CPAC: IONA - Tropical Storm - Discussion

[Subtrop]

EP, 97, 2025072512, , BEST, 0, 106N, 1377W, 25, 1009, DB, 34, NEQ, 0, 0, 0, 0, 1012, 210, 90, 0, 0, E, 0, , 0, 0, INVEST, S, 0, , 0, 0, 0, 0, genesis-num, 013, SPAWNINVEST, ep742025 to ep972025,

https://ftp.nhc.noaa.gov/atcf/btk/bep972025.dat

[cycloneye]

Looking good and maybe close to TD.

[cycloneye]

Well Southeast of the Hawaiian Islands(EP97):
An area of low pressure located well southeast of the Hawaiian
Islands is producing a large area of showers and thunderstorms.
Some gradual development of this system is possible, and a tropical
depression could form this weekend or early next week as it
moves generally westward at 10 to 15 mph across the far western
portion of the eastern Pacific and into the Central Pacific basin.
* Formation chance through 48 hours...medium...40 percent.
* Formation chance through 7 days...medium...60 percent.

[Kingarabian]

So far... its looking really good. It just needs to find a brief pocket of favorable conditions. This is looking like a CPAC classic.

[StormWeather]

So far... its looking really good. It just needs to find a brief pocket of favorable conditions. This is looking like a CPAC classic.

A Central Pacific classic? What does that mean? All I know is that a July CPAC storm is rare, I think the last time it happened was 2015

[cycloneye]

Is now passed 140W so it changed identification and number.

Eastern portion of Central Pacific (CP90):
An area of low pressure located well southeast of the Hawaiian
Islands is producing a large area of showers and thunderstorms.
Some gradual development of this system is possible, and a tropical
depression could form this weekend or early next week as it moves
generally westward at 10 to 15 mph across the Central Pacific basin.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 7 days...medium...60 percent.

[cycloneye]

CP, 90, 2025072600, , BEST, 0, 106N, 1409W, 25, 1008, LO

[cycloneye]

Eastern portion of Central Pacific (CP90):
Showers and thunderstorms remain limited and disorganized in
association with an area of low pressure located about 1000 miles
east-southeast of the Hawaiian Islands. Some gradual development of
this system is possible, and a tropical depression could form
before it moves into less conducive environmental conditions by
the middle of next week. This system is forecast to move westward
during the next several days well south of the Hawaiian Islands.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 7 days...medium...60 percent.

[Kingarabian]

It looks like the system behind it seems to have a better shot. Still good structure on it. Dry air taking a toll.

[cycloneye]

. Eastern portion of Central Pacific (CP90):
Showers and thunderstorms have become slightly better organized in
association with an area of low pressure located about 950 miles
southeast of the Hawaiian Islands. Additional development of this
system is expected as it moves generally westward staying well south
of the Hawaiian Islands, and a short-lived tropical depression is
likely to form during the next few days.
* Formation chance through 48 hours...medium...60 percent.
* Formation chance through 7 days...high...70 percent.

[Subtrop]

01C ONE 250727 0600 10.9N 145.1W CPAC 30 1009

NRL.

[cycloneye]

BULLETIN
Tropical Depression One-C Advisory Number 1
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 PM HST Sat Jul 26 2025

...TROPICAL DEPRESSION ONE-C FORMS WELL TO THE SOUTHEAST OF THE
HAWAIIAN ISLANDS...


SUMMARY OF 1100 PM HST...0900 UTC...INFORMATION
-----------------------------------------------
LOCATION...10.9N 145.6W
ABOUT 1085 MI...1745 KM SE OF HONOLULU HAWAII
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1009 MB...29.80 INCHES




Tropical Depression One-C Discussion Number 1
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 PM HST Sat Jul 26 2025

Deep convection has persisted over the area of low pressure well
southeast of the Hawaiian Islands for the last 12 hours or so, with
the latest satellite images showing improved organization and a
better defined banding structure. The latest subjective Dvorak
current intensity estimates came in at 2.0/30 knots from PHFO,
1.5/25 knots from SAB, and 1.0/25 knots from JTWC. A recent
27/0620Z Metop-B ASCAT pass showed a well-defined low-level
circulation with a large swath of 25-knot winds and a few embedded
25- to 30-knot wind barbs. Given the improved structure and
organization of the system, advisories are being initiated on the
first tropical cyclone of the season in the central Pacific,
Tropical Depression One-C. The initial intensity will be set at 30
knots.

Tropical Depression One-C is moving toward the west, or 270 degrees,
at 10 knots. This general westward motion is expected to continue
over the next couple of days as the system moves along the southern
periphery of a subtropical ridge to the north. Beyond 48 hours, the
forward speed is expected to increase considerably in response to a
low- to mid-level ridge strengthening to the north of cyclone. The
track forecast closely follows a blend of the TVCE and HCCA
consensus guidance.

The environment will remain conducive for intensification over the
next couple of days, as One-C remains over warm waters between 28
and 29C, with adequate mid-level moisture and light to moderate
easterly shear. As a result, the intensity forecast calls for
steady strengthening during this time, with One-C becoming Tropical
Storm Iona on Sunday. Beyond 60 hours, the cyclone will begin to
experience increasing westerly vertical wind shear while also
entraining drier mid-level air. This is expected to lead to steady
weakening, with the system becoming a tropical depression again
around day 5. The intensity forecast is best aligned with the HCCA
intensity consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 27/0900Z 10.9N 145.6W 30 KT 35 MPH
12H 27/1800Z 10.9N 146.7W 35 KT 40 MPH
24H 28/0600Z 11.2N 148.2W 40 KT 45 MPH
36H 28/1800Z 11.5N 150.0W 50 KT 60 MPH
48H 29/0600Z 11.5N 152.2W 50 KT 60 MPH
60H 29/1800Z 11.5N 154.8W 45 KT 50 MPH
72H 30/0600Z 11.7N 157.7W 40 KT 45 MPH
96H 31/0600Z 12.2N 164.7W 35 KT 40 MPH
120H 01/0600Z 12.9N 171.6W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Jelsema (CPHC)

[Hurricane2022]

The 06Z runs of HWRF, HMON, HAFS-A and HAFS-B are in agreement in showing 01C rapidly intensifying into a weak/moderate hurricane before shear begins in 50 - 60 hours. HAFS-B is the most bullish model there: it shows 01C becoming a high-end C2 before rapid weakening.

