ATL: ISAAC - Advisories

[cycloneye]

el centro de la Tormenta Tropical
Isaac se localizó cerca de la latitud 37.1 Norte, longitud 54.1
Oeste. Isaac se está moviendo hacia el este cerca de 12 mph (19
km/h) y se anticipa un movimiento general hacia el este a este-
noreste a una velocidad ligeramente más rápida durante los próximos
días.

Los datos de viento por satélite recientes indican que los vientos
máximos sostenidos están cerca de 50 mph (85 km/h) con ráfagas más
fuertes. Se pronostica algún fortalecimiento durante los próximos
dos días y Isaac podría estar cerca de la intensidad de huracán para
el final de la semana.


Tropical Storm Isaac Discussion Number 1
NWS National Hurricane Center Miami FL AL102024
1100 PM AST Wed Sep 25 2024

The non-tropical area of low pressure we have been monitoring to the
northeast of Bermuda over the last day or so has become
significantly better organized today, with deep central convection
forming and now wrapping around a small surface circulation. While
the system had previously been attached to an occluded frontal
boundary to its northwest, recently received scatterometer data now
shows that the circulation has fully detached form this boundary,
and is producing winds of 40-45 kt in a small circular area embedded
within the organizing convection. These structural changes indicate
the the system has undergone and has completed tropical
transition. Thus, the system is now being designated Tropical Storm
Isaac this advisory, and the initial intensity is set to 45 kt based
on the earlier scatterometer data.

Isaac is moving just north of due east at 85/10 kt. This motion
should continue with a gentle turn more east-northeastward over the
next several days as the system is steered primarily by mid-level
ridging present to its south. However, there does not appear to be a
mid-latitude trough early on to help the storm to accelerate much
over the next few days. The initial NHC track forecast is in good
agreement with the simple and corrected consensus aids TCVN and
HCCA.

While Isaac is traversing only marginally warm waters, it is also
still embedded within the base of an upper-level trough which has
reduced tropospheric stability due to colder upper-level
temperatures. This is partially why Isaac has been able to develop
organized deep convection this evening. The shear is not
anticipated to increase much over the next couple of days, which
could allow for some gradual intensification. The NHC intensity
forecast is in good agreement the with HCCA and ECMWF forecasts and
shows a peak intensity of 60 kt in 48 h, followed by gradual
weakening after 72 h as sea-surface temperatures cool further and
shear begins to increase more markedly.


FORECAST POSITIONS AND MAX WINDS

INIT 26/0300Z 37.1N 54.1W 45 KT 50 MPH
12H 26/1200Z 37.1N 52.3W 50 KT 60 MPH
24H 27/0000Z 37.3N 49.8W 50 KT 60 MPH
36H 27/1200Z 37.9N 46.8W 55 KT 65 MPH
48H 28/0000Z 38.8N 43.2W 60 KT 70 MPH
60H 28/1200Z 40.1N 39.5W 60 KT 70 MPH
72H 29/0000Z 41.4N 35.8W 60 KT 70 MPH
96H 30/0000Z 42.7N 29.4W 55 KT 65 MPH
120H 01/0000Z 42.5N 21.0W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin

[cycloneye]

Tropical Storm Isaac Discussion Number 2
NWS National Hurricane Center Miami FL AL102024
500 AM AST Thu Sep 26 2024

Isaac's structure on satellite looks fairly healthy for a
high-latitude tropical cyclone. There are plenty of curved banding
features rotating around the estimated center on IR satellite,
matched by a recent AMSR2 microwave pass also showing convective
banding. While the cloud top temperatures are not that cold, only
-50 to -60 C, that is rather typical for a tropical cyclone along
the axis of an upper-level trough with a depressed tropopause. Based
on the earlier scatterometer wind data, plus a DMINT estimate from
the AMSR2 pass of 46 kt, the initial intensity is held at 45 kt
this advisory.

The tropical storm is continuing eastward this morning at 90/10 kt.
This general motion, albeit with a gradual turn more
east-northeastward is expected over the next several days as Isaac
slowly accelerates under the influence of mid-level ridging to the
southeast of the storm. The track guidance is in pretty good
agreement in the short-term. However, differences primarily in the
along-track direction begin to be evident beyond 48 h, which could
ultimately play a role in more notable across-track differences
shown between the GFS and ECMWF solutions by early next week. For
now, the NHC track forecast is very similar to the previous track
forecast, and favors a solution very similar to the HFIP corrected
consensus approach (HCCA) and ECMWF track.

Given the storm's healthy current structure, there is an opportunity
for it to intensify over the next couple of days. During this time
span, sea-surface temperatures (SSTs) hover between 26-27 C, colder
than normal upper-level temperatures should aid tropospheric
instability, and vertical wind shear is forecast to remain in the
15-20 kt range, in the same direction as the storm motion. The
simulated IR imagery from the latest ECMWF and GFS runs also show
Isaac attempting to clear out an eye occasionally in the 36-60 h
period. These signals suggest that Isaac could become a hurricane,
and that is now explicitly reflected in the latest NHC forecast,
which is very close to the latest HCCA consensus aid. After 60 h,
shear increases markedly as SSTs also decrease, which should lead to
weakening, and Isaac is forecast to become post-tropical sometime
near the end of the forecast as it passes by to the north of the
Azores.


FORECAST POSITIONS AND MAX WINDS

INIT 26/0900Z 37.1N 52.8W 45 KT 50 MPH
12H 26/1800Z 37.2N 50.9W 50 KT 60 MPH
24H 27/0600Z 37.6N 48.1W 55 KT 65 MPH
36H 27/1800Z 38.3N 44.9W 60 KT 70 MPH
48H 28/0600Z 39.4N 41.2W 65 KT 75 MPH
60H 28/1800Z 40.5N 37.9W 65 KT 75 MPH
72H 29/0600Z 41.8N 34.0W 60 KT 70 MPH
96H 30/0600Z 43.0N 27.2W 55 KT 65 MPH
120H 01/0600Z 43.4N 19.4W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin

[cycloneye]

Tropical Storm Isaac Discussion Number 3
NWS National Hurricane Center Miami FL AL102024
1100 AM AST Thu Sep 26 2024

Isaac's structure has remained mostly steady state during the past
6 hours. Cloud tops associated with the tropical storm are still
not particularly cold, but as noted in previous discussions, this
is not unexpected for its synoptic environment. A blend of
subjective Dvorak estimates from TAFB and SAB plus objective
estimates from UW-CIMSS still supports an intensity of 45 kt for
this advisory.

For the next 48-60 h, the track forecast appears to be mostly
straightforward. All reliable guidance indicates that Isaac will
continue moving eastward to east-northeastward with a mid-latitude
trough during that period. However, there is a significant
bifurcation in the dynamical models after that. A majority of the
guidance, including most of the ECMWF ensemble, indicate that Isaac
will turn back eastward, continuing with the mid-latitude flow
associated with another deep-layer trough. However, a few members
of the global ensembles, plus the deterministic GFS, indicate Isaac
could move slightly slower, which could cause the tropical storm to
interact with a different trough, currently located over eastern
Canada. This may cause the tropical cyclone to move northward after
60 h, on a very different track. The NHC forecast treats the GFS as
an outlier, and is instead near the consensus of the remaining
guidance. It is therefore very similar to the previous forecast.
However, confidence in this forecast is particularly low, and we
can't rule out needing to make an unusually large change to later
forecasts.

Isaac appears to be in an environment that should be generally
conducive for some strengthening, especially during the next day or
two. After that, unusually cold upper-tropospheric temperatures
could help sustain the tropical cyclone for a while, even as it
moves over colder SSTs in the 24-25 deg C. By the end of the
forecast period, much colder SSTs and a dramatic increase in shear
should cause Isaac to become post-tropical. No changes of note were
made to the NHC forecast, which still shows Isaac reaching
hurricane strength during the next couple of days.


FORECAST POSITIONS AND MAX WINDS

INIT 26/1500Z 37.3N 51.7W 45 KT 50 MPH
12H 27/0000Z 37.4N 49.8W 50 KT 60 MPH
24H 27/1200Z 37.9N 46.9W 55 KT 65 MPH
36H 28/0000Z 38.7N 43.5W 60 KT 70 MPH
48H 28/1200Z 39.9N 39.9W 65 KT 75 MPH
60H 29/0000Z 41.2N 36.7W 65 KT 75 MPH
72H 29/1200Z 42.3N 33.3W 60 KT 70 MPH
96H 30/1200Z 43.5N 25.5W 55 KT 65 MPH
120H 01/1200Z 43.5N 18.0W 45 KT 50 MPH...POST-TROP/EXTRATROP

$$
Forecaster D. Zelinsky

[cycloneye]

Tropical Storm Isaac Discussion Number 4
NWS National Hurricane Center Miami FL AL102024
500 PM AST Thu Sep 26 2024

Little change with the structure of Isaac has been noted this
afternoon. Episodic bursts of deep convection have been observed
over the western semi-circle throughout the day, but cloud tops have
largely failed to break -60C. There remains a relative void of
thunderstorms over the eastern half of Isaac's circulation as it
traverses across 26C water. ASCAT passes, combined with a blend of
subjective Dvorak estimates from TAFB and SAB, plus objective
estimates from UW-CIMSS collectively support maintaining an initial
intensity of 45 kt for this advisory.

For the next couple of days, the track guidance is tightly clustered
as Isaac continues on a general east to east-northeast heading,
remaining embedded within a well established mid-latitude trough.
Beyond Saturday, there has been a large change in the dynamic model
suite. A vast majority of the the guidance, including the
deterministic ECMWF and several of its ensemble members, have shown
a rather dramatic poleward shift, similar to previous runs of the
GFS. This is ultimately induced by a slight decrease in forward
speed, which in turn allows Isaac to be steered by a strong closed
mid-latitude trough digging out of eastern Canada into the north
Atlantic. This would force Isaac to the northeast and eventually
northward. Isaac's medium range track confidence is quite low, owing
to lack of model consistency during the past several runs. To avoid
making too large of a change, the forecast track after Day 3 has
shifted to the left for this advisory. However, it falls well to the
east of the guidance envelope and further large changes may be
needed.

Despite the longer range track uncertainty, Isaac appears to be in
an environment that should be generally conducive for some
strengthening, especially during the next day or two. After that,
unusually cold upper-tropospheric temperatures should help sustain
the tropical cyclone for a while, even as it moves over colder SSTs
in the 24-25 deg C range. By the end of the forecast period, much
colder SSTs and a dramatic increase in shear should cause Isaac to
become post-tropical. This transition could occur more quickly than
forecast if the system takes a more northerly turn early next week.
No changes of note were made to the official intensity forecast,
which still shows Isaac reaching hurricane strength during the next
couple of days.


FORECAST POSITIONS AND MAX WINDS

INIT 26/2100Z 37.5N 50.6W 45 KT 50 MPH
12H 27/0600Z 37.6N 48.7W 50 KT 60 MPH
24H 27/1800Z 38.2N 45.5W 60 KT 70 MPH
36H 28/0600Z 39.2N 42.0W 65 KT 75 MPH
48H 28/1800Z 40.5N 38.7W 65 KT 75 MPH
60H 29/0600Z 41.9N 35.9W 60 KT 70 MPH
72H 29/1800Z 43.0N 33.8W 60 KT 70 MPH
96H 30/1800Z 44.2N 27.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 01/1800Z 45.5N 22.5W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster D. Zelinsky/Bookbinder

[cycloneye]

Tropical Storm Isaac Discussion Number 5
NWS National Hurricane Center Miami FL AL102024
1100 PM AST Thu Sep 26 2024

Isaac's structure has quickly improved this evening on satellite
imagery. Deeper cold convection down to -60 C has been fully
wrapping around the center, and the last few frames show a warm
spot, which could very well be a formative eye. While the
subjective Dvorak classifications hasn't changed much this evening,
the objective estimates, especially DPRINT and DMINT from UW-CIMSS
have increased to 54 and 51 kt respectively. Similar to last night,
a late arriving set of scatterometer passes showed a tight vortex
with derived-winds up to 49 kt. Assuming some undersampling of the
small circulation, the initial intensity is raised to 55 kt this
advisory.

Isaac appears to have made a wobble south of due east earlier this
evening, but more recently has resumed an eastward motion at
90/10 kt. This motion with a gradual turn to the east-northeast
should occur over the next couple of days. Afterwards, the forward
motion of Isaac is likely to become very critical to its future
track trajectory. The poleward shift in the guidance that started
earlier today has continued, possibly due to the slower forecasted
forward motion of the storm, making it more likely to be captured by
an upstream mid-latitude trough digging in from the west. The latest
NHC track forecast has continued the northward shift started on the
prior cycle, but is still not as far north and east as the bulk of
the guidance suite, and further adjustments in that direction may be
needed in future forecasts.

Given the improvement in Isaac's structure this evening, it's
looking increasingly likely the storm will soon become a hurricane.
Vertical wind shear remains under 20 kt while sea-surface
temperatures (SSTs) between 26-27 C for the next 24 h should allow
additional intensification. After that time, shear begins to
increase more as SSTs also begin to cool. These factors are expected
to result in a leveling off in intensity, though cold upper-level
temperatures should maintain enough instability to keep moderate to
deep convection near the system. By 72 h, weakening should begin,
with extratropical transition likely to be underway, finishing
sometime early next week over the high-latitudes of the North
Atlantic. The latest NHC intensity forecast is above much of
the guidance in the short-term due to recent satellite trends, but
falls back towards the reliable HCCA consensus aid after 24 hours.


FORECAST POSITIONS AND MAX WINDS

INIT 27/0300Z 37.1N 49.8W 55 KT 65 MPH
12H 27/1200Z 37.1N 47.6W 65 KT 75 MPH
24H 28/0000Z 38.1N 44.1W 70 KT 80 MPH
36H 28/1200Z 39.5N 40.5W 70 KT 80 MPH
48H 29/0000Z 41.1N 37.9W 65 KT 75 MPH
60H 29/1200Z 42.6N 36.4W 65 KT 75 MPH
72H 30/0000Z 43.7N 35.1W 60 KT 70 MPH
96H 01/0000Z 46.0N 30.4W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 02/0000Z 50.0N 26.0W 40 KT 45 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin

[cycloneye]

Hurricane Isaac Discussion Number 6
NWS National Hurricane Center Miami FL AL102024
500 AM AST Fri Sep 27 2024

Isaac has continued to become better organized since the prior
advisory. The earlier warm spot noted on IR satellite imagery has
persisted and warmed further, with sufficently cold -55 to -60 C
convective cloud tops encircling the feature. A helpful AMSR2 pass
that became available after the prior advisory also showed an eye
feature on both the 89-GHz and 37-GHz channels, the latter showing a
cyan ring. These features suggest that Isaac has become a hurricane,
and the initial intensity has been raised to 65 kt for this
advisory.

Isaac still is moving generally eastward this morning, estimated at
090/10 kt. This motion with a gradual turn to the east-northeast
should occur over the next couple of days as the hurricane remains
steered mostly by deep-layer flow along the northern periphery of a
mid-level ridge centered to the southeast of Isaac. Once again, how
quick this forward motion ends up being will be critical for its
ultimate track down the road, with a track bifurcation continuing
between the GFS and ECMWF solutions occuring beyond 60 h as to how
much the upstream trough located to its east is able to interact
and pick up the cyclone. Despite the spread in the guidance, the
consensus aids are not that much altered from the prior cycle, and
the NHC track forecast is quite similar to the prior advisory.

Additional intensification is expected while the vertical wind shear
remains only low to moderate and Isaac continues to be over
sufficently warm sea-surface temperatures (SSTs) with instability
being aided by cold upper-level temperatures. After 36 h, SSTs drop
to 24 C and lower as shear increases markedly, which should induce
a gradual weakening trend. Both the GFS and ECMWF suggest the
cyclone should lose its deep convection sometime in the 72-96 h
period, marking its transition to post-tropical in that time frame.
The NHC intensity forecast is on the high side of the guidance, but
is generally in line with the latest GFS-based SHIPS and LGEM
guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 37.0N 48.5W 65 KT 75 MPH
12H 27/1800Z 37.5N 45.9W 70 KT 80 MPH
24H 28/0600Z 38.8N 42.4W 75 KT 85 MPH
36H 28/1800Z 40.4N 39.3W 75 KT 85 MPH
48H 29/0600Z 41.8N 37.3W 70 KT 80 MPH
60H 29/1800Z 43.1N 35.9W 60 KT 70 MPH
72H 30/0600Z 44.0N 34.3W 55 KT 65 MPH
96H 01/0600Z 46.9N 30.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 02/0600Z 50.4N 26.2W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin

[cycloneye]

Hurricane Isaac Discussion Number 7
NWS National Hurricane Center Miami FL AL102024
1100 AM AST Fri Sep 27 2024

Isaac is holding steady this morning. Geostationary satellite data
has shown consistent periodic bursts of convection wrapping around a
small eye. Subjective and objective Dvorak estimates range widely
from 35 kt to 73 kt and the initial intensity is held at 65 kt for
this advisory, closest to the SAB classification.

