAR 14.4N 43.8W AT 15/1500Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE NORTHWEST OR 320 DEGREES AT 10 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1007 MB
MAX SUSTAINED WINDS 30 KT WITH GUSTS TO 40 KT.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 14.4N 43.8W AT 15/1500Z
AT 15/1200Z CENTER WAS LOCATED NEAR 14.0N 43.5W

FORECAST VALID 16/0000Z 15.5N 44.7W
MAX WIND 35 KT...GUSTS 45 KT.
34 KT... 40NE 0SE 0SW 30NW.

FORECAST VALID 16/1200Z 17.2N 46.0W
MAX WIND 40 KT...GUSTS 50 KT.
34 KT... 90NE 40SE 0SW 70NW.

FORECAST VALID 17/0000Z 18.9N 47.9W
MAX WIND 50 KT...GUSTS 60 KT.
50 KT... 30NE 0SE 0SW 0NW.
34 KT...100NE 60SE 50SW 100NW.

FORECAST VALID 17/1200Z 20.5N 49.6W
MAX WIND 60 KT...GUSTS 75 KT.
50 KT... 50NE 40SE 0SW 40NW.
34 KT...110NE 80SE 60SW 100NW.

FORECAST VALID 18/0000Z 22.0N 51.2W
MAX WIND 70 KT...GUSTS 85 KT.
50 KT... 50NE 40SE 30SW 40NW.
34 KT...130NE 110SE 80SW 100NW.

FORECAST VALID 18/1200Z 23.4N 52.9W
MAX WIND 85 KT...GUSTS 105 KT.
50 KT... 60NE 50SE 40SW 50NW.
34 KT...140NE 130SE 100SW 130NW.

EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 125 NM
ON DAY 4 AND 175 NM ON DAY 5...AND FOR INTENSITY NEAR 15 KT EACH DAY

OUTLOOK VALID 19/1200Z 26.0N 56.8W
MAX WIND 105 KT...GUSTS 130 KT.

OUTLOOK VALID 20/1200Z 29.0N 60.0W
MAX WIND 100 KT...GUSTS 120 KT.

REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 14.4N 43.8W

NEXT ADVISORY AT 15/2100Z

$$
FORECASTER CHURCHILL/BLAKE/LAMERS




BULLETIN
Tropical Depression Fifteen Advisory Number 1
NWS National Hurricane Center Miami FL AL152023
Issued by the NWS Weather Prediction Center College Park MD
1100 AM AST Fri Sep 15 2023

...NEW TROPICAL DEPRESSION FORMS IN THE CENTRAL ATLANTIC...
...EXPECTED TO BECOME A HURRICANE WELL NORTHEAST OF THE LESSER
ANTILLES...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...14.4N 43.8W
ABOUT 1170 MI...1880 KM E OF THE LESSER ANTILLES
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...NW OR 320 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1007 MB...29.74 INCHES




Tropical Depression Fifteen Discussion Number 1
NWS National Hurricane Center Miami FL AL152023
Issued by the NWS Weather Prediction Center College Park MD
1100 AM AST Fri Sep 15 2023

1-min GOES-East visible imagery indicates improved organization of
Invest 97, with a well-defined surface center forming since
sunrise, along with large curved banding features. Thus the system
is designated as a tropical depression, and the initial intensity
is set to 30 kt, in closest agreement with the TAFB Dvorak
classification.

Our estimate of initial motion is northwestward at 10 kt, though
this is uncertain due to the recent center formation. A
subtropical ridge over the central Atlantic should be the dominant
steering mechanism during the next several days, sending the system
on a steady northwestward track. For a first forecast, guidance is
in remarkably good agreement, and the official forecast lies near
or west of the model consensus. Despite the good model agreement,
there is still some long-range uncertainty in how far west this
system moves, which partially depends on the strength of the
subtropical ridge in the wake of Tropical Storm Margot.

The depression is forecast to gradually intensify over the next
couple of days as moderate northeasterly shear and the initial broad
cyclone structure could provide a check on the intensification
rate. By early next week, this system is forecast to move over
near record warm sea-surface temperatures for the region in light
shear conditions. It probably sounds like a broken record at this
point in the season, but rapid intensification is a significant
possibility, and the official forecast could be conservative below.
The intensity forecast is near or above the intensity consensus,
only leveling off at 120 hours due to gradual SST cooling and the
possibility of eyewall replacement cycles, as suggested by the HAFS
model suite.


FORECAST POSITIONS AND MAX WINDS

INIT 15/1500Z 14.4N 43.8W 30 KT 35 MPH
12H 16/0000Z 15.5N 44.7W 35 KT 40 MPH
24H 16/1200Z 17.2N 46.0W 40 KT 45 MPH
36H 17/0000Z 18.9N 47.9W 50 KT 60 MPH
48H 17/1200Z 20.5N 49.6W 60 KT 70 MPH
60H 18/0000Z 22.0N 51.2W 70 KT 80 MPH
72H 18/1200Z 23.4N 52.9W 85 KT 100 MPH
96H 19/1200Z 26.0N 56.8W 105 KT 120 MPH
120H 20/1200Z 29.0N 60.0W 100 KT 115 MPH

$$
Forecaster Churchill/Blake/Lamers

[cycloneye]

Tropical Depression Fifteen Discussion Number 2
NWS National Hurricane Center Miami FL AL152023
500 PM AST Fri Sep 15 2023

The depression has lost some convective organization since this
morning, but the low-level circulation appears to be better defined
in recent visible satellite imagery. There are a couple of bursts of
convection just north of the center, but there is a lack of overall
banding at this time. The initial intensity is held at 30 kt, which
is in agreement with the latest Dvorak Data T-number from TAFB.

The center appears to have reformed to the north since this
morning, which had lead to a rather uncertain initial motion
estimate of 320/11 kt. The global model guidance suggests that
there could be some additional center reformations during the next
12-24 hours until an inner-core becomes better established. The
early portion of the track forecast was shifted northward, based on
the farther north initial position. A deep-layer ridge over the
central Atlantic is expected to steer the system northwestward
during the next several days. By day 4, a mid-latitude trough
moving over the western Atlantic should allow the cyclone to turn
north-northwestward near the end of the period. The 12Z dynamical
model guidance made a noticeable eastward shift, and the NHC
forecast has been adjusted in that direction. The updated track
forecast is close to the consensus aids through 72 hours, but is
along the left side of the model envelope at days 4 and 5.

The depression is likely to only gradually intensify during the
next 12 hours or so as it is still in the formative stage and lacks
inner-core structure. After that time, a faster rate of
intensification is indicated in the official forecast while the
system remains in an environment of low vertical wind shear and it
traverses warm sea surface temperatures. The intensity guidance
is somewhat lower this cycle, but the NHC forecast remains generally
the same as before. This is near the upper-end of the latest
guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 15/2100Z 15.4N 44.0W 30 KT 35 MPH
12H 16/0600Z 16.7N 45.0W 30 KT 35 MPH
24H 16/1800Z 18.5N 46.6W 40 KT 45 MPH
36H 17/0600Z 20.4N 48.4W 50 KT 60 MPH
48H 17/1800Z 22.0N 50.1W 60 KT 70 MPH
60H 18/0600Z 23.6N 51.7W 70 KT 80 MPH
72H 18/1800Z 25.1N 53.6W 85 KT 100 MPH
96H 19/1800Z 28.0N 57.3W 100 KT 115 MPH
120H 20/1800Z 32.0N 59.0W 100 KT 115 MPH

$$
Forecaster Brown

[cycloneye]

Tropical Depression Fifteen Discussion Number 3
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Fri Sep 15 2023

Deep convection has increased over the northern portion of the
depression's broad circulation this evening, however it is still
not particularly well organized. A recent ASCAT passed indicated
that the circulation is still broad and that the center may be
trying to reform farther north closer to the convection. The
scatterometer data revealed peak winds of 25-30 kt, and the initial
wind speed is maintained at 30 kt for this advisory.

The initial motion is a still somewhat uncertain 320/12 kt. A
deep-layer ridge over the central Atlantic should steer the
depression northwestward through early next week. By late
Tuesday, a mid-latitude trough moving westward over the western
Atlantic is expected to cause the cyclone to turn north-
northwestward and northward near the end of the forecast period.
The track guidance is in relatively good agreement on this
scenario, and the latest NHC track forecast lies near the middle of
the guidance envelope.

Given the current structure of the system, only gradual
strengthening is likely overnight. Once an inner core become
established a faster rate of strengthening seems likely as the
system traverses warm waters and remains in a low vertical wind
shear conditions. Steady to potentially rapid strengthening is
predicted later this weekend, and the system is forecast to become
a hurricane by late Sunday or Monday. The NHC intensity forecast is
near the higher-end of the guidance, close to the HCCA model
through 72 hours, and is then near the higher dynamical guidance at
96 h. By late in the period, increasing southwesterly vertical wind
shear is likely to cause some gradual weakening.


FORECAST POSITIONS AND MAX WINDS

INIT 16/0300Z 16.3N 44.7W 30 KT 35 MPH
12H 16/1200Z 17.9N 45.8W 35 KT 40 MPH
24H 17/0000Z 20.1N 47.7W 45 KT 50 MPH
36H 17/1200Z 21.7N 49.4W 55 KT 65 MPH
48H 18/0000Z 23.3N 50.9W 70 KT 80 MPH
60H 18/1200Z 24.7N 52.6W 80 KT 90 MPH
72H 19/0000Z 26.0N 54.6W 90 KT 105 MPH
96H 20/0000Z 29.3N 57.9W 100 KT 115 MPH
120H 21/0000Z 34.1N 58.3W 90 KT 105 MPH

$$
Forecaster Brown

[cycloneye]

BULLETIN
Tropical Depression Fifteen Advisory Number 4
NWS National Hurricane Center Miami FL AL152023
500 AM AST Sat Sep 16 2023

...TROPICAL DEPRESSION EXPECTED TO BECOME A TROPICAL STORM THIS
WEEKEND...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...17.9N 45.3W
ABOUT 1070 MI...1725 KM ENE OF THE LESSER ANTILLES
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...NNW OR 330 DEGREES AT 14 MPH...22 KM/H
MINIMUM CENTRAL PRESSURE...1007 MB...29.74 INCHES


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


DISCUSSION AND OUTLOOK
----------------------
At 500 AM AST (0900 UTC), the center of Tropical Depression Fifteen
was located near latitude 17.9 North, longitude 45.3 West. The
depression is moving toward the north-northwest near 14 mph (22
km/h), and a general northwest or north-northwest motion is
expected to continue for the next few days.

Maximum sustained winds are near 35 mph (55 km/h) with higher gusts.
Strengthening is expected during the next few days, and the
depression is forecast to become a tropical storm later today and
it could become a hurricane by early next week.

The estimated minimum central pressure is 1007 mb (29.74 inches).


HAZARDS AFFECTING LAND
----------------------
None


NEXT ADVISORY
-------------
Next complete advisory at 1100 AM AST.

$$
Forecaster Papin

[cycloneye]

Tropical Depression Fifteen Discussion Number 5
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Sat Sep 16 2023

The tropical depression has not changed much this morning. The
center remains poorly defined, and visible satellite imagery
indicates a broad region of low- and mid-level rotation persisting
over a large area. Most of the deep convection is occurring in the
northern semicircle of the broad circulation. Subjective Dvorak
estimates from SAB and TAFB were 25 kt and 35 kt, respectively, and
are similar to the objective Dvorak estimates. Based on the
disorganized structure and the current intensity estimates, the
intensity is held at 30 kt for this advisory. A late arriving ASCAT
pass suggests that this estimate may be a bit generous.

Since the center of circulation is broad, the initial motion is
somewhat uncertain and estimated at 330/16 kt. This general motion
is expected to continue for the next couple days as the depression
moves along the southern periphery of a mid-level ridge located to
the northeast of the system. Track guidance, including global and
regional hurricane models, is in general agreement that the system
will continue on a northwestward trajectory for the next several
days. After 72 hours, the track forecast uncertainty increases
somewhat as the system will begin to interact with a deep-layer
trough. This trough interaction is expected to induce a more
poleward shift in the track and eventually recurve the system toward
the northeast by 120 hours. The NHC track forecast is similar to the
prior forecast and in agreement with the consensus aids, TVCN and
HCCA.

The system is currently in a favorable environment for
intensification, with low vertical wind shear and warm sea surface
temperatures expected to persist during the next several days.
However, the timing of intensification will depend on how quickly
the system is able to consolidate into a more compact structure. The
current intensity forecast assumes that it will take 24-36 hours for
this to happen, with the system expected to become a hurricane in 48
hours. The current forecast reaches 95 kt by 72 h, although some of
the regional hurricane models, including HWRF and HMON, indicate a
more rapid intensification. ECMWF-SHIPS guidance continues to
indicate a greater than a 40 percent chance of rapid intensification
in the next 72 hours. By mid-week, as the system recurves to the
northeast, it is expected to gradually weaken as southwesterly shear
increases. The NHC forecast is lower than the regional model
guidance and is similar to both the prior forecast and HCCA.