[cycloneye]

Tropical Depression One-C Discussion Number 2
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
500 AM HST Sun Jul 27 2025

There has not been much change with the depression over the past
several hours. Deep convection has been persisting in some curved
bands near and generally to the west of the center. Earlier
microwave images suggest that the circulation is quite tilted in the
vertical, with the low-level center located to the southwest of the
mid-level vortex. All of the satellite intensity estimates are
around 30 kt, and therefore, the initial wind speed is held at that
value.

The system is moving nearly due westward at about 11 kt. A
continued westward motion is expected through the week as the
cyclone moves on the south side of a narrow mid-level ridge.
Although there is some spread in the guidance, there is high
confidence that the system should stay well south of the Hawaiian
Islands. The NHC track forecast leans toward the southern side of
the guidance envelope, in best agreement with the HCCA consensus
aid. This forecast is quite close to the previous one.

The environmental conditions favor slow strengthening during the
next couple of days, and therefore, the NHC forecast follows the
previous one and shows strengthening to a mid-grade tropical storm
during that time period. However, this intensity forecast is a
little lower than the HCCA and IVCN models since it might take some
time for the vortex to align vertically and organize. Weakening
should begin beyond a couple of days when the system moves over
slightly cooler waters and into an environment of drier air and
moderate to strong shear. The cyclone is forecast to become a
remnant low by day 5, but it could open into a trough by then.


FORECAST POSITIONS AND MAX WINDS

INIT 27/1500Z 11.0N 146.6W 30 KT 35 MPH
12H 28/0000Z 11.1N 147.8W 35 KT 40 MPH
24H 28/1200Z 11.2N 149.3W 45 KT 50 MPH
36H 29/0000Z 11.4N 151.4W 50 KT 60 MPH
48H 29/1200Z 11.6N 153.9W 50 KT 60 MPH
60H 30/0000Z 11.7N 156.7W 45 KT 50 MPH
72H 30/1200Z 11.8N 159.8W 40 KT 45 MPH
96H 31/1200Z 12.2N 167.0W 35 KT 40 MPH
120H 01/1200Z 13.2N 174.2W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Cangialosi

[Hurricane2022]

12z...HAFS-A shows a low-end C2 peak for 01, while HAFS-B is now up to a C3 peak.

[Hurricane2022]

Looks like we're seeing a RI'ing storm now.

[Sciencerocks]

Almost certainly a tropical storm with winds around 40 knts in my opinion. Nice banding.

[cycloneye]

They have not upgraded yet.

CP, 01, 2025072718, , BEST, 0, 110N, 1470W, 30, 1008, TD

[cycloneye]

Hurricane2022, Sciencerocks, they are waiting for a scatterometer.

Tropical Depression One-C Discussion Number 3
NWS Central Pacific Hurricane Center Honolulu HI CP012025
Issued by NWS National Hurricane Center Miami FL
1100 AM HST Sun Jul 27 2025

The system continues to become better organized this morning, with
deep convection in curved banding generally to the west of the
center. Recent imagery shows that convection has been developing the
last few hours near the low-level center, with some increased
curvature. Subjective Dvorak satellite intensity estimates from HFO
and SAB were T/2.0 this cycle. A scatterometer pass is expected
shortly which should give a better idea of the surface winds. Using
the Dvorak estimates, the intensity is held at 30 kt.

The system is moving westward at about 270/10 kt. A westward motion
is expected the next several days as the cyclone moves on the south
side of a mid-level ridge. There continues to be some spread in the
along track guidance, although there is high confidence on the
system remaining well south of the Hawaiian islands. The NHC track
forecast is similar to the previous, and lies closest to the HCCA
corrected consensus aids.

Environmental conditions appear favorable for some gradual
strengthening the next couple of days. The hurricane regional models
remain at the higher end of the guidance envelope, and the latest
NHC forecast is near the previous. Beyond 48 h, a gradual weakening
trend should begin as the system moves over slightly cooler sea
surface temperatures, a drier airmass, and vertical wind shear is
expected to increase. The gradual weakening trend will cause the
system to become a remnant low by day 5, potentially even opening
into a trough.


FORECAST POSITIONS AND MAX WINDS

INIT 27/2100Z 11.0N 147.4W 30 KT 35 MPH
12H 28/0600Z 11.1N 148.6W 35 KT 40 MPH
24H 28/1800Z 11.2N 150.4W 45 KT 50 MPH
36H 29/0600Z 11.2N 152.7W 50 KT 60 MPH
48H 29/1800Z 11.2N 155.3W 50 KT 60 MPH
60H 30/0600Z 11.5N 158.2W 45 KT 50 MPH
72H 30/1800Z 11.8N 161.6W 40 KT 45 MPH
96H 31/1800Z 12.5N 168.8W 35 KT 40 MPH
120H 01/1800Z 13.9N 176.0W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Kelly

[StormWeather]

Why is the NHC issuing advisories on a CPAC storm? Isn’t that the CPHC’s responsibility?