EPAC: ISELLE - Post-Tropical

[Yellow Evan]

Wow has it been 6 years already since Iselle 2014? Man time flys.

For all intents and purposes, 2014 was a century ago.

[Yellow Evan]

685
WTPZ44 KNHC 270849
TCDEP4

Tropical Storm Iselle Discussion Number 4
NWS National Hurricane Center Miami FL EP142020
200 AM PDT Thu Aug 27 2020

A sheared band of deep convection has persisted near and to the
southwest and west of Iselle's center. Night-visible satellite
imagery indicates that the circulation has tightened up somewhat,
and the latest UW-CIMSS SATCON intensity estimate is 41 kt. Based
on these satellite data, the initial intensity has been increased
to 40 kt. Unfortunately, all three scatterometer passes missed
Iselle's inner-core wind field.

The initial motion estimate is 030/04 kt. Similar to Tropical Storm
Hernan farther to the east, Iselle is forecast to remain embedded
within a large monsoon gyre, which will gradually contract down as
Hernan moves toward southern Baja California and weakens over the
next few days. This will result in the current southwesterly flow on
the south side of Iselle to become more southerly, which will
gradually turn the cyclone northward and then northwestward. By 72
hours, Iselle is expected to interact with and possibly absorb the
remnants of Hernan when the two systems are west-southwest of Baja
California. Thereafter, Iselle is forecast to move westward to
west-southwestward and slowly weaken. The new NHC forecast track is
close to a blend of the HCCA corrected-consensus model and the
simple consensus models GFEX and TVCE, and is similar to the
previous advisory track.

Although short-lived convective bursts near the center will likely
continue for the next few days, no significant strengthening is
forecast for the next 48 hours due to moderate-to-strong easterly
vertical wind shear that will persist across the system. At 60
hours and beyond, strong shear is expected to gradually weaken
Iselle, with degeneration into a post-tropical remnant low forecast
to occur by day 4, with the cyclone possibly even dissipating by 120
hours. The NHC intensity forecast is similar to the previous
advisory, and closely follows the IVCN intensity consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 16.6N 116.1W 40 KT 45 MPH
12H 27/1800Z 17.2N 115.7W 40 KT 45 MPH
24H 28/0600Z 17.8N 115.2W 40 KT 45 MPH
36H 28/1800Z 18.4N 114.6W 40 KT 45 MPH
48H 29/0600Z 19.3N 114.1W 40 KT 45 MPH
60H 29/1800Z 20.5N 113.9W 35 KT 40 MPH
72H 30/0600Z 21.7N 114.6W 35 KT 40 MPH
96H 31/0600Z 22.4N 115.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
120H 01/0600Z 22.4N 117.0W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

[EquusStorm]

EPac clearly inspired by the Laura/Marco duo.

[Yellow Evan]

771
WTPZ44 KNHC 272032
TCDEP4

Tropical Storm Iselle Discussion Number 6
NWS National Hurricane Center Miami FL EP142020
200 PM PDT Thu Aug 27 2020

Since the previous advisory, a new burst of deep convection
developed near the center of Iselle the persisted for much of the
morning. It was not until recently that the easterly shear has begun
to push the convection to the west of the center. The latest Dvorak
estimates from TAFB and SAB as well as data from a recent ASCAT
overpass all agree that the initial intensity has increased to 45
kt. Moderate-to-strong easterly to northeasterly vertical wind shear
should prevent Iselle from further strengthening over the next
couple of days. After that time, the shear is forecast to relax
somewhat. However, by that time the cyclone will then be moving over
SSTs of only 26 C and into a more stable atmospheric environment.
This should cause the deep convection to gradually dissipate, and
the cyclone is expected to weaken late this weekend into early next
week before degenerating into a remnant low. The latest NHC
intensity forecast is close to the LGEM guidance.