The hurricane is moving eastward at 12 kt. Isaac is moving along
the northern periphery of a mid-level ridge and should gradually
turn to the east-northeast over the next couple of days. Models are
more in agreement this cycle and show a more northeastward turn on
days 4 and 5 as Isaac interacts with an upper-level trough located
to its east. The latest track forecast is similar to the previous
forecast through 72 h and slightly to the north at forecast periods
beyond.

Isaac could still strengthen further over the next day or so. While
the vertical wind shear is expected to be strong-to-moderate, the
upper-level divergence is forecast to increase and sea surface
temperature should be relatively sufficient. Beyond a day or so,
oceanic and environmental conditions become less conducive, which
should begin to weaken Isaac by Saturday night or Sunday. The
official intensity forecast is very similar to previous advisory,
with a peak intensity of 75 kt by Saturday and the cyclone becoming
extratropical by early next week. However, global models show the
system losing its deep convection by Monday, and the official
forecast now predicts Isaac becoming a post-tropical cyclone at that
point.


FORECAST POSITIONS AND MAX WINDS

INIT 27/1500Z 37.4N 46.8W 65 KT 75 MPH
12H 28/0000Z 38.1N 44.2W 70 KT 80 MPH
24H 28/1200Z 39.4N 40.9W 75 KT 85 MPH
36H 29/0000Z 41.0N 38.4W 75 KT 85 MPH
48H 29/1200Z 42.4N 36.6W 70 KT 80 MPH
60H 30/0000Z 43.7N 35.1W 60 KT 70 MPH
72H 30/1200Z 44.8N 33.0W 55 KT 65 MPH...POST-TROPICAL
96H 01/1200Z 48.0N 29.6W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 02/1200Z 52.1N 25.0W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Bucci

[cycloneye]

Hurricane Isaac Discussion Number 8
NWS National Hurricane Center Miami FL AL102024
500 PM AST Fri Sep 27 2024

Isaac's satellite presentation has improved since this morning. A
small ragged eye can be seen on infrared imagery with cooler cloud
tops surrounding it. An AMSR2 microwave pass from earlier today
showed the mid- and low-level centers co-located with a primary
band in the northwestern quadrant. Both the SAB and TAFB subjective
Dvorak classifications suggested that Isaac's intensity has
increased. Objective guidance aids are also suggesting the same.
Taking all of these into consideration, the intensity has been
increased to 75 kt.

The hurricane is moving eastward at 14 kt. Isaac is moving in the
mid-latitude zonal flow and will gradual turn to the east-northeast
later today. In about a day or so, Isaac will begin interacting with
an upper-level trough to its east and turn more northeastward and
slow down. The latest track forecast is very similar to the previous
forecast, however, the day four and five track positions were
adjusted westward as the model guidance has been shifting in this
direction.

Isaac is close to its peak intensity as the window for strengthening
is ending soon. By Saturday, it will move over cooler waters, into
an area of strong vertical wind shear, and a dry mid-level airmass.
The peak intensity has been adjusted upward to 80 kt because of the
recent strengthening. On Saturday, Isaac is expect to gradually
weaken and this trend will continue for the reminder of the forecast
period. Simulated satellite imagery from the global models show
Isaac losing its deep convection by Monday and the official forecast
now predicts the hurricane to become a post-tropical cyclone by 60
h.

FORECAST POSITIONS AND MAX WINDS

INIT 27/2100Z 37.8N 45.3W 75 KT 85 MPH
12H 28/0600Z 38.8N 42.7W 80 KT 90 MPH
24H 28/1800Z 40.3N 39.8W 75 KT 85 MPH
36H 29/0600Z 41.8N 37.9W 70 KT 80 MPH
48H 29/1800Z 43.1N 36.3W 65 KT 75 MPH
60H 30/0600Z 44.4N 34.5W 60 KT 70 MPH...POST-TROPICAL
72H 30/1800Z 45.7N 32.7W 50 KT 60 MPH...POST-TROPICAL
96H 01/1800Z 50.1N 29.9W 45 KT 50 MPH...POST-TROP/EXTRATROP
120H 02/1800Z 52.9N 27.3W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Bucci/Rosado

[cycloneye]

BULLETIN
Hurricane Isaac Advisory Number 9
NWS National Hurricane Center Miami FL AL102024
300 AM GMT Sat Sep 28 2024

...ISAAC CONTINUES TO STRENGTHEN...


SUMMARY OF 300 AM GMT...0300 UTC...INFORMATION
----------------------------------------------
LOCATION...38.2N 43.4W
ABOUT 890 MI...1430 KM W OF THE AZORES
MAXIMUM SUSTAINED WINDS...90 MPH...150 KM/H
PRESENT MOVEMENT...ENE OR 75 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...973 MB...28.74 INCHES



Hurricane Isaac Discussion Number 9
NWS National Hurricane Center Miami FL AL102024
300 AM GMT Sat Sep 28 2024

Isaac has maintained a well-defined eye on both visible and infrared
imagery for the past 12 hours or so. The latest subjective Dvorak
classifications are a consensus T-4.5/77 kt from TAFB and SAB. The
latest objective intensity estimates from UW-CIMSS range from 72 to
92 kt. The initial intensity is adjusted upward to 80 kt based on a
blend of the subjective and objective estimates.