FORECAST POSITIONS AND MAX WINDS

INIT 16/1500Z 19.7N 46.2W 30 KT 35 MPH
12H 17/0000Z 21.3N 47.4W 35 KT 40 MPH
24H 17/1200Z 23.2N 49.0W 45 KT 50 MPH
36H 18/0000Z 24.7N 50.4W 55 KT 65 MPH
48H 18/1200Z 26.2N 52.2W 65 KT 75 MPH
60H 19/0000Z 27.5N 54.1W 75 KT 85 MPH
72H 19/1200Z 28.9N 56.0W 95 KT 110 MPH
96H 20/1200Z 32.8N 58.3W 90 KT 105 MPH
120H 21/1200Z 37.0N 55.2W 80 KT 90 MPH

$$
Forecaster Hogsett/Bucci

[cycloneye]

Tropical Depression Fifteen Discussion Number 6
NWS National Hurricane Center Miami FL AL152023
500 PM AST Sat Sep 16 2023

The tropical depression remains poorly organized this afternoon.
However, visible satellite imagery continues to indicate a broad
region of low- and mid-level rotation, and deep convection has
increased somewhat in the northern semicircle of the disturbance.
Subjective Dvorak estimates from SAB and TAFB remain steady at 25 kt
and 35 kt, respectively. Based on the current structure and the
various intensity estimates, the intensity is held at 30 kt for this
advisory.

The initial motion is estimated at 330/14 kt. This general motion
is expected to continue for the next couple days as the depression
moves along the southern periphery of a mid-level ridge. Consensus
track guidance, as well as the global and regional hurricane models,
are in general agreement during the early portion of the forecast
period. Early next week, the track forecast uncertainty increases
somewhat as the system will begin to interact with a deep-layer
trough. This trough interaction is expected to induce a more
poleward shift in the track and eventually recurve the system toward
the northeast after 96 hours. The NHC track forecast is in agreement
with the consensus aids, TVCN and HCCA, and slightly faster than
the prior forecast.

Environmental conditions remain favorable for intensification during
the next several days, with low vertical wind shear and warm sea
surface temperatures expected to persist along the forecast track.
The timing of intensification will depend on how quickly the system
is able to consolidate into a more compact structure. The current
intensity forecast assumes that it will take 24-36 hours for this to
happen, with the system expected to become a hurricane in 48 hours.
The current forecast reaches 95 kt by 72 h, although the regional
hurricane models HWRF and HAFS indicate that a major hurricane is
not out of the question. ECMWF-SHIPS guidance continues to indicate
the possibility of rapid intensification in the next 72 hours. By
mid-week, as the system recurves to the northeast, it is expected to
gradually weaken as southwesterly shear increases. The NHC forecast
is in the middle of the guidance envelope and is similar to both the
prior forecast and HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT 16/2100Z 20.8N 47.1W 30 KT 35 MPH
12H 17/0600Z 22.3N 48.2W 35 KT 40 MPH
24H 17/1800Z 24.2N 49.7W 45 KT 50 MPH
36H 18/0600Z 25.6N 51.1W 55 KT 65 MPH
48H 18/1800Z 26.9N 52.9W 65 KT 75 MPH
60H 19/0600Z 28.3N 54.8W 80 KT 90 MPH
72H 19/1800Z 30.0N 56.4W 95 KT 110 MPH
96H 20/1800Z 34.6N 57.3W 90 KT 105 MPH
120H 21/1800Z 39.6N 52.0W 80 KT 90 MPH

$$
Forecaster Hogsett/Bucci

[cycloneye]

BULLETIN
Tropical Storm Nigel Advisory Number 7
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Sat Sep 16 2023

...TROPICAL STORM NIGEL FORMS IN THE CENTRAL ATLANTIC...
...EXPECTED TO BECOME A HURRICANE ON MONDAY...


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...22.0N 48.0W
ABOUT 980 MI...1580 KM ENE OF THE LESSER ANTILLES
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...NW OR 325 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...1005 MB...29.68 INCHES


Tropical Storm Nigel Discussion Number 7
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Sat Sep 16 2023

The satellite presentation of the cyclone has improved this evening
as there has been an increase in convective banding over the
northern portion of the circulation. The center appears to be
located near the southern portion of the convective mass. Dvorak
Data T-numbers from both SAB and TAFB were T2.5 (35 kt) at 0000 UTC
and objective satellite estimates from UW/CIMSS are in the 35 to 40
kt range. Based on these data, the initial intensity has been
raised to 35 kt. Nigel becomes the 10th named storm to form in
the Atlantic basin since August 20th.

Nigel is moving northwestward at about 14 kt. A mid-level ridge
to the northeast of the cyclone should steer the storm
northwestward during the next few days. After that time, Nigel is
expected to turn northward around the western side of the ridge. By
the middle of next week, the global model guidance indicates that
the storm should turn northeast ahead of a broad mid-latitude
trough over the western Atlantic. There is fairly good agreement
in the global model guidance on the overall scenario, but there are
some timing or forward speed difference later in the period. The
NHC forecast track is near the HFIP corrected consensus which is
close to a blend of the latest GFS and ECMWF models.

Environmental conditions are expected to be conducive for steady
intensification during the next few days, with low vertical wind
shear and warm sea surface temperatures along the forecast track.
A period of rapid intensification is is certainly possible within
the next two to three days. Although the SHIP RI probabilities are
not very high at the moment, DTOPS gives a 50 percent chance of a 65
kt increase in intensity during the next 72 hours. The NHC forecast
is close to that, showing a 60 kt increase in strength during the
time. After 72 hours, increasing southwesterly shear is likely to
cause gradual weakening. The NHC wind speed forecast is closest to
the HFIP corrected consensus.


