WPAC: KROSA - Post-Tropical - Discussion

[cycloneye]

URGENT - MARINE WEATHER MESSAGE
National Weather Service Tiyan GU
749 AM ChST Thu Jul 24 2025

PMZ152>154-241200-
/O.NEW.PGUM.TR.A.4012.250723T2149Z-000000T0000Z/
/O.CON.PGUM.SC.Y.0040.000000T0000Z-250726T2000Z/
Rota Coastal Waters-Tinian Coastal Waters-Saipan Coastal Waters-
749 AM ChST Thu Jul 24 2025

...TROPICAL STORM WATCH IN EFFECT...
...SMALL CRAFT ADVISORY REMAINS IN EFFECT UNTIL 6 AM CHST SUNDAY...

* WHAT...For the Small Craft Advisory, southwest winds 20 to 30 kt
with gusts up to 40 kt and seas 8 to 12 feet. For the Tropical
Storm Watch, south winds 20 to 30 kt with gusts up to 40 kt and
seas 8 to 12 feet, with higher values possible depending on the
track and development of Tropical Depression 12W.

* WHERE...Coastal Waters of Rota, Tinian and Saipan.

* WHEN...For the Small Craft Advisory, until 6 AM ChST Sunday.
For the Tropical Storm Watch, until further notice.

* IMPACTS...Very strong winds will cause hazardous seas which
could capsize or damage vessels and reduce visibility.

* ADDITIONAL DETAILS...The center of Tropical Depression 12W is
already west of the Marianas and is expected to remain so. This
means that as it consolidates the higher winds will be west of
the islands. Therefore, the watches are primarily for the waters
west of the islands. The Small Craft Advisory is for the entire
coastal waters.

[cycloneye]

JMA now at TD.

https://www.jma.go.jp/bosai/map.html#5/ ... on&lang=en

[StormWeather]

Should this get named, the next name I believe is Krosa

[cycloneye]

[Hayabusa]

T2509(Krosa)
Issued at 2025/07/24 07:20 UTC
Analysis at 07/24 06 UTC
Grade TS
Scale -
Intensity -
Center position N14°30′ (14.5°)
E143°40′ (143.7°)
Direction and speed of movement NNW Slow
Central pressure 1002 hPa
Maximum wind speed 18 m/s (35 kt)
Maximum wind gust speed 25 m/s (50 kt)
Radius of 30-kt wind area S280 km (150 NM)
N165 km (90 NM)

[cycloneye]

FORECAST DISCUSSION: TD 12W IS EXPECTED TO CONTINUE ON A GENERALLY
NORTHWARD TRACK THROUGHOUT THE 120 HOUR FORECAST PERIOD. IN THE
SHORT-TERM, A BUILDING SUBTROPICAL RIDGE TO THE EAST WILL INITIATE
SLIGHT VARIATIONS IN THE AFOREMENTIONED TRACK, WITH A
NORTH-NORTHEASTWARD MOVEMENT BETWEEN TAU 0 AND TAU 48. FOLLOWING
TAU 48, TD 12W WILL APPROACH THE WEAKER NORTHWESTERN EXTENT OF THE
STEERING RIDGE AND BEGIN TRACKING NORTH-NORTHWESTWARD UNTIL TAU
120. AT THE SAME TIME, A BUILDING RIDGE EXTENDING FROM THE
NORTHWEST PACIFIC INTO THE SEA OF JAPAN WILL GRADUