CPAC: IONA - Tropical Depression - Discussion

[cycloneye]

Tropical Storm Iona Discussion Number 21
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 PM HST Thu Jul 31 2025

Satellite imagery indicates that convection associated with Iona
remains fragmented and poorly organized, with the low-level
circulation center nearly exposed or located near the western edge
of the deep convection due to persistent west-northwest shear. A
recent AMSR2 microwave pass revealed an elongated low-level
structure oriented from north to south, with no well-defined core.
Subjective Dvorak current intensity estimates from HFO, SAB, and
JTWC ranged from 2.5/35 kt to 3.0/45 kt. Based on these estimates
and the poor satellite presentation, the initial intensity is
lowered to 40 kt.

The initial motion is estimated at 285/16 kt. Iona is expected to
continue moving west-northwestward over the next couple of days
while gradually slowing, steered along the southern periphery of a
subtropical ridge to the north. A turn toward the northwest is
forecast by late this weekend or early next week as the system
approaches a weakness in the ridge. The updated track forecast
remains close to the previous one, with a slight rightward
adjustment that aligns with the consensus guidance.

Little change in intensity is expected over the next day or two, but
a steady weakening trend should begin by Sunday as the system
interacts with an upper-level trough and entrains mid- to
upper-level dry air. Most dynamical guidance suggests that Iona will
gradually open into a trough by early next week. As such, the
official forecast now shows the system dissipating by 96 hours, or
Tuesday. This forecast assumes Iona will maintain sufficient
convection to remain a tropical cyclone through the weekend. If
convection fails to persist, however, dissipation could occur sooner
than currently indicated.


FORECAST POSITIONS AND MAX WINDS

INIT 01/0900Z 14.2N 174.4W 40 KT 45 MPH
12H 01/1800Z 14.8N 176.7W 40 KT 45 MPH
24H 02/0600Z 15.8N 179.6W 40 KT 45 MPH
36H 02/1800Z 17.0N 177.9E 40 KT 45 MPH
48H 03/0600Z 18.1N 175.7E 35 KT 40 MPH
60H 03/1800Z 19.5N 173.6E 35 KT 40 MPH
72H 04/0600Z 21.3N 171.4E 30 KT 35 MPH
96H 05/0600Z...DISSIPATED

$$
Forecaster Gibbs (CPHC)

[Hayabusa]

JMA 06Z sfc weather chart

WWJP27 RJTD 010600
WARNING AND SUMMARY 010600.
WARNING VALID 020600.
WARNING IS UPDATED EVERY 6 HOURS.
REMARKS.
SEVERE TROPICAL STORM 2509 KROSA (2509) 975 HPA AT 33.6N 141.5E : SEE
TROPICAL CYCLONE WARNING.
TROPICAL STORM IONA: SEE TROPICAL CYCLONE WARNING(WTPA21 PHFO).
JAPAN METEOROLOGICAL AGENCY.=

[StormWeather]

252
WTPA41 PHFO 011438
TCDCP1

Tropical Storm Iona Discussion Number 22
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 AM HST Fri Aug 01 2025

Satellite imagery shows that the convection associated with Iona
remains fragmented and disorganized. Scatterometer data received
since the last advisory indicates the maximum winds are near 35 kt
and that the circulation has weakened to the point that it is
uncertain whether it is still closed. The system will be maintained
as a 35-kt tropical storm for this advisory.

The initial motion is estimated at 285/18 kt. As mentioned
previously, Iona is expected to continue moving west-northwestward
over the next couple of days with a gradual decrease in forward
speed as it moves along the southern periphery of the subtropical
ridge to the north. A turn toward the northwest is forecast after
36-48 h as the system approaches a developing break in the ridge.
The new forecast track lies near the consensus models and the center
of the guidance envelope, and it is an update of the previous
forecast.

Iona continues to be affected by westerly shear, which should
persist for the next 12-18 h. After that, the system is forecast to
interact with an upper-level trough, and it will likely enter an
area of upper-level convergence by 60-72 h. Based on this scenario
and the dynamical guidance, the new intensity forecast calls for
Iona remain a tropical storm for 36 h or so, followed by weakening
to a depression by 48 h and degenerating into a trough by 96 h.
Given the current state of organization and the generally
unfavorable environment, the system could weaken to a trough at any
time during the next 72 h.


FORECAST POSITIONS AND MAX WINDS

INIT 01/1500Z 14.6N 176.3W 35 KT 40 MPH
12H 02/0000Z 15.4N 178.5W 35 KT 40 MPH
24H 02/1200Z 16.6N 178.8E 35 KT 40 MPH
36H 03/0000Z 17.8N 176.5E 35 KT 40 MPH
48H 03/1200Z 19.0N 174.3E 30 KT 35 MPH
60H 04/0000Z 20.6N 172.1E 30 KT 35 MPH
72H 04/1200Z 22.5N 170.2E 30 KT 35 MPH
96H 05/1200Z...DISSIPATED

$$
Forecaster Beven

[TomballEd]

Is CPHC short staffed? I've noticed NHC has issued several of the advisories.

[StormWeather]

Is CPHC short staffed? I've noticed NHC has issued several of the advisories.

I mean the CPHC is the same agency that takes years to issue TCRs for storms that form in their area (see Hone for a recent example, who TCR is still not out and late this month will be a year since Hone)

It is possible that they are short staffed.

[cycloneye]

Tropical Storm Iona Discussion Number 23
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 AM HST Fri Aug 01 2025

Convection associated with Iona remains disorganized, with clusters
of convection now occurring only in the southern semicircle. While
satellite imagery shows that the circulation is becoming elongated,
it is still uncertain whether it is actually closed. An upcoming
scatterometer overpass should help provide more information on
whether the circulation still exists and on the maximum winds. For
the time being, the initial intensity is held at 35 kt as a blend
of the objective and subjective satellite intensity estimates.

The initial motion is now at 285/16 kt. Iona is expected to move
west-northwestward during the next couple of days with a gradual
decrease in forward speed along the southern periphery of the
subtropical ridge to the north. A turn toward the northwest is
forecast after 36-48 h as the system approaches a developing break
in the ridge west of 165E. The track guidance has shifted a little
to the west on this cycle, and the new forecast track is slightly
to the west of the previous track.

The intensity forecast scenario is unchanged from earlier. Iona
continues to be affected by westerly shear, which should persist
for the next 12-18 h. After that, the system is forecast to
interact with an upper-level trough, and it will likely enter an
area of upper-level convergence by 60-72 h. Based on this scenario
and the dynamical guidance, the intensity forecast calls for Iona
remain a tropical storm for 12 h or so, followed by weakening to a
depression by 24 h and degenerating into a remnant low by 72 h.
Given the current state of organization and the generally
unfavorable environment, there remains the possibility the system
could weaken to a trough at any time during the next 60-72 h.


FORECAST POSITIONS AND MAX WINDS

INIT 01/2100Z 15.0N 177.6W 35 KT 40 MPH
12H 02/0600Z 15.8N 179.6W 35 KT 40 MPH
24H 02/1800Z 17.0N 177.8E 30 KT 35 MPH
36H 03/0600Z 18.2N 175.5E 30 KT 35 MPH
48H 03/1800Z 19.4N 173.4E 30 KT 35 MPH
60H 04/0600Z 21.1N 171.3E 30 KT 35 MPH
72H 04/1800Z 22.7N 169.5E 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 05/1800Z...DISSIPATED

$$
Forecaster Beven

[StormWeather]

Iona’s a fighter, that’s for sure! I personally wasn’t expecting her to survive this long.

[cycloneye]

The long journey is almost over.

CP, 01, 2025080200, , BEST, 0, 155N, 1788W, 30, 1008, TD

[StormWeather]

The long journey is almost over.

CP, 01, 2025080200, , BEST, 0, 155N, 1788W, 30, 1008, TD

Question is, can Iona survive another 6-12 hours to make it into the Western Pacific?

[cycloneye]

Last advisory from NHC and now JMA / JTWC take over.

Tropical Depression Iona Discussion Number 24
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 PM HST Fri Aug 01 2025

Satellite imagery indicates that convection associated with Iona
remains fragmented and disorganized, with the low-level circulation
becoming increasingly broad and diffuse under persistent
west-northwest shear. The center is exposed and becoming less
defined, and a recent ASCAT pass detected peak winds of only around
25 kt in the northeastern quadrant. Subjective Dvorak current
intensity estimates from SAB, HFO, and JTWC ranged from 1.5/25 kt to
2.5/35 kt. Based on these data and the degraded satellite
appearance, the initial intensity is lowered to 30 kt.

The initial motion is estimated at 295/15 kt. Iona is expected to
continue west-northwestward through the weekend while gradually
slowing, steered by the southern flank of a low- to mid-level
subtropical ridge to the north. A northwestward turn remains likely
by late this weekend or early next week as the system approaches a
weakness in the ridge. The latest track forecast remains similar to
the previous advisory and is close to the consensus aids, with only
minor adjustments.

Given the increasingly degraded structure of Iona and the lack of
deep convection, little to no reorganization is anticipated.
Persistent westerly shear, interaction with an upper-level trough,
and the intrusion of mid- to upper-level dry air should continue to
inhibit strengthening. While most dynamical model guidance depicts
Iona gradually opening into a trough by early next week, the current
disorganized state suggests that degeneration into a remnant low or
trough could occur at any time over the weekend. The updated
forecast maintains Iona as a tropical cyclone briefly, but the
system is expected to dissipate by 72 hours or sooner.

This is the last advisory issued by the Central Pacific Hurricane
Center on this system. Future advisories will be issued by RSMC
Tokyo, Japan. Advisories issued by RSMC Tokyo are found on the web
at:

www.jma.go.jp/jma/jma-eng/jma-center/rs ... SMC_HP.htm

For U.S. interests, see advisories issued by the Joint Typhoon
Warning Center (JTWC) in Honolulu, Hawaii. Advisories issued by JTWC
can be found on the web at:

www.metoc.navy.mil/jtwc/jtwc.html


FORECAST POSITIONS AND MAX WINDS

INIT 02/0300Z 15.8N 179.5W 30 KT 35 MPH
12H 02/1200Z 16.8N 178.3E 30 KT 35 MPH
24H 03/0000Z 18.0N 175.8E 30 KT 35 MPH
36H 03/1200Z 19.1N 173.6E 30 KT 35 MPH
48H 04/0000Z 20.6N 171.6E 30 KT 35 MPH
60H 04/1200Z 22.1N 169.7E 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 05/0000Z...DISSIPATED

$$
Forecaster Gibbs (CPHC)