CPAC: IONA - Tropical Storm - Discussion

[REDHurricane]

So does anyone know whether or not this tsunami is going to have any effect on Iona? Has this scenario (tsunami from an 8.0+ magnitude earthquake hitting a major hurricane/super typhoon) ever even occurred before?

[cycloneye]

Hurricane Iona Discussion Number 12
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 PM HST Tue Jul 29 2025

Iona likely reached peak intensity earlier today, with satellite
images now showing the eye obscured by a central dense overcast.
The cyclone has begun to weaken as it feels the influence of
increasing westerly vertical wind shear while also moving over
cooler sea surface temperatures below 27C, as depicted by the
Real-time, Global, Sea Surface Temperature Analysis. The latest
subjective Dvorak current intensity estimates from PHFO and JTWC
were 5.5/102 knots, while SAB was a somewhat lower 4.5/77 knots.
Meanwhile, the latest objective intensity estimates range from 85 to
96 knots. Given that the system has just recently begun to weaken,
the initial intensity will be lowered to 100 knots for this
advisory.

Iona is moving due west, or 270 degrees, at 15 knots. This general
motion is expected to continue during the next couple of days as the
cyclone moves along the southern periphery of a subtropical ridge to
the north. A turn toward the west-northwest and a gradual decrease
in forward speed is expected late in the week and over the weekend,
as the system is steered into a weakness in the mid-level ridge to
the north. The official track forecast remains near the center of
the tightly clustered guidance envelope and is nearly identical to
the previous advisory. Confidence remains high that Iona will stay
well south of the Hawaiian Islands.

The cyclone is expected to undergo rapid weakening tonight and
Wednesday as it experiences increasing westerly vertical wind shear
while also moving over cooler sea surface temperatures of 26.5 to
27C. A slower rate of weakening is then forecast Wednesday night
and Thursday. By Friday, the shear begins to relax, sea surface
temperatures begin to increase to 28C or above, and the mid-levels
begin to moisten. The spread among the intensity guidance is
considerable, and the cyclone’s intensity will likely depend on how
intact the system remains after moving through the hostile
environment over the next couple of days. The official forecast
shows the intensity leveling off after 48 hours, but confidence in
this is low. Overall, the intensity forecast lies pretty close to
the middle of the intensity guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0300Z 11.0N 157.1W 100 KT 115 MPH
12H 30/1200Z 11.1N 159.5W 85 KT 100 MPH
24H 31/0000Z 11.3N 162.6W 70 KT 80 MPH
36H 31/1200Z 11.6N 166.0W 65 KT 75 MPH
48H 01/0000Z 12.1N 169.5W 60 KT 70 MPH
60H 01/1200Z 12.8N 172.7W 55 KT 65 MPH
72H 02/0000Z 13.7N 175.7W 55 KT 65 MPH
96H 03/0000Z 15.9N 179.2E 55 KT 65 MPH
120H 04/0000Z 17.9N 174.1E 50 KT 60 MPH

$$
Forecaster Jelsema (CPHC)

[gib]

So does anyone know whether or not this tsunami is going to have any effect on Iona? Has this scenario (tsunami from an 8.0+ magnitude earthquake hitting a major hurricane/super typhoon) ever even occurred before?

Tsunamis have long wavelengths and low amplitude in the open ocean. They aren't very noticeable until they reach shallow water. Iona won't be affected.

[cycloneye]

Hurricane Iona Discussion Number 13
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 PM HST Tue Jul 29 2025

The satellite presentation has continued to deteriorate this
evening, with the low-level center of Iona obscured by a central
dense overcast with gradually warming cloud top temperatures. The
cyclone is continuing to be influenced by increasing westerly
vertical wind shear while also moving over cooler sea surface
temperatures around or slightly below 27C, as depicted by the
Real-time, Global, Sea Surface Temperature Analysis. The latest
subjective Dvorak current intensity estimates ranged from
4.5/77 knots to 5.5/102 knots, while the latest objective intensity
estimates range from 77 to 87 knots. Taking a blend of these data
the initial intensity has been lowered to 90 knots for this
advisory.