FORECAST POSITIONS AND MAX WINDS

INIT 17/0300Z 22.0N 48.0W 35 KT 40 MPH
12H 17/1200Z 23.6N 49.2W 45 KT 50 MPH
24H 18/0000Z 25.3N 50.6W 55 KT 65 MPH
36H 18/1200Z 26.7N 52.2W 65 KT 75 MPH
48H 19/0000Z 27.8N 54.0W 75 KT 85 MPH
60H 19/1200Z 29.4N 55.7W 85 KT 100 MPH
72H 20/0000Z 31.3N 56.7W 95 KT 110 MPH
96H 21/0000Z 36.0N 55.6W 85 KT 100 MPH
120H 22/0000Z 41.9N 47.3W 80 KT 90 MPH

$$
Forecaster Brown

[cycloneye]

Tropical Storm Nigel Discussion Number 8
NWS National Hurricane Center Miami FL AL152023
500 AM AST Sun Sep 17 2023

Nigel's satellite structure has improved overnight as deep
convection has taken on a distinct curved band appearance to the
north of the estimated center. With that said, we have not
received any recent microwave imagery to confirm if the low-level
circulation has tightened up or has become more embedded in the deep
convection, though derived motion winds from the 1-minute meso
sector over the system do suggest the center has likely tucked
underneath the convective cirrus canopy. Subjective Dvorak intensity
estimates from TAFB and SAB were a consensus T3.0/45 kt, and the
recent objective estimates from UW-CIMSS are in the 44-50 kt range.
The initial intensity will be set at 45 kt, on the lower end given
the uncertainty on the center position.

Based on the estimated position, Nigel has been moving somewhat
right of the prior track forecast, off to the north-northwest at
330/12 kt. This continued rightward motion relative to the track
forecast over the past 24 hours could be related to Margot eroding
the mid-level ridging located northeast of Nigel, in addition to a
mid- to upper-level trough favoring more convection on Nigel's
eastern flank overall. However, as Margot continues to weaken, this
mid-level ridging should rebuild, allowing Nigel to bend a bit more
northwestward in the 24-48 hour time frame. By the middle of next
week, the global model guidance is in good agreement that Nigel
should recurve to the northeast as it gets picked up by a
mid-latitude trough ejecting off the eastern U.S. coastline. The NHC
track forecast continues to lie near the TVCA and HCCA consensus
aids, which is a little east of the prior forecast, but not far off
of a blend of the latest GFS and ECMWF models.

While it is unclear to know without recent microwave imagery if
Nigel is in the early stages of developing an inner core, the
improvement of the larger-scale structure on satellite suggests
further intensification is likely. Both GFS- and ECMWF-based SHIPS
guidance show vertical wind shear should remain low (under 10 kt)
for the next 48-72 hours, while sea-surface temperatures remain
between 28-30C. While mid-level relative humidity is a bit on the
low side, this may only help to tighten up the core from its initial
large origin assuming the shear remains low. SHIPS-RII and DTOPS
continue to suggest a significant chance of rapid intensification
over the next 2-3 days, and the latest NHC intensity forecast now
shows Nigel peaking as a category 3 hurricane again. This is a bit
higher than the IVCN and HCCA consensus aids, but close to the raw
model output of the HAFS, HWRF, and COAMPS-TC models. By the end of
the forecast, Nigel will likely be undergoing extratropical
transition, though this process might not be complete until just
beyond the 120 h point.


FORECAST POSITIONS AND MAX WINDS

INIT 17/0900Z 23.0N 48.6W 45 KT 50 MPH
12H 17/1800Z 24.5N 49.7W 55 KT 65 MPH
24H 18/0600Z 26.0N 51.0W 65 KT 75 MPH
36H 18/1800Z 27.5N 52.5W 80 KT 90 MPH
48H 19/0600Z 28.8N 54.0W 95 KT 110 MPH
60H 19/1800Z 30.5N 55.4W 100 KT 115 MPH
72H 20/0600Z 32.7N 55.8W 100 KT 115 MPH
96H 21/0600Z 38.0N 52.0W 85 KT 100 MPH
120H 22/0600Z 44.5N 40.0W 70 KT 80 MPH

$$
Forecaster Papin

[cycloneye]

Tropical Storm Nigel Discussion Number 9
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Sun Sep 17 2023

Based on satellite imagery, Nigel appears to be steadily
strengthening. A prominent curved band of deep convection wraps
around the northern half of the tropical storm, with additional
convective activity recently observed very near its center.
Satellite intensity estimates range from around 45 to 55 kt, so the
initial intensity is set at a near-consensus value of 50 kt.

Nigel is in an environment that should support strengthening, and in
fact the SHIPS-RII and DTOPS rapid intensification models indicate
it has a near 50/50 chance of strengthening 25 kt in the next 24 h
and/or 55 kt in the next 48 h. Recent SSMIS imagery at 0911 UTC
indicated that the inner core of Nigel is probably not yet
well-enough developed to support immediate rapid intensification,
but that could happen at some point later today. Most of the
intensity models indicate that Nigel's intensity will peak in about
60 h, before increasing wind shear and cooler waters begin to affect
it. Extratropical transition should then begin in about 4 days, and
that process will likely be near complete at the end of the forecast
period. Only very small adjustments were made to the NHC intensity
forecast, which is very near the IVCN intensity consensus.

Likewise, nearly no changes were made to the official track
forecast. For the next 4-5 days, Nigel should generally move between
a ridge centered between Nigel and Margot, and a mid-latitude trough
that is forecast to move off the eastern coasts of the U.S. and
Canada early this week. These features will likely steer Nigel
generally northwestward for the next 2 days, and cause it to
accelerate northeastward after that. The NHC track forecast
continues to be based primarily on the HCCA and TVCN models.


FORECAST POSITIONS AND MAX WINDS

INIT 17/1500Z 24.0N 49.1W 50 KT 60 MPH
12H 18/0000Z 25.3N 50.2W 60 KT 70 MPH
24H 18/1200Z 26.8N 51.7W 75 KT 85 MPH
36H 19/0000Z 28.2N 53.1W 85 KT 100 MPH
48H 19/1200Z 29.8N 54.6W 95 KT 110 MPH
60H 20/0000Z 31.8N 55.4W 100 KT 115 MPH
72H 20/1200Z 34.2N 54.9W 95 KT 110 MPH
96H 21/1200Z 39.6N 49.0W 85 KT 100 MPH
120H 22/1200Z 46.5N 35.5W 65 KT 75 MPH

$$
Forecaster D. Zelinsky

[cycloneye]

Tropical Storm Nigel Discussion Number 10
NWS National Hurricane Center Miami FL AL152023
500 PM AST Sun Sep 17 2023

Nigel's organization continues to increase, with several convective
bands noted over the northern portion of the circulation.
Although microwave imagery shows that an eyewall structure has not
yet formed, it seems likely that an inner core will develop fairly
soon. The current intensity is set at 55 kt based on a subjective
Dvorak satellite intensity estimate from TAFB although objective
estimates are a little lower.