The hurricane is moving east-northeastward, or 075/14 kt. Isaac is
currently moving in the zonal flow with the mid-latitude westerlies.
A powerful deep-layer trough over Atlantic Canada will approach
Isaac by Saturday night as a narrow mid-level ridge builds to the
north of Isaac. These steering features will cause Isaac to bend a
bit more toward the northeast while slowing down slightly. On
Monday, after Isaac becomes post-tropical, a turn more toward the
north-northeast is expected, as the deep-layer trough moves
eastward and the ridge moves northward. No significant changes
have been made to the official forecast track forecast through the
first 60-72 h. After that time, a sizable westward adjustment is
made to the official forecast track, but the new NHC forecast track
is well east of the latest consensus models beyond 72 h.

It is possible Isaac could strengthen a little more tonight,
although the hurricane is likely near its peak intensity. The
cyclone is crossing the 26 degree C isotherm now, but is forecast to
remain over 24-26 degree C sea-surface temperatures for another 36 h
or so. The cold upper-level temperatures that Isaac is forecast to
traverse should help maintain enough instability such that only slow
weakening is forecast through 36 h, and Isaac is expected to
maintain hurricane strength through that time. Also, although
moderate southwesterly shear is forecast to increase slightly, the
shear direction is the same as the direction of Isaac's motion, so
this should not be too unfavorable. Beyond 48 h, sea-surface
temperatures decrease significantly and wind shear is forecast to
become very strong. This should cause the cyclone to lose its deep
convection by Monday and then become extratropical. Little to no
change was made to the previous official intensity forecast, while
lies near the middle of the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 28/0300Z 38.2N 43.4W 80 KT 90 MPH
12H 28/1200Z 39.5N 40.9W 80 KT 90 MPH
24H 29/0000Z 41.0N 38.6W 75 KT 85 MPH
36H 29/1200Z 42.5N 37.0W 70 KT 80 MPH
48H 30/0000Z 43.8N 35.4W 60 KT 70 MPH
60H 30/1200Z 45.0N 33.7W 55 KT 65 MPH...POST-TROPICAL
72H 01/0000Z 46.9N 32.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 02/0000Z 50.9N 31.0W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 03/0000Z 53.8N 29.5W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Hagen

[cycloneye]

Hurricane Isaac Discussion Number 10
NWS National Hurricane Center Miami FL AL102024
900 AM GMT Sat Sep 28 2024

Isaac has continued to strengthen this morning. The satellite
presentation consists of a 20 nm (WMG 8C) clear eye and an
impressive curve band in the north side of the cyclone beneath the
upper diffluent westerlies. The initial intensity is bumped up to
90 kt for this advisory and is based on a UW-CIMSS SATCON analysis
of 93 kt and the Dvorak intensity estimates for TAFB and SAB.

Some fluctuations in strength are possible during the day, after
which Isaac will be traversing cooler oceanic surface temperatures
and experiencing increasing deep-layer shear. Subsequently,
gradual weakening is forecast, and the cyclone is expected to lose
its deep core convection and become a post-tropical cyclone in 48
hrs. After that time, the global model fields and the FSU Cyclone
Phase Evolution forecast show Isaac transitioning from a symmetric
warm-core system to an asymmetric cold-core, more frontal thermal
structure. The NHC intensity forecast is similar to the consensus
aids and shows Isaac completing its extratropical cyclone
transition by Tuesday.

Isaac has turned toward the northeast, or 055/17 kt, in response
to a mid-latitude shortwave ridge temporarily building to the north
of the cyclone. By early next week, as Isaac loses its tropical
characteristics and completes an extratropical transition, the
cyclone should turn toward the north-northeast while a major
shortwave trough approaches from the northwest over the central
Atlantic. The official track forecast is adjusted a little to the
right of the previous forecast beyond 48 hrs to agree more with
the TVCA and HCCA consensus models.


FORECAST POSITIONS AND MAX WINDS

INIT 28/0900Z 39.3N 41.3W 90 KT 105 MPH
12H 28/1800Z 40.4N 39.2W 85 KT 100 MPH
24H 29/0600Z 42.1N 37.0W 75 KT 85 MPH
36H 29/1800Z 43.5N 35.2W 65 KT 75 MPH
48H 30/0600Z 44.6N 33.1W 55 KT 65 MPH...POST-TROPICAL
60H 30/1800Z 46.0N 31.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 01/0600Z 48.0N 30.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 02/0600Z 52.5N 30.0W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 03/0600Z 55.7N 28.6W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Roberts

[cycloneye]

Hurricane Isaac Discussion Number 11
NWS National Hurricane Center Miami FL AL102024
300 PM GMT Sat Sep 28 2024

Isaac's strengthening trend from the past day or so appears to have
leveled off. Isaac still has a clear eye and a relatively symmetric
appearance, although some dry air entrainment is evident coming in
from the southwest. Intensity estimates range from 77 to 93 kt, and
initial intensity has been held at 90 kt, in agreement with TAFB's
Dvorak classification of 5.0. The wind radii have been modified to
reflect values from 1245 UTC ASCAT-B pass.

Isaac is already over relatively cool water and will reach
progressively lower SSTs in the coming day or two, all while
encountering increasingly hostile shear. Thus, Isaac's intensity has
most likely peaked, and gradual weakening today is forecast to
accelerate through the weekend. This weakening will coincide with an
extratropical transition, which based on global models, will
complete in about 48 hours. The NHC intensity forecast remains
unchanged from the previous forecast and is in close agreement with
consensus aids through the forecast period.

Isaac is moving east-northeastward, 060/17 kt. Steering flow
should decrease as Isaac passes in the vicinity of a mid- to upper-
level ridge over the next day or so, which should lead to slower
forward motion through the remainder of its warm-core existence. As
Isaac becomes extratropical, it should turn more northerly as a
shortwave trough digs to the west of the cyclone. The official track
forecast remains unchanged through 48 hours but has again trended to
the right of the previous forecast beyond 48 hours to adjust closer
to consensus models.