Iselle continues moving northeastward at 4 kt, embedded in a large
monsoon gyre. The steering flow is forecast to become more
southeasterly this weekend inside the gyre, resulting in a gradual
turn to the northwest. As the cyclone breaks north of the gyre and
becomes shallow, a turn to the west is anticipated while the system
becomes steered by a low- to mid- level ridge to its north. The
latest NHC track forecast was nudged just a little to the right of
the previous one due to a shift in the guidance, and lies in between
the previous forecast and the various track consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 27/2100Z 17.2N 115.8W 45 KT 50 MPH
12H 28/0600Z 17.8N 115.3W 45 KT 50 MPH
24H 28/1800Z 18.6N 114.7W 45 KT 50 MPH
36H 29/0600Z 19.6N 114.4W 40 KT 45 MPH
48H 29/1800Z 20.8N 114.3W 35 KT 40 MPH
60H 30/0600Z 21.9N 114.6W 30 KT 35 MPH
72H 30/1800Z 22.8N 115.4W 25 KT 30 MPH
96H 31/1800Z 23.2N 116.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 01/1800Z 23.1N 117.8W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Latto

[hurricanes1234]

EPac clearly inspired by the Laura/Marco duo.

Ikr! EPAC is on fire!

[Yellow Evan]

078
WTPZ44 KNHC 281447
TCDEP4

Tropical Storm Iselle Discussion Number 9
NWS National Hurricane Center Miami FL EP142020
800 AM PDT Fri Aug 28 2020

GOES-16 shortwave infrared imagery and a recent AMSR2 microwave
overpass indicate that deep convection has been developing near the
surface center during the past several hours. In fact, the
microwave image showed a small, compact inner core defined by a
partially closed eye-like feature. My initial thoughts were that
this cloud feature is in the mid-portion of the atmosphere, but the
lower 37 GHZ frequency confirmed very little vertical tilt.
Subjective and objective satellite intensity estimates are based on
the shear scene-type which would yield a slightly lower intensity
estimation. Consequently, the initial intensity is held at 50 kt,
but it could certainly be a little stronger based on the
aforementioned polar low-orbiter pass.

The FV3/GFS and ECMWF SHIPS statistical-dynamical intensity models
show 20 to 30 kt of northeasterly shear persisting through the next
few days, however, the UW-CIMSS shear analysis reveals less than 20
kt. For now, based on what the large-scale models and the SHIPS
models agree on, gradual weakening should begin by early Saturday
morning and continue through Monday morning as the cyclone
traverses decreasing oceanic temperatures and moves into a more
thermodynamically stable surrounding environment. The NHC intensity
is an update of the previous advisory, and calls for Iselle
to weaken to a tropical depression on Sunday and degenerate into a
remnant low on Sunday evening.

Based on the 0920 UTC AMSR2 pass, the initial position was adjusted
to the northwest of the previous position and the forward motion is
estimated to be northeastward, or 035/4 kt. A northward direction
should commence by early Saturday morning, then a turn
north-northwestward to northwestward is forecast during the 48-60
hr period. As Iselle continues to weaken and become a more
shallower system, a turn toward the west-northwest, well offshore
of the southern coast of the Baja California peninsula, is forecast
to occur Monday morning. The NHC forecast is nudged to the left of
the previous track forecast due the adjusted position, and is based
on the HCCA and TVCE multi-model consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 28/1500Z 18.3N 115.3W 50 KT 60 MPH
12H 29/0000Z 19.2N 114.8W 50 KT 60 MPH
24H 29/1200Z 20.6N 114.6W 45 KT 50 MPH
36H 30/0000Z 22.2N 114.9W 35 KT 40 MPH
48H 30/1200Z 23.5N 115.7W 30 KT 35 MPH
60H 31/0000Z 24.1N 116.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 31/1200Z 24.4N 117.3W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/1200Z...DISSIPATED

$$
Forecaster Roberts

[Kingarabian]

Nice discussion by Roberts!

[Yellow Evan]

28/1731 UTC 18.5N 115.4W T3.0/3.5 ISELLE -- East Pacific

[Yellow Evan]

000
WTPZ44 KNHC 282032
TCDEP4

Tropical Storm Iselle Discussion Number 10
NWS National Hurricane Center Miami FL EP142020
200 PM PDT Fri Aug 28 2020

First light visible imagery shows that Iselle's center of
circulation has once again reappeared to the northeast of the
deep convective mass. Last night's briefly diminished shear
interlude probably resulted in Iselle's peak intensity. For this
advisory, the initial intensity is lowered to 45 kt which is based
on a blend of the Dvorak intensity estimates from TAFB and SAB,
a METOP-B ASCAT overpass containing 40-45 kt winds and a SATCON
analysis of 42 kt.