Nigel continues northwestward with an estimated motion of 325/11 kt.
The system should continue on a generally northwestward track over
the next couple of days while a mid-level high builds to its
east-northeast. By Wednesday, Nigel should turn northward and move
along the western side of the high. Later in the forecast period,
the system is expected to accelerate northeastward in the flow on
the southeast side of a strong mid-latitude trough. The NHC
forecast is roughly in the middle of the track guidance envelope and
lies between the simple and corrected dynamical model consensus
solutions.

The system's environment appears to be very conducive for
strengthening. The vertical wind shear is quite weak and there is
well-defined upper-level anticyclonic outflow over the area. The
various rapid intensification (RI) indices are in agreement on a
significantly higher-than-normal likelihood of RI. This is also
reflected in the official intensity forecast, which shows Nigel
becoming a major hurricane within the next couple of days. By the
end of the forecast period, the system will likely become embedded
within a frontal zone and transition into a strong extratropical
cyclone.


FORECAST POSITIONS AND MAX WINDS

INIT 17/2100Z 25.1N 49.5W 55 KT 65 MPH
12H 18/0600Z 26.3N 50.6W 70 KT 80 MPH
24H 18/1800Z 27.8N 52.1W 85 KT 100 MPH
36H 19/0600Z 29.2N 53.6W 100 KT 115 MPH
48H 19/1800Z 31.0N 55.0W 100 KT 115 MPH
60H 20/0600Z 33.1N 55.4W 100 KT 115 MPH
72H 20/1800Z 35.7N 54.2W 90 KT 105 MPH
96H 21/1800Z 41.3N 45.9W 75 KT 85 MPH
120H 22/1800Z 47.8N 31.7W 60 KT 70 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

[cycloneye]

Tropical Storm Nigel Discussion Number 11
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Sun Sep 17 2023

Proxy-vis and infrared satellite imagery show that Nigel is
becoming better organized and continues to strengthen this evening.
Deep convection continues to burst near the center, with cold cloud
tops to -80 degrees Celsius. An earlier SSMI/S microwave pass
depicts that an inner core has developed and it is vertically
aligned with the low-level center. Subjective Dvorak estimates from
TAFB and SAB were T4.0/T3.5, respectively. Given a blend of these
estimates, the initial intensity is increased to 60 kt for this
advisory.

Nigel is moving north-northwestward with an estimated motion of
330/10 kt. This general north-northwestward heading should continue
for the next couple of days as a mid-level ridge builds to the
northeast of the system. In about 2 to 3 days, Nigel should begin
to round the western edge of the ridge and move more northward. By
the end of the forecast period, a strong trough will pick up the
system, and Nigel should accelerate towards the northeast. The NHC
track forecast is similar to the previous advisory, and lies near
the simple and corrected consensus aids.

The environment around Nigel is conducive for steady to rapid
intensification. Vertical wind shear is fairly low, with warm sea
surface temperatures, and outflow aloft continues to become more
pronounced. The SHIPS and DTOPS rapid intensification (RI) indices
continue to show above climatological normal chances of RI. Given
the improving structure and the favorable environment, the NHC
intensity forecast reflects this and forecasts rapid
intensification over the next 24-36 hrs, with Nigel expected to
become a hurricane overnight, and a major hurricane on Tuesday.
Towards the end of the period, Nigel will likely become a strong
extratropical cyclone as the aforementioned trough picks up the
system, and it becomes embedded within a frontal zone. The NHC
intensity forecast lies near the upper-end of the guidance envelope,
closest to the HCCA and IVCN intensity consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 18/0300Z 25.8N 50.2W 60 KT 70 MPH
12H 18/1200Z 27.0N 51.3W 75 KT 85 MPH
24H 19/0000Z 28.5N 52.8W 90 KT 105 MPH
36H 19/1200Z 30.1N 54.2W 100 KT 115 MPH
48H 20/0000Z 32.1N 55.2W 100 KT 115 MPH
60H 20/1200Z 34.4N 54.8W 95 KT 110 MPH
72H 21/0000Z 37.4N 52.5W 90 KT 105 MPH
96H 22/0000Z 43.1N 42.1W 75 KT 85 MPH
120H 23/0000Z 50.8N 28.0W 60 KT 70 MPH...POST-TROP/EXTRATROP

$$
Forecaster Kelly

[abajan]

Hurricane Nigel Advisory Number 12
NWS National Hurricane Center Miami FL AL152023
500 AM AST Mon Sep 18 2023

...NIGEL FORECAST TO RAPIDLY INTENSIFY INTO A MAJOR HURRICANE ON
TUESDAY...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...26.5N 50.7W
ABOUT 935 MI...1510 KM ESE OF BERMUDA
MAXIMUM SUSTAINED WINDS...80 MPH...130 KM/H
PRESENT MOVEMENT...NW OR 325 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...984 MB...29.06 INCHES


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


DISCUSSION AND OUTLOOK
----------------------
At 500 AM AST (0900 UTC), the center of Hurricane Nigel was located
near latitude 26.5 North, longitude 50.7 West. Nigel is moving
toward the northwest near 12 mph (19 km/h), and this general motion
is expected to continue for the next couple of days. Nigel is then
forecast to turn northward late Tuesday, and then accelerate
northeastward through the rest of the week.

Maximum sustained winds have increased to near 80 mph (130 km/h)
with higher gusts. Nigel is forecast to rapidly strengthen into a
major hurricane on Tuesday. A gradual weakening trend could start
late Wednesday.

Hurricane-force winds extend outward up to 25 miles (35 km) from the
center and tropical-storm-force winds extend outward up to 160 miles
(260 km).

The estimated minimum central pressure is 984 mb (29.06 inches).


HAZARDS AFFECTING LAND
----------------------
None


NEXT ADVISORY
-------------
Next complete advisory at 1100 AM AST.

$$
Forecaster Blake

[abajan]

Tropical Storm Nigel Discussion Number 11
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Sun Sep 17 2023

Proxy-vis and infrared satellite imagery show that Nigel is
becoming better organized and continues to strengthen this evening.
Deep convection continues to burst near the center, with cold cloud
tops to -80 degrees Celsius. An earlier SSMI/S microwave pass
depicts that an inner core has developed and it is vertically
aligned with the low-level center. Subjective Dvorak estimates from
TAFB and SAB were T4.0/T3.5, respectively. Given a blend of these
estimates, the initial intensity is increased to 60 kt for this
advisory.