FORECAST POSITIONS AND MAX WINDS

INIT 28/1500Z 40.2N 39.7W 90 KT 105 MPH
12H 29/0000Z 41.4N 37.8W 85 KT 100 MPH
24H 29/1200Z 42.9N 36.1W 75 KT 85 MPH
36H 30/0000Z 44.0N 33.9W 65 KT 75 MPH
48H 30/1200Z 45.4N 31.5W 55 KT 65 MPH...POST-TROPICAL
60H 01/0000Z 47.1N 29.4W 45 KT 50 MPH...POST-TROPICAL
72H 01/1200Z 49.6N 28.7W 45 KT 50 MPH...POST-TROP/EXTRATROP
96H 02/1200Z 53.5N 28.1W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 03/1200Z 56.6N 25.0W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Konarik/Cangialosi

[cycloneye]

Hurricane Isaac Discussion Number 12
NWS National Hurricane Center Miami FL AL102024
900 PM GMT Sat Sep 28 2024

The expected weakening of Isaac appears to have started. The eye of
the hurricane has been filling some during the past few hours, and
the convective pattern is losing symmetry with dry air entraining
into the southwestern side of the circulation. The Dvorak
classifications are dropping, and accordingly, the initial intensity
is nudged downward to 85 kt. The hurricane is already over cool 24
C waters and it is headed for progressively cooler waters during the
next several days. These unfavorable oceanic conditions coupled
with a sharp increase in vertical wind shear should cause steady
weakening, and a transition into a post-tropical cyclone in about
36 hours.

Isaac is moving relatively quickly northeastward at 16 kt. The
system is forecast to move a little slower to the east-northeast or
northeast during the next couple of days within the mid-latitude
flow. After that, a turn to the north is expected on the eastern
side of an extratropical low. No significant changes were made to
the previous track forecast, and this one lies fairly close to the
various consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 28/2100Z 41.3N 38.4W 85 KT 100 MPH
12H 29/0600Z 42.5N 37.0W 75 KT 85 MPH
24H 29/1800Z 43.7N 35.1W 65 KT 75 MPH
36H 30/0600Z 44.8N 32.3W 55 KT 65 MPH...POST-TROPICAL
48H 30/1800Z 46.1N 30.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
60H 01/0600Z 48.3N 28.2W 45 KT 50 MPH...POST-TROP/EXTRATROP
72H 01/1800Z 50.4N 27.4W 40 KT 45 MPH...POST-TROP/EXTRATROP
96H 02/1800Z 54.0N 26.5W 40 KT 45 MPH...POST-TROP/EXTRATROP
120H 03/1800Z 56.9N 23.6W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Cangialosi

[cycloneye]

Hurricane Isaac Discussion Number 13
NWS National Hurricane Center Miami FL AL102024
300 AM GMT Sun Sep 29 2024

Isaac continues to weaken tonight. Southwesterly shear and
intrusions of drier mid-level air have eroded convection over the
southern portion of the hurricane, and there is no longer a coherent
eye feature in satellite images. A partial scatterometer pass shows
a frontal zone in close proximity to the east of Isaac, signaling
extratropical transition could begin soon. A blend of the TAFB and
SAB Dvorak Current Intensity estimates with objective UW-CIMSS
estimates supports lowering the intensity to 70 kt.

As Isaac moves deeper into the mid-latitudes, it will cross even
cooler waters and encounter increasing deep-layer shear. Thus,
continued weakening is likely, and Isaac is expected to lose
tropical characteristics soon and complete extratropical transition
by Monday while becoming entangled with a nearby frontal system. The
updated NHC forecast shows a faster rate of weakening in the near
term. The wind radii of Isaac are likely to become increasingly
asymmetric as the cyclone takes on frontal characteristics.

Isaac is moving northeastward at about 13 kt. This general motion
should continue for the next couple of days while Isaac remains
embedded in deep-layer southwesterly flow. The storm is forecast to
turn more north-northeastward by midweek as an upper-level trough
digs to the west of Isaac. There is more track spread in the
guidance related to this interaction, and the extratropical portion
of the NHC track forecast was adjusted slightly to the right of the
previous prediction, following the latest multi-model consensus
trends.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0300Z 42.1N 37.6W 70 KT 80 MPH
12H 29/1200Z 43.1N 36.3W 65 KT 75 MPH
24H 30/0000Z 44.2N 33.7W 55 KT 65 MPH...POST-TROPICAL
36H 30/1200Z 45.4N 30.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
48H 01/0000Z 47.1N 28.3W 45 KT 50 MPH...POST-TROP/EXTRATROP
60H 01/1200Z 49.2N 27.1W 40 KT 45 MPH...POST-TROP/EXTRATROP
72H 02/0000Z 51.1N 26.3W 40 KT 45 MPH...POST-TROP/EXTRATROP
96H 03/0000Z 55.0N 24.4W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 04/0000Z 58.0N 20.0W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Reinhart

[cycloneye]

Hurricane Isaac Discussion Number 14
NWS National Hurricane Center Miami FL AL102024
900 AM GMT Sun Sep 29 2024

The strong deep-layer shear and sub-24C sea surface temperatures
continue to disrupt Isaac's cloud pattern, although recent images
show a small burst of deep convection developing near the surface
center. The outer convective curve bands are diminishing and
have dissipated in the east semicircle of the cyclone. Based on
the latest Dvorak satellite intensity estimates and a UW-CIMSS
SATCON analysis, the initial intensity is set at 70 kt.