The northeasterly shear is forecast to persist through the 24 hour
period and then decrease and veer from the southeast. At that
time, however, Iselle will be traversing sub 25C sea surface
temperatures and will be moving into a more dry and stable
surrounding environment. As a result, Iselle should weaken to a
tropical depression Saturday night and degenerate to a remnant
low on Sunday.

The initial motion is estimated to be north-northeastward, or,
020/6 kt. A northward direction should commence Saturday
morning, then a turn north-northwestward is forecast Sunday morning.
As Iselle continues to weaken and degenerates to a remnant low, a
turn toward the northwest, well offshore of the southern coast of
the Baja California peninsula, is expected to occur Sunday
late night. The NHC forecast is an update of the previous advisory
and is close to the GFEX (FV3/ECMWF mean) and the NOAA HCCA
consensus model.


FORECAST POSITIONS AND MAX WINDS

INIT 28/2100Z 18.8N 115.2W 45 KT 50 MPH
12H 29/0600Z 19.6N 114.9W 40 KT 45 MPH
24H 29/1800Z 21.0N 114.7W 35 KT 40 MPH
36H 30/0600Z 22.6N 115.0W 30 KT 35 MPH
48H 30/1800Z 23.7N 115.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 31/0600Z 24.4N 116.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 31/1800Z 25.1N 117.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/1800Z...DISSIPATED

$$
Forecaster Roberts

[Yellow Evan]

555
WTPZ44 KNHC 290232
TCDEP4

Tropical Storm Iselle Discussion Number 11
NWS National Hurricane Center Miami FL EP142020
800 PM PDT Fri Aug 28 2020

Persistent bursts of deep convection continue to develop just to
the west of Iselle's center, which occasionally becomes hidden
beneath the cirrus outflow (as it is now). Dvorak final-T numbers
from TAFB and SAB are 3.0/45 kt based on a shear pattern, and along
with what was shown by an afternoon ASCAT pass, Iselle's intensity
remains 45 kt. Strong shear from the east-northeast is forecast to
continue for the next 24 hours or so. After that time, the shear
should relax, but then the thermodynamic environment becomes more
limiting with Iselle moving over sub-26C waters and into a much more
stable environment. All told, Iselle is expected to steadily weaken
during the next several days, and it is likely to degenerate into a
remnant low on Sunday. The NHC forecast is very similar to the
previous prediction and is close to HCCA and the IVCN intensity
consensus aid. The remnant low is expected to dissipate in about 4
days.

Iselle is moving slowly north-northeastward (015/5 kt) on the
western periphery of a low- to mid-level area of high pressure
centered over Mexico. This high, along with a trough extending off
the coast of California, should cause Iselle to gradually turn
toward the north and then north-northwest with some increase in
forward speed during the next couple of days. There is a typical
amount of spread among the track models, and they generally agree
on this northward motion with a slight bend to the left through the
forecast period. The regional hurricane models appear almost in a
cluster all to themselves on the eastern side of the guidance
envelope, and the NHC forecast therefore closely follows the TVDG
model consensus aid, which gives double weight to the global models.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0300Z 19.3N 115.2W 45 KT 50 MPH
12H 29/1200Z 20.2N 115.0W 40 KT 45 MPH
24H 30/0000Z 21.7N 115.0W 35 KT 40 MPH
36H 30/1200Z 23.1N 115.4W 30 KT 35 MPH
48H 31/0000Z 24.2N 115.9W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 31/1200Z 24.9N 116.4W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/0000Z 25.6N 116.8W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 02/0000Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Tropical Storm Iselle Discussion Number 12
NWS National Hurricane Center Miami FL EP142020
200 AM PDT Sat Aug 29 2020

Iselle has become less organized during the past several hours.
The low-level center is exposed to the east of a decaying and
disheveled area of deep convection. An ASCAT-B overpass from around
0430 UTC showed maximum winds in the 40-45 kt range, but since the
system's appearance has degraded since that time, the initial
intensity is set at 40 kt. This value is also in agreement
with an average of the latest satellite intensity estimates, which
range from 33 to 45 kt. Iselle is feeling the effects of about 25 kt
of easterly vertical wind shear. Since the shear is expected to
remain strong while Iselle heads for cooler waters and a drier air
mass, steady weakening is anticipated. Iselle will likely become a
tropical depression by tonight and a remnant low on Sunday. Most of
the models show the remnant low dissipating in 3 days, and so does
the official forecast.