Nigel is moving north-northwestward with an estimated motion of
330/10 kt. This general north-northwestward heading should continue
for the next couple of days as a mid-level ridge builds to the
northeast of the system. In about 2 to 3 days, Nigel should begin
to round the western edge of the ridge and move more northward. By
the end of the forecast period, a strong trough will pick up the
system, and Nigel should accelerate towards the northeast. The NHC
track forecast is similar to the previous advisory, and lies near
the simple and corrected consensus aids.

The environment around Nigel is conducive for steady to rapid
intensification. Vertical wind shear is fairly low, with warm sea
surface temperatures, and outflow aloft continues to become more
pronounced. The SHIPS and DTOPS rapid intensification (RI) indices
continue to show above climatological normal chances of RI. Given
the improving structure and the favorable environment, the NHC
intensity forecast reflects this and forecasts rapid
intensification over the next 24-36 hrs, with Nigel expected to
become a hurricane overnight, and a major hurricane on Tuesday.
Towards the end of the period, Nigel will likely become a strong
extratropical cyclone as the aforementioned trough picks up the
system, and it becomes embedded within a frontal zone. The NHC
intensity forecast lies near the upper-end of the guidance envelope,
closest to the HCCA and IVCN intensity consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 18/0300Z 25.8N 50.2W 60 KT 70 MPH
12H 18/1200Z 27.0N 51.3W 75 KT 85 MPH
24H 19/0000Z 28.5N 52.8W 90 KT 105 MPH
36H 19/1200Z 30.1N 54.2W 100 KT 115 MPH
48H 20/0000Z 32.1N 55.2W 100 KT 115 MPH
60H 20/1200Z 34.4N 54.8W 95 KT 110 MPH
72H 21/0000Z 37.4N 52.5W 90 KT 105 MPH
96H 22/0000Z 43.1N 42.1W 75 KT 85 MPH
120H 23/0000Z 50.8N 28.0W 60 KT 70 MPH...POST-TROP/EXTRATROP

$$
Forecaster Kelly

[cycloneye]

Hurricane Nigel Discussion Number 13
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Mon Sep 18 2023

Nigel is exhibiting a somewhat ragged-looking eye on satellite
imagery, surrounded by bands of very deep convection with cloud
tops to -80 deg C. High-level outflow has become a bit restricted
over the northeastern quadrant of the circulation, but overall the
system continues to become better organized. The current intensity
estimate of 70 kt is a blend of subjective and objective
satellite-derived estimates.

During the next day or so, the hurricane should remain in an
environment that is quite conducive for strengthening. Vertical
shear is forecast to remain very low and SSTs are near 28 deg C.
The SHIPS Rapid Intensification (RI) Index continues to show above
normal probabilities of RI. Accordingly, the official forecast
calls for a 30-kt increase in intensity during the ensuing 24
hours. In a couple of days, increasing shear and cooler waters are
likely to result in the beginning of a gradual weakening trend. In
4-5 days, global forecast models depict Nigel becoming involved with
a frontal zone over the North Atlantic. Therefore, the official
forecast shows the system as extratropical in 5 days, although this
transition could occur a little sooner than that.

Nigel's motion continues toward the northwest, or at about 325/10
kt. The hurricane is currently situated on the southwestern edge
of a mid-level high pressure area. Over the next day or two, the
system should move around the western periphery of the high.
On this track, Nigel will turn northward in about 36 hours.
Thereafter, the cyclone should accelerate in the flow to the
southeast and south of a strong mid-latitude trough, and head into
the higher latitudes of the Atlantic. The track guidance is
fairly tightly clustered, and the new official forecast is
very similar to the previous one. This is also in close agreement
with the dynamical model consensus aids, HCCA and TVCA.


FORECAST POSITIONS AND MAX WINDS

INIT 18/1500Z 27.2N 51.4W 70 KT 80 MPH
12H 19/0000Z 28.4N 52.5W 85 KT 100 MPH
24H 19/1200Z 30.0N 53.9W 100 KT 115 MPH
36H 20/0000Z 32.0N 54.8W 100 KT 115 MPH
48H 20/1200Z 34.6N 54.5W 95 KT 110 MPH
60H 21/0000Z 37.4N 52.4W 90 KT 105 MPH
72H 21/1200Z 40.3N 47.7W 85 KT 100 MPH
96H 22/1200Z 46.5N 33.0W 70 KT 80 MPH
120H 23/1200Z 53.5N 20.0W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 14
NWS National Hurricane Center Miami FL AL152023
500 PM AST Mon Sep 18 2023

The center of Nigel is embedded within a central dense overcast of
cold infrared cloud tops this afternoon. Recent GMI microwave images
do not show much improvement to the hurricane's inner core
structure, as it appears some drier air could be intruding into the
north side of the circulation. There is no eye feature apparent in
conventional satellite imagery at this time. The latest subjective
Dvorak estimates are T4.0 (SAB) and T4.5 (TAFB). Meanwhile, the
latest UW-CIMSS ADT and SATCON intensity estimates are 63 kt and 71
kt, respectively. A blend of these data support holding the initial
intensity at 70 kt for this advisory.

Despite seemingly conducive environmental conditions for
strengthening, Nigel's intensification appears to have been
disrupted, possibly by dry air entrainment related to some mid-level
shear. If the hurricane can solidify its inner core during the next
day or so, the very warm SSTs and relatively low shear should allow
for some strengthening in the near term. The NHC forecast has been
slightly lowered from the previous one but still shows Nigel peaking
near major hurricane intensity in 36-48 h. This prediction lies
above the latest intensity guidance, closest to HCCA and some of the
regional hurricane models (HAFS-A). Beyond 48 h, increasing
deep-layer shear associated with the upper trough and rapidly
cooling SSTs are forecast to induce steady weakening through the
rest of the period.

A mid-level ridge over the central Atlantic continues to steer Nigel
northwestward (320/10 kt). The hurricane is expected to turn
northward over the next couple of days while it moves around the
western periphery of this steering ridge. Then, an upper-level
trough moving across the western Atlantic is forecast to cause Nigel
to accelerate northeastward and move deeper in the mid-latitudes
through late this week. The NHC forecast remains near the center of
the tightly clustered track guidance, with a faster forward motion
shown later in the period in agreement with the TVCA and HCCA aids.
This takes Nigel more quickly into the mid-latitudes, and the latest
global model fields and phase diagrams suggest its extratropical
transition should be complete by 96 h.