Iona is moving westward, or 275 degrees, at 15 knots. This general
motion is expected to continue during the next couple of days as the
cyclone moves along the southern periphery of a subtropical ridge to
the north. A turn toward the west-northwest and a gradual decrease
in forward speed is expected late in the week and over the weekend,
as the system is steered into a weakness in the mid-level ridge to
the north. The official track forecast remains near the center of
the tightly clustered guidance envelope and is very close to the
previous advisory. Confidence remains high that Iona will stay well
south of the Hawaiian Islands.

The cyclone is expected to undergo rapid weakening tonight and
Wednesday as it experiences increasing westerly vertical wind shear
while also moving over cooler sea surface temperatures of 26.5 to
27C. The intensity forecast then shows little change in strength
Thursday through Friday as sea surface temperatures begin to warm,
vertical wind shear gradually eases, and mid-level moisture
increases. Iona should then begin to gradually weaken by days 4
and 5 as the mid-levels dry out and the system becomes influenced by
increasing westerly and southwesterly vertical wind shear. The
official forecast was adjusted to account for the latest intensity
guidance trends, and lies pretty close to the middle of the guidance
envelope.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0900Z 11.3N 158.6W 90 KT 105 MPH
12H 30/1800Z 11.5N 161.0W 70 KT 80 MPH
24H 31/0600Z 11.8N 164.5W 55 KT 65 MPH
36H 31/1800Z 12.2N 168.2W 50 KT 60 MPH
48H 01/0600Z 12.9N 171.7W 50 KT 60 MPH
60H 01/1800Z 13.7N 175.1W 50 KT 60 MPH
72H 02/0600Z 14.5N 177.9W 50 KT 60 MPH
96H 03/0600Z 16.5N 177.3E 45 KT 50 MPH
120H 04/0600Z 18.8N 172.6E 40 KT 45 MPH

$$
Forecaster Jelsema (CPHC)

[cycloneye]

HAFs-A continues to intensify Iona at WPAC.

[StormWeather]

000
WTPA41 PHFO 301433
TCDCP1

Hurricane Iona Discussion Number 14
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 AM HST Wed Jul 30 2025

Iona has continued to rapidly weaken since the last advisory. A GPM
microwave overpass showed an exposed low-level center located well
to the northwest of the mid-level center apparent in geostationary
imagery, with little or no organized convection near the low-level
center. Subjective and objective satellite intensity estimates have
decreased significantly and currently are in the 45-75 kt range.
Based on a blend of these estimates, the initial intensity is
reduced to a likely generous 65 kt.

The initial motion is now 280/17 kt. This general motion is
expected to continue during the next day or two as the cyclone
moves along the southern periphery of a low- to mid-level
subtropical ridge to the north. A turn toward the west-northwest
and a gradual decrease in forward speed are expected late in the
week and over the weekend, as the system is steered into a weakness
in the ridge. The official forecast is nudged just a little north
of the previous track and lies near the center of the track
guidance envelope. Iona has stayed well south of the Hawaiian
Islands, and the cyclone is forecast to move farther away from the
islands during the next couple of days.

Rapid weakening is expected to continue for the next 12-24 h due to
increasing westerly vertical shear and a track over cooler sea
surface temperatures. After that, the intensity forecast shows a
period of little change in strength as sea surface temperatures
begin to warm along the forecast track and the atmospheric
conditions become a little more favorable. By the end of the
forecast period, Iona should again weaken as it encounters mid-level
dry air and another bout of vertical shear. The intensity forecast
was lowered below the previous forecast mainly in response to the
current intensity trends.