The compact tropical storm is moving northward at about 5 kt. A
continued northward motion is expected for another day or so,
followed by a turn to the northwest when Iselle becomes a weak and
shallow system and moves in the low-level flow. The new NHC track
forecast is a little to the east of the previous one to come into
better agreement with the latest consensus aids.

The initial 34-kt wind radii have been modified slightly based on
the aforementioned ASCAT data.


FORECAST POSITIONS AND MAX WINDS

INIT 29/0900Z 19.7N 115.1W 40 KT 45 MPH
12H 29/1800Z 20.7N 114.9W 35 KT 40 MPH
24H 30/0600Z 22.2N 114.9W 30 KT 35 MPH
36H 30/1800Z 23.5N 115.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/0600Z 24.5N 115.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 31/1800Z 25.3N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/0600Z...DISSIPATED

$$
Forecaster Cangialosi

[Yellow Evan]

29/1130 UTC 19.9N 114.8W T2.5/3.0 ISELLE -- East Pacific

[cycloneye]

Tropical Storm Iselle Discussion Number 13
NWS National Hurricane Center Miami FL EP142020
900 AM MDT Sat Aug 29 2020

Iselle has changed little in organization since the last advisory,
as the storm has a classic shear pattern in satellite imagery with
the low-level center near the northeastern edge of the convective
mass. Satellite intensity estimates have changed little in the
last 6 h, so the initial intensity remains 40 kt.

The cyclone has tracked a little to the right with the initial
motion now north-northeastward or 025/6 kt. This should be
short lived, and Iselle is expected to resume a northward motion
later today or tonight. The northward motion should continue for
another day or so, followed by a turn toward the northwest as
Iselle becomes a weak and shallow system. The new forecast track
is tweaked a little from the previous forecast based on the initial
position and motion.

Iselle is moving over decreasing sea surface temperatures, and it
continues to experience easterly vertical shear. This combination
should cause steady weakening, and the new intensity forecast
follows the previous forecast in calling for the system to weaken
to a depression by 24 h, to degenerate to a remnant low by 36 h,
and to weaken to a trough by 72 h. The new intensity forecast lies
at the upper edge of the intensity guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 29/1500Z 20.3N 114.7W 40 KT 45 MPH
12H 30/0000Z 21.3N 114.6W 35 KT 40 MPH
24H 30/1200Z 22.8N 114.7W 30 KT 35 MPH
36H 31/0000Z 23.9N 115.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/1200Z 24.7N 115.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 01/0000Z 25.4N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/1200Z...DISSIPATED

$$
Forecaster Beven

[Yellow Evan]

29/1730 UTC 20.5N 114.4W T2.5/3.0 ISELLE -- East Pacific

[cycloneye]

Tropical Storm Iselle Discussion Number 14
NWS National Hurricane Center Miami FL EP142020
300 PM MDT Sat Aug 29 2020

Iselle has become less organized during the past several hours, as
the area of convection southwest of the center is now smaller and
farther from the center than previously. This is likely due to the
combined effects of continued easterly shear and decreasing sea
surface temperatures under the storm. A recent ASCAT overpass
shows that maximum sustained winds remain near 40 kt, but the
tropical-storm-force winds are now occurring only over the
southeastern quadrant.

The initial motion is north-northeastward or 025/6 kt. Iselle
should turn toward the north during the next several hours as it
is steered by a mid-level ridge to the east. The northward motion
should continue for another day or so, followed by a turn toward
the northwest as Iselle becomes a weak and shallow system. The new
forecast track again has minor adjustment from the previous track
based mainly on the initial position and motion.

The shear and cooler water along the forecast track should cause
Iselle to steadily weaken, and the new forecast track has no
changes from the previous forecast. The forecast, which calls for
the cyclone to weaken to a depression by 24 h, to degenerate to a
remnant low by 36 h, and to weaken to a trough by 72 h, again lies
at the upper edge of the intensity guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 29/2100Z 21.0N 114.5W 40 KT 45 MPH
12H 30/0600Z 22.1N 114.5W 35 KT 40 MPH
24H 30/1800Z 23.4N 114.8W 30 KT 35 MPH
36H 31/0600Z 24.2N 115.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 31/1800Z 24.8N 115.6W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 01/0600Z 25.4N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 01/1800Z...DISSIPATED

$$
Forecaster Beven

[cycloneye]