FORECAST POSITIONS AND MAX WINDS

INIT 18/2100Z 28.0N 52.2W 70 KT 80 MPH
12H 19/0600Z 29.2N 53.3W 80 KT 90 MPH
24H 19/1800Z 31.0N 54.5W 90 KT 105 MPH
36H 20/0600Z 33.4N 54.8W 95 KT 110 MPH
48H 20/1800Z 36.2N 53.5W 95 KT 110 MPH
60H 21/0600Z 39.1N 50.0W 85 KT 100 MPH
72H 21/1800Z 42.1N 43.9W 75 KT 85 MPH
96H 22/1800Z 48.0N 29.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
120H 23/1800Z 56.0N 18.0W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster B Reinhart/A Reinhart

[cycloneye]

Hurricane Nigel Discussion Number 15
NWS National Hurricane Center Miami FL AL152023
1100 PM AST Mon Sep 18 2023

Nigel has become a little better organized this evening, with a
ragged eye becoming more apparent in infrared satellite imagery. An
earlier SSMIS microwave overpass revealed a fairly well-defined
low-level inner-core structure, but the eye was open to the
northwest. Subjective Dvorak intensity estimates from TAFB and SAB
were a consensus T4.5 (77 kt), and the various objective estimates
were in the 70-80 kt range at 0000 UTC. Based on a blend of these
intensity estimates, the initial wind speed is set at 75 kt for
this advisory.

Although Nigel's strengthening appears to have been disrupted by
dry air entrainment today, the hurricane has about another 24-36
hours within low shear conditions and over warm SSTs in which
to strengthen. The NHC intensity forecast calls for additional
strengthening during that time, but the peak intensity has been
lowered slightly. After 36-48 hours, increasing southwesterly
vertical wind shear and cooler SSTS along the track of Nigel are
expected to cause gradual weakening. The NHC wind speed forecast
is in best agreement with the HFIP corrected-consensus aid, which
is at the higher end of the guidance in the short term. The global
models indicate that extratropical transition is likely to begin by
72 hours, and that process should be complete by day 4.

Nigel continues to move northwestward (310 degrees) at about 10 kt.
The hurricane should continue on that general heading and speed
through midday Tuesday, but as it nears the western extent of the
ridge, Nigel is predicted to turn northwestward and northward
shortly thereafter. By 48 hours, an approaching mid- to upper-level
trough is forecast to cause Nigel to accelerate northeastward over
the central and northeastern Atlantic through the remainder of the
forecast period. The track guidance remains tightly clustered, with
the primary differences being how much Nigel accelerates
northeastward later in the period. The NHC track forecast is a
blend of the typically reliable GFS and ECMWF models, nd is close
to the various consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 19/0300Z 28.6N 52.7W 75 KT 85 MPH
12H 19/1200Z 29.9N 53.8W 85 KT 100 MPH
24H 20/0000Z 32.1N 54.5W 90 KT 105 MPH
36H 20/1200Z 34.7N 54.1W 90 KT 105 MPH
48H 21/0000Z 37.6N 51.6W 85 KT 100 MPH
60H 21/1200Z 40.5N 46.6W 80 KT 90 MPH
72H 22/0000Z 43.4N 39.5W 70 KT 80 MPH
96H 23/0000Z 50.0N 24.7W 55 KT 65 MPH...POST-TROP/EXTRATROP
120H 24/0000Z 57.0N 17.4W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Brown

[cycloneye]

Hurricane Nigel Discussion Number 16
NWS National Hurricane Center Miami FL AL152023
500 AM AST Tue Sep 19 2023

Nigel is sending mixed signals this morning. While the large eye of
the hurricane has become better defined and warmer, convection
around the eyewall has weakened (and AMSR2 microwave data show the
eyewall is open on the north side). This convective disruption
appears to be due to persistent dry air and just enough shear to
entrain that air into the inner core. Satellite intensity estimates
remain about the same as 6 h ago, so the initial wind speed is
unchanged at 75 kt. Both of the NOAA Hurricane Hunter P-3 aircraft
are scheduled to be in the hurricane late this morning, so we should
have a more precise assessment of Nigel this afternoon.

The hurricane still has another day or so to intensify while it
remains in light-shear and warm-water conditions, which could
facilitate mixing out the dry air that has plagued the internal
convective structure. However, it seems like changes with Nigel will
be gradual since it has formed a large eye (which tends to be more
stable). Nigel should undergo a rapid extratropical transition from
48 to 72 h as it comes under the influence of a strong mid-latitude
trough. Little change was made to the previous intensity forecast,
above the intensity consensus at short range and near the model
average at longer range. SAR data from yesterday evening did
suggest Nigel that has grown in size, so that's reflected in the
current and near-term forecast wind radii.

The track forecast seems straightforward with Nigel, as the current
northwestward motion should bend to northward late today while the
hurricane moves along the western periphery of the subtropical
ridge. Nigel should then move rather quickly northeastward through
late week due to fast flow ahead of a mid-latitudes trough dropping
out of the Canadian Maritimes, then northward as an extratropical
low this weekend as it is captured by the trough south of Iceland.
The new forecast is shifted a touch left to come into better
agreement with the consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 19/0900Z 29.6N 53.6W 75 KT 85 MPH
12H 19/1800Z 31.2N 54.5W 85 KT 100 MPH
24H 20/0600Z 33.8N 54.8W 90 KT 105 MPH
36H 20/1800Z 36.7N 53.2W 85 KT 100 MPH
48H 21/0600Z 39.6N 49.2W 80 KT 90 MPH
60H 21/1800Z 42.5N 42.7W 70 KT 80 MPH
72H 22/0600Z 45.0N 34.7W 60 KT 70 MPH...POST-TROP/EXTRATROP
96H 23/0600Z 51.8N 21.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 24/0600Z 58.5N 21.0W 45 KT 50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Blake

[cycloneye]

Hurricane Nigel Discussion Number 17
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Tue Sep 19 2023

Nigel continues to exhibit a large eye about 50 n mi across on
satellite images this morning. The surrounding deep convection has
intensified somewhat over the southern and western portions of the
circulation, and the overall cloud pattern is fairly symmetrical.
The current intensity estimate is increased to 80 kt in agreement
with the latest objective estimates from UW-CIMSS. A NOAA
Hurricane Hunter aircraft will investigate the hurricane shortly
and should provide a more accurate estimate of Nigel's intensity.