FORECAST POSITIONS AND MAX WINDS

INIT 30/1500Z 11.5N 160.2W 65 KT 75 MPH
12H 31/0000Z 11.8N 162.8W 55 KT 65 MPH
24H 31/1200Z 12.2N 166.4W 45 KT 50 MPH
36H 01/0000Z 12.8N 169.8W 45 KT 50 MPH
48H 01/1200Z 13.5N 173.2W 45 KT 50 MPH
60H 02/0000Z 14.5N 176.3W 45 KT 50 MPH
72H 02/1200Z 15.4N 179.2W 45 KT 50 MPH
96H 03/1200Z 17.3N 176.0E 40 KT 45 MPH
120H 04/1200Z 19.2N 171.1E 35 KT 40 MPH

$$
Forecaster Beven

[wxman57]

Looking more like a disorganized and weak TS than a hurricane. Jack is much more "generous" than I would be with its intensity.

[cycloneye]

Tropical Storm Iona Discussion Number 15
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 AM HST Wed Jul 30 2025

Iona continues to rapidly weaken, as the low-level center is now
seen in visible imagery exposed well to the west-northwest of the
remaining poorly-organized convection. Subjective and objective
satellite intensity estimates again have decreased significantly
and currently are in the 40-65 kt range. Based on a blend of these
estimates, the initial intensity is reduced to a possibly generous
55 kt, and Iona is downgraded to a tropical storm.

The initial motion is 275/19 kt. There is little change in the
forecast philosophy from the past couple of forecasts, as a general
westward motion is expected to continue during the next day or two
while Iona moves along the southern periphery of a low- to mid-level
subtropical ridge to the north. A turn toward the west-northwest
and a gradual decrease in forward speed are expected after 48 h as
the system is steered into a weakness in the ridge. There has been
little change in the forecast guidance since the last advisory, and
the new forecast track is similar to the previous track.

Rapid weakening should continue for the next 12 h or so due to
westerly vertical shear and a track over cooler sea surface
temperatures. After that, the intensity forecast shows a period of
little change in strength as sea surface temperatures begin to warm
along the forecast track and the atmospheric conditions become a
little more favorable. After 72 h, Iona should again weaken as it
encounters mid-level dry air and another bout of vertical shear.
The intensity forecast was again lowered below the previous
forecast based on the current intensity trends. Note that the
intensity forecast is based on the premise that Iona will re-develop
enough convection to sustain the system as a tropical cyclone. If
this does not happen, the cyclone could degenerate to a remnant low
or trough before 120 h.


FORECAST POSITIONS AND MAX WINDS

INIT 30/2100Z 11.7N 162.6W 55 KT 65 MPH
12H 31/0600Z 12.1N 165.5W 45 KT 50 MPH
24H 31/1800Z 12.7N 169.3W 40 KT 45 MPH
36H 01/0600Z 13.4N 172.7W 40 KT 45 MPH
48H 01/1800Z 14.1N 175.9W 40 KT 45 MPH
60H 02/0600Z 15.1N 178.8W 40 KT 45 MPH
72H 02/1800Z 16.0N 178.7E 40 KT 45 MPH
96H 03/1800Z 18.1N 174.1E 35 KT 40 MPH
120H 04/1800Z 20.2N 169.1E 30 KT 35 MPH

$$
Forecaster Beven

[StormWeather]

I’m just going say this: Adios Iona!

[cycloneye]

Tropical Storm Iona Discussion Number 16
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 PM HST Wed Jul 30 2025

Iona continues to weaken, with the low-level circulation center
remaining mostly exposed. Recent satellite imagery showed a
convective burst on the north side of the center, with cloud tops
reaching as cold as -75°C. However, this burst has not been
sustained, and convection remains generally disorganized. An earlier
ASCAT pass sampled peak winds of around 40 kt in the northern
semicircle of the circulation. Subjective and objective satellite
intensity estimates remain in the 40–65 kt range. Taking into
account the degraded satellite presentation, ASCAT data, and the
blend of available intensity estimates, the initial intensity is set
at 45 kt.