Decreasing oceanic surface temperatures and a sharp increase in
west-southwesterly vertical shear support further weakening as an
extratropical cyclone through day 5. Isaac could lose what remains
of its organized deep convection later tonight and become a
post-tropical cyclone, as suggested by the global model
simulated/IR forecast. In any event, extratropical transition is
anticipated by Monday evening due to baroclinic forcing from a
middle-latitude trough and the proximity of a frontal boundary.

Isaac's initial motion is estimated to be northeastward, or 045/11
kt as it continues to be steered by the mid-latitude southwesterly
flow. A turn toward the north-northeast by the middle part of this
week is forecast in response to a mid- to upper-level trough
approaching the cyclone from the west-northwest. Around the 72 hr
period, Isaac should turn northward while moving around the eastern
periphery of a larger baroclinic low-pressure system. An
adjustment slightly to the right of the previous track advisory is
again needed beyond the 24 hr period to conform with the latest
consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 42.6N 36.5W 70 KT 80 MPH
12H 29/1800Z 43.5N 34.9W 60 KT 70 MPH
24H 30/0600Z 44.5N 31.8W 50 KT 60 MPH...POST-TROPICAL
36H 30/1800Z 46.0N 28.7W 45 KT 50 MPH...POST-TROP/EXTRATROP
48H 01/0600Z 47.8N 26.3W 40 KT 45 MPH...POST-TROP/EXTRATROP
60H 01/1800Z 49.9N 25.2W 35 KT 40 MPH...POST-TROP/EXTRATROP
72H 02/0600Z 52.0N 24.6W 35 KT 40 MPH...POST-TROP/EXTRATROP
96H 03/0600Z 56.4N 20.8W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 04/0600Z 60.3N 16.8W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Roberts

[cycloneye]

Hurricane Isaac Discussion Number 15
NWS National Hurricane Center Miami FL AL102024
300 PM GMT Sun Sep 29 2024

Hurricane Isaac continues to maintain a small inner-core, which has
proved resilient in an increasingly unfavorable environment
characterized by strong deep-layer vertical wind shear near 30 kt
and sea surface temperatures (SSTs) around 23C. The satellite
appearance of the hurricane has been relatively steady this morning,
with consistent bursts of deep convection near the center. The
initial intensity is set to a possibly generous 65 kt, which is on
the higher end of the subjective and objective intensity estimates.

The tropical cyclone is forecast to track over cooler SSTs and into
increasing west-southwesterly vertical wind shear, which will induce
steady weakening through the forecast period. The intensity forecast
is similar to the previous forecast. There is evidence on visible
satellite this morning that Isaac has begun to interact with a
baroclinic zone to its north and east, in addition to wrapping dry
air from the northwest around the southern side of the circulation.
These factors are likely to result in Isaac becoming an
extratropical cyclone by Monday.

Isaac's forward motion continues to be northeastward (045/10 kt) as
it is steered by deep-layer southwesterly flow. This general motion
should continue for the next day or so, with a turn toward the
north-northeast expected by the middle part of this week in response
to a mid- to upper-level trough approaching from the west. The
cyclone is then forecast to turn back towards the northeast beyond
72 hr as it tracks along the southeastern periphery of an
upper-level trough. Once again, the track forecast is adjusted
eastward from the previous advisory beyond the 24 hr period, in
agreement with the latest consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 29/1500Z 43.2N 35.6W 65 KT 75 MPH
12H 30/0000Z 44.0N 33.6W 55 KT 65 MPH
24H 30/1200Z 45.1N 30.3W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 01/0000Z 46.6N 26.8W 45 KT 50 MPH...POST-TROP/EXTRATROP
48H 01/1200Z 48.5N 25.0W 40 KT 45 MPH...POST-TROP/EXTRATROP
60H 02/0000Z 50.6N 24.3W 35 KT 40 MPH...POST-TROP/EXTRATROP
72H 02/1200Z 52.8N 23.1W 35 KT 40 MPH...POST-TROP/EXTRATROP
96H 03/1200Z 57.1N 18.6W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 04/1200Z 61.2N 14.2W 35 KT 40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Mahoney/Papin

[cycloneye]

Tropical Storm Isaac Discussion Number 16
NWS National Hurricane Center Miami FL AL102024
900 PM GMT Sun Sep 29 2024

The increasingly hostile sea surface temperatures, strong vertical
wind shear, and dry air wrapping around the southern half of Isaac
has weakened the cyclone. The last few visible stallite images
before sundown suggest that the vortex could be tilted, with the
low-level center displaced southwest of the upper-level vortex.
Additionally, deep convection has become less symmetric since the
last advisory, with convection in the southern semicircle likely
being cut off by the aformentioned dry air. This structure is
indicative of a progressing extratropical transition, as Isaac
continues to interact with a baroclinic frontal zone to its
northeast. The initial intensity is set to 60 kt for this advisory,
which is a blend of the subjective and objective satellite
classifications.

Tropical Storm Isaac is forecast to continue its extratropical
transition and become a fully extratropical cyclone on Monday. This
is in agreement with model-diagnosed thermodynamics that maintain a
warm core until this time, in addition to simulated IR satellite
data from the GFS and ECMWF models. The intensity forecast is
similar to the previous forecast and shows a steady decrease in
maximum sustained winds through the forecast period.

The forward motion of the cyclone continues to be northeastward
(050/10 kt). This general motion is forecast to continue for the
next day or so as Isaac continues to be steered by a mid-level
trough to its west. This mid-level trough will steer Isaac towards
the north-northeast beyond 36 hr. Beyond 96 hr, global model data
indicates Isaac will be absorbed by a more potent extratropical
cyclone just southeast of Iceland. The track forecast is again
adjusted eastward from the previous advisory.