There is still about one more day for the hurricane to intensify
while it remains over SSTs near 28 deg C with weak vertical wind
shear. Therefore, the official forecast shows some short-term
strengthening and this is generally above the model guidance. In
48 to 72 hours, the dynamical model guidance shows a large increase
in shear which, along with cooler waters, should result in
weakening. By 72 hours, the global model forecast indicates that
Nigel will become a frontal low over the northern Atlantic and this
is shown in the official forecast.

As noted earlier, the track forecast for Nigel appears to be
straightforward. The system is currently moving northwestward
along the western periphery of a mid-level anticyclone. In 12
to 24 hours, Nigel should turn northward while it moves around
the high. Afterwards, the system is likely to accelerate
northeastward to the southeast and south of a strong mid-level
trough. In 3-5 days, Nigel is expected to rotate around the eastern
side of a large extratropical low over the North Atlantic. The NHC
track forecast is close to the model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT 19/1500Z 30.5N 54.3W 80 KT 90 MPH
12H 20/0000Z 32.3N 54.7W 90 KT 105 MPH
24H 20/1200Z 35.0N 54.2W 90 KT 105 MPH
36H 21/0000Z 38.0N 51.4W 85 KT 100 MPH
48H 21/1200Z 41.0N 46.2W 75 KT 85 MPH
60H 22/0000Z 43.8N 39.0W 65 KT 75 MPH
72H 22/1200Z 46.5N 31.3W 55 KT 65 MPH...POST-TROP/EXTRATROP
96H 23/1200Z 54.0N 20.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 24/1200Z 59.0N 21.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 17
NWS National Hurricane Center Miami FL AL152023
1100 AM AST Tue Sep 19 2023

Nigel continues to exhibit a large eye about 50 n mi across on
satellite images this morning. The surrounding deep convection has
intensified somewhat over the southern and western portions of the
circulation, and the overall cloud pattern is fairly symmetrical.
The current intensity estimate is increased to 80 kt in agreement
with the latest objective estimates from UW-CIMSS. A NOAA
Hurricane Hunter aircraft will investigate the hurricane shortly
and should provide a more accurate estimate of Nigel's intensity.

There is still about one more day for the hurricane to intensify
while it remains over SSTs near 28 deg C with weak vertical wind
shear. Therefore, the official forecast shows some short-term
strengthening and this is generally above the model guidance. In
48 to 72 hours, the dynamical model guidance shows a large increase
in shear which, along with cooler waters, should result in
weakening. By 72 hours, the global model forecast indicates that
Nigel will become a frontal low over the northern Atlantic and this
is shown in the official forecast.

As noted earlier, the track forecast for Nigel appears to be
straightforward. The system is currently moving northwestward
along the western periphery of a mid-level anticyclone. In 12
to 24 hours, Nigel should turn northward while it moves around
the high. Afterwards, the system is likely to accelerate
northeastward to the southeast and south of a strong mid-level
trough. In 3-5 days, Nigel is expected to rotate around the eastern
side of a large extratropical low over the North Atlantic. The NHC
track forecast is close to the model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT 19/1500Z 30.5N 54.3W 80 KT 90 MPH
12H 20/0000Z 32.3N 54.7W 90 KT 105 MPH
24H 20/1200Z 35.0N 54.2W 90 KT 105 MPH
36H 21/0000Z 38.0N 51.4W 85 KT 100 MPH
48H 21/1200Z 41.0N 46.2W 75 KT 85 MPH
60H 22/0000Z 43.8N 39.0W 65 KT 75 MPH
72H 22/1200Z 46.5N 31.3W 55 KT 65 MPH...POST-TROP/EXTRATROP
96H 23/1200Z 54.0N 20.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 24/1200Z 59.0N 21.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Pasch

[cycloneye]

Hurricane Nigel Discussion Number 18
NWS National Hurricane Center Miami FL AL152023
500 PM AST Tue Sep 19 2023

The satellite structure of Nigel has improved this afternoon.
Satellite images indicate a solid ring of deep convection surrounds
the large, 45 to 50-n-mi wide eye of the hurricane. The NOAA
Hurricane Hunters have conducted research missions into Nigel today
and provided helpful data to better assess the hurricane's structure
and intensity. The aircraft recorded peak 700-mb flight-level winds
of around 100 kt, which reduces to a surface intensity of 90 kt
using a standard reduction factor. However, peak SFMR winds from
both planes were around 75 kt, and dropsonde data indicate only
modest pressure falls to around 974 mb. Based on a blend of these
data, the initial intensity is set at 85 kt for this advisory. The
wind radii were adjusted slightly outward based on the aircraft data
and scatterometer data received just after the previous advisory.

Given its improved structure, additional near-term strengthening is
forecast as Nigel moves over 28C SSTs in a weak vertical shear
environment. By late Wednesday, the guidance indicates deep-layer
shear will increase while the hurricane quickly moves into a drier
environment over much cooler waters. So, the NHC forecast shows
steady weakening through late week that closely follows the latest
multi-model consensus aids. The global model fields indicate that
Nigel will complete its extratropical transition by 60 h, which is
reflected in this advisory.

It appears that Nigel has begun to turn more northward, and its
initial motion is now north-northwestward (330/12 kt). The steering
ridge to the northeast of Nigel will continue to slide eastward
through tonight. As a result, the hurricane is expected to turn
northward around the western periphery of this ridge. Then, Nigel
should turn northeastward and accelerate within the flow ahead of a
deep-layer trough moving across the northeastern United States and
Atlantic Canada. The track guidance remains well clustered around
this scenario, and the latest NHC track forecast was only nudged
slightly west based on the latest TVCA and HCCA aids. As Nigel moves
deeper into the mid-latitudes, the extratropical low is forecast to
become captured within a broader cyclonic circulation over the north
Atlantic on days 4 and 5, which introduces increased track
uncertainty late in the period.


FORECAST POSITIONS AND MAX WINDS

INIT 19/2100Z 31.8N 54.7W 85 KT 100 MPH
12H 20/0600Z 33.7N 54.9W 95 KT 110 MPH
24H 20/1800Z 36.7N 53.3W 90 KT 105 MPH
36H 21/0600Z 39.8N 49.3W 80 KT 90 MPH
48H 21/1800Z 42.8N 42.9W 70 KT 80 MPH
60H 22/0600Z 45.7N 35.0W 60 KT 70 MPH...POST-TROP/EXTRATROP
72H 22/1800Z 48.6N 28.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 23/1800Z 56.0N 20.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 24/1800Z 58.0N 21.5W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Reinhart