The initial motion is 275/20 kt. There is little change in the
forecast philosophy from the past couple of forecasts, as a general
westward motion is expected to continue during the next day or so
while Iona moves along the southern periphery of a low- to mid-level
subtropical ridge to the north. A turn toward the west-northwest and
a gradual decrease in forward speed are expected after 36 h as the
system is steered into a weakness in the ridge. There has been
little change in the forecast guidance since the last advisory, and
the new forecast track is similar to the previous track.

The weakening trend appears to have stabilized, with only slight
additional weakening expected tonight. By Thursday, Iona's intensity
is forecast to level off as it moves over warmer sea surface
temperatures and into a marginally more favorable atmospheric
environment. The system is expected to resume weakening by early
next week as it encounters mid-level dry air and increasing vertical
shear. The intensity forecast has been adjusted slightly downward to
reflect the recent trends. This forecast assumes that Iona will
maintain enough convection to remain a tropical cyclone. Should
convection fail to persist, the system could degenerate into a
remnant low or trough before day five.


FORECAST POSITIONS AND MAX WINDS

INIT 31/0300Z 11.8N 164.8W 45 KT 50 MPH
12H 31/1200Z 12.2N 167.7W 40 KT 45 MPH
24H 01/0000Z 12.8N 171.3W 35 KT 40 MPH
36H 01/1200Z 13.4N 174.8W 35 KT 40 MPH
48H 02/0000Z 14.2N 177.7W 35 KT 40 MPH
60H 02/1200Z 15.0N 179.6E 35 KT 40 MPH
72H 03/0000Z 16.0N 177.3E 35 KT 40 MPH
96H 04/0000Z 18.2N 172.6E 35 KT 40 MPH
120H 05/0000Z 20.6N 168.1E 30 KT 35 MPH

$$
Forecaster Gibbs (CPHC)

[cycloneye]

Tropical Storm Iona Discussion Number 17
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 PM HST Wed Jul 30 2025

Satellite imagery indicates that convection has become more
concentrated over the center of Iona, with a recent convective burst
producing cloud tops as cold as -85 C. This burst has obscured the
previously exposed low-level circulation center, though the overall
structure generally remains disorganized. Subjective Dvorak current
intensity estimates ranged from 2.5/35 kt (JTWC) to 3.0/45 kt (SAB),
with HFO at 3.5/55 kt. These values are mostly consistent with
objective satellite intensity estimates, which ranged from 30 to 40
kt. Based on a blend of these estimates and the current satellite
presentation, the initial intensity is held at 45 kt.

The initial motion is estimated at 280/20 kt. Iona is expected to
continue moving westward over the next day or two along the southern
periphery of a low- to mid-level subtropical ridge located to the
north. By 48 to 60 hours, a gradual turn toward the west-northwest
and a decrease in forward speed are anticipated as the system
approaches a weakness in the ridge. The forecast track remains
consistent with the previous advisory and is well supported by the
latest consensus guidance.

Only slight additional weakening is expected in the short term as
Iona remains embedded within a moderate westerly shear environment
of 20 to 25 kt. By around 60 hours (Saturday), vertical shear is
forecast to relax, which could allow the cyclone to hold steady or
even briefly restrengthen as it moves over slightly warmer waters.
However, conditions are expected to deteriorate again late this
weekend through early next week, with increasing shear and mid-level
dry air likely leading to weakening. The intensity forecast has been
held steady from the the previous advisory, supported by the recent
burst of convection and the latest guidance. This forecast assumes
that Iona will maintain enough convection to remain a tropical
cyclone, but if convection fails to persist, the system could
degenerate into a remnant low sooner than currently indicated.