Tropical Storm Iselle Discussion Number 15
NWS National Hurricane Center Miami FL EP142020
900 PM MDT Sat Aug 29 2020

Most of the deep convection associated with Iselle has been
displaced to the west of the center due to strong easterly shear
caused by a large upper-level anticyclone to the northeast. Based
on the decreased organization, it is assumed that the cyclone has
been gradually spinning down this evening, and the current
intensity estimate is set at 35 kt. Although the shear may relax a
bit in a day or so, Iselle will be moving over increasingly cooler
waters during that time. Therefore, continued weakening is
anticipated, and the system will likely degenerate into a remnant
low in 12 to 24 hours. The official intensity forecast is at the
high end of the model guidance.

The cyclone has been moving north-northeastward, at around 020/8
kt, on the west side of a mid-level high pressure area. The
weakening and increasingly shallow system should gradually turn to
the left under the influence of the lower-level flow. The official
track forecast is about in the middle of the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0300Z 21.8N 114.2W 35 KT 40 MPH
12H 30/1200Z 22.9N 114.3W 30 KT 35 MPH
24H 31/0000Z 23.9N 114.6W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 31/1200Z 24.5N 114.9W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 01/0000Z 25.0N 115.3W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 01/1200Z 25.5N 115.7W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 02/0000Z...DISSIPATED

$$
Forecaster Pasch

[cycloneye]

Tropical Depression Iselle Discussion Number 16
NWS National Hurricane Center Miami FL EP142020
300 AM MDT Sun Aug 30 2020

Iselle has produced very little deep convection since around 00Z.
Earlier partial ASCAT data indicated that the maximum winds have
decreased to 30 kt, which is not surprising given Iselle's lack of
organization. Iselle is moving into a drier and more stable
environment and over progressively cooler SSTs. While some slight
redevelopment of convection is possible during the next few hours,
it is unlikely to be very well organized and Iselle is forecast
become post-tropical later today. The remnants of Iselle will then
continue to gradually spin down for another day or two until it
dissipates entirely in a couple of days.

The cyclone has moved generally north-northeastward to northward
during the pas few hours and is slowing down. The guidance is in
good agreement that a northward to north-northwestward motion is
likely until Iselle dissipates. No changes of note were made to the
NHC track or intensity forecasts.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0900Z 22.3N 114.0W 30 KT 35 MPH
12H 30/1800Z 23.3N 114.1W 30 KT 35 MPH...POST-TROP/REMNT LOW
24H 31/0600Z 24.2N 114.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 31/1800Z 24.9N 114.4W 20 KT 25 MPH...POST-TROP/REMNT LOW
48H 01/0600Z 26.0N 115.0W 15 KT 15 MPH...POST-TROP/REMNT LOW
60H 01/1800Z...DISSIPATED

$$
Forecaster Zelinsky

[Yellow Evan]

000
WTPZ44 KNHC 301436
TCDEP4

Tropical Depression Iselle Discussion Number 17
NWS National Hurricane Center Miami FL EP142020
900 AM MDT Sun Aug 30 2020

Iselle is skirting the line between tropical cyclone and remnant
low, producing a small amount of deep convection during the past few
hours. While this isn't particularly organized thunderstorm
activity, for now it is enough to keep advisories going another 6
hours. The initial intensity is set to 30 kt, near the latest TAFB
T-number. Iselle is still expected to degenerate into a remnant
low later today due to persistent easterly shear and a dry and
stable environment. Model guidance all show the system becoming a
trough by Tuesday, so the dissipation phase has been pushed ahead
from the last forecast. The guidance also remains in good agreement
on a northward track today, followed by a north-northwest turn
overnight. The track is shifted a bit to the east because of a more
eastward initial position, otherwise it is unchanged.


FORECAST POSITIONS AND MAX WINDS

INIT 30/1500Z 23.0N 113.3W 30 KT 35 MPH
12H 31/0000Z 24.0N 113.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 31/1200Z 25.0N 113.7W 20 KT 25 MPH...POST-TROP/REMNT LOW
36H 01/0000Z...DISSIPATED

$$
Forecaster Blake

[Yellow Evan]

30/1130 UTC 22.7N 113.4W TOO WEAK ISELLE -- East Pacific

[Yellow Evan]

Will probably be our last storm for a couple weeks. This has no deep convection so the plug is going to be pulled very soon.