FORECAST POSITIONS AND MAX WINDS

INIT 29/2100Z 43.8N 34.7W 60 KT 70 MPH
12H 30/0600Z 44.6N 32.4W 55 KT 65 MPH
24H 30/1800Z 45.8N 28.6W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 01/0600Z 47.4N 25.8W 45 KT 50 MPH...POST-TROP/EXTRATROP
48H 01/1800Z 49.3N 24.2W 40 KT 45 MPH...POST-TROP/EXTRATROP
60H 02/0600Z 51.4N 23.2W 35 KT 40 MPH...POST-TROP/EXTRATROP
72H 02/1800Z 53.1N 22.3W 35 KT 40 MPH...POST-TROP/EXTRATROP
96H 03/1800Z 57.9N 17.9W 35 KT 40 MPH...POST-TROP/EXTRATROP
120H 04/1800Z...DISSIPATED

$$
Forecaster Mahoney/Blake

[cycloneye]

Tropical Storm Isaac Discussion Number 17
NWS National Hurricane Center Miami FL AL102024
300 AM GMT Mon Sep 30 2024

Isaac's expected transition to an extratropical cyclone continues.
The tropical storm's cloud pattern has become asymmetrical, with
cold clouds limited mostly to the north of the surface center, and
it is beginning to take the form of an extratropical rain shield.
Partial ASCAT data indicated maximum winds of 45-50 kt, supporting
an intensity of 55 kt after accounting for undersampling. This also
agrees with the most recent tropical Dvorak analysis from TAFB.

The tropical storm should become fully extratropical later today.
Models unanimously indicated that Isaac will continue to gradually
spin down as a post-tropical low during the next several days. By
96 h, the center of Isaac is expected to become poorly-defined as
it interacts with another large non-tropical low pressure system
over the north Atlantic.

Models are in reasonably good agreement for Isaac's track forecast.
It should stay on a generally northeastward track for the next day
or so. Beyond that time, there is some disagreement on exactly how
soon Isaac's post-tropical remnants will turn north-northeastward,
however all of the guidance shows the same generally idea that Isaac
will turn toward a broad trough over the north Atlantic. No
significant changes were made to the NHC forecast, which is based on
the IVCN consensus for intensity and the TVCN consensus for the
track.

FORECAST POSITIONS AND MAX WINDS

INIT 30/0300Z 44.4N 32.8W 55 KT 65 MPH
12H 30/1200Z 45.1N 30.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
24H 01/0000Z 46.7N 26.4W 45 KT 50 MPH...POST-TROP/EXTRATROP
36H 01/1200Z 48.6N 24.1W 40 KT 45 MPH...POST-TROP/EXTRATROP
48H 02/0000Z 50.7N 23.1W 40 KT 45 MPH...POST-TROP/EXTRATROP
60H 02/1200Z 52.4N 22.4W 35 KT 40 MPH...POST-TROP/EXTRATROP
72H 03/0000Z 54.4N 20.8W 35 KT 40 MPH...POST-TROP/EXTRATROP
96H 04/0000Z...DISSIPATED

$$
Forecaster D. Zelinsky

[cycloneye]

Tropical Storm Isaac Discussion Number 18
NWS National Hurricane Center Miami FL AL102024
900 AM GMT Mon Sep 30 2024

Isaac continues to show signs of transitioning into an extratropical
cyclone. The central convection has mostly dissipated, and the
system is taking on more of a frontal appearance, with a
zonally-oriented cloud and rain shield over the cyclone's northern
semicircle. The current intensity is kept at 55 kt in accordance
with a Dvorak estimate from TAFB, although this is somewhat
uncertain since Isaac no longer appears to be a true tropical
cyclone.

The cyclone continues to move east-northeastward or at about 060/13
kt within a southern branch of the mid-latitude westerlies. Isaac
is expected to turn toward the northeast during the next couple
of days as it approaches a shortwave ridge just west of the
British Isles. The official track forecast has been nudged only
slightly eastward in the 2-3 day time frame, towards the latest
corrected dynamical model consensus.

Isaac is moving over cool waters and should complete its
extratropical transition today. The global models suggest that the
system will gradually weaken during the next few days as it becomes
absorbed within a larger low over the North Atlantic. The official
intensity forecast is similar to the previous NHC prediction.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0900Z 44.7N 31.3W 55 KT 65 MPH
12H 30/1800Z 45.8N 28.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
24H 01/0600Z 47.4N 25.1W 45 KT 50 MPH...POST-TROP/EXTRATROP
36H 01/1800Z 49.4N 23.3W 40 KT 45 MPH...POST-TROP/EXTRATROP
48H 02/0600Z 51.3N 22.3W 40 KT 45 MPH...POST-TROP/EXTRATROP
60H 02/1800Z 53.0N 21.2W 35 KT 40 MPH...POST-TROP/EXTRATROP
72H 03/0600Z 54.5N 20.0W 30 KT 35 MPH...POST-TROP/EXTRATROP
96H 04/0600Z...DISSIPATED

$$
Forecaster Pasch

[cycloneye]

BULLETIN
Post-Tropical Cyclone Isaac Advisory Number 19
NWS National Hurricane Center Miami FL AL102024
300 PM GMT Mon Sep 30 2024

...ISAAC BECOMES A POST-TROPICAL CYCLONE...
...THIS IS THE LAST ADVISORY...


SUMMARY OF 300 PM GMT...1500 UTC...INFORMATION
----------------------------------------------
LOCATION...44.8N 29.1W
ABOUT 480 MI...775 KM NNW OF THE AZORES
MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H
PRESENT MOVEMENT...ENE OR 65 DEGREES AT 17 MPH...28 KM/H
MINIMUM CENTRAL PRESSURE...983 MB...29.03 INCHES