FORECAST POSITIONS AND MAX WINDS

INIT 31/0900Z 12.3N 166.9W 45 KT 50 MPH
12H 31/1800Z 12.7N 169.8W 40 KT 45 MPH
24H 01/0600Z 13.5N 173.3W 35 KT 40 MPH
36H 01/1800Z 14.1N 176.7W 35 KT 40 MPH
48H 02/0600Z 14.9N 179.6W 35 KT 40 MPH
60H 02/1800Z 15.8N 177.9E 35 KT 40 MPH
72H 03/0600Z 16.6N 175.7E 35 KT 40 MPH
96H 04/0600Z 18.4N 171.3E 30 KT 35 MPH
120H 05/0600Z 20.5N 167.1E 30 KT 35 MPH

$$
Forecaster Gibbs (CPHC)

[cycloneye]

Hanging on.

[cycloneye]

Tropical Storm Iona Discussion Number 18
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 AM HST Thu Jul 31 2025

Iona continues to generate a strong burst of convection over the
low-level center, although there are no recent microwave overpasses
to shows if there has been an increase in the system's organization.
The various subjective and objective satellite intensity estimates
range from 30-50 kt, and based on a blend of these and the current
convection, the initial intensity is held at 45 kt. This intensity
is supported by a recent ASCAT overpass with 35-40 kt winds, and
the scatterometer data has also been used to revise the wind radii.

The initial motion is 280/19 kt. Iona is expected to continue
moving westward over the next day or so along the southern
periphery of a low- to mid-level subtropical ridge located to the
north. By 48 to 60 hours, a gradual turn toward the west-northwest
and a decrease in forward speed are anticipated as the system
approaches a weakness in the ridge. How much of a turn may be
dependent on the strength of Iona. The GFS and UKMET have a weaker
cyclone and show a more westerly track, while the ECMWF and Canadian
models have a stronger system and a more northward motion. The new
forecast track splits the difference between these extremes and lies
near the various consensus models.

Iona continues to be affected by 25 kt of northwesterly vertical
wind shear, and due to this it is uncertain how long the current
convective burst will persist. The intensity guidance shows some
weakening during the next 24-36 h while the shear persists, and the
official forecast follows that trend. After that, the shear is
forecast to decrease, and there may be a day or two where the
environment becomes favorable enough for Iona to at least maintain
its intensity. By the end of the forecast period, the global models
diverge on the forecast environment near the cyclone, with the GFS
forecast strong subsidence that would suppress the convection
while the ECMWF forecasts a less hostile environment. The new
forecast will lean toward the GFS and the previous forecast in
showing gradual weakening, with Iona forecast to decay to a remnant
low by 120 h.


FORECAST POSITIONS AND MAX WINDS

INIT 31/1500Z 12.7N 168.7W 45 KT 50 MPH
12H 01/0000Z 13.3N 171.5W 40 KT 45 MPH
24H 01/1200Z 14.0N 174.9W 35 KT 40 MPH
36H 02/0000Z 14.7N 178.0W 35 KT 40 MPH
48H 02/1200Z 15.5N 179.2E 35 KT 40 MPH
60H 03/0000Z 16.4N 176.5E 35 KT 40 MPH
72H 03/1200Z 17.2N 174.2E 35 KT 40 MPH
96H 04/1200Z 19.0N 169.7E 30 KT 35 MPH
120H 05/1200Z 21.4N 165.9E 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Beven

[Kingarabian]

Banding has returned to the south of the system. Also growing in size.

[sasha_B]

The latest ADT fixes for TS Iona suggest that it's reintensifying fairly quickly, but subjective Dvorak fixes from JTWC and NOAA are substantially lower; JTWC had T2.5/3.0 and NOAA T3.0/3.0 on their respective 1700z and 1730z fixes, whereas ADT's estimates have risen from T3.2 to T3.7 (CI/FT) over the past six hours, with raw T#s ranging between 3.7～4.5 during that period.

Intensity was held at 45 kts / 1002 hPa at 18z today, per ATCF best track:

01C IONA 250731 1800 13.0N 169.8W CPAC 45 1002

Whereas UW-CIMSS ADT has:

CI# / Pressure / Vmax
3.7 / 992.1mb / 59.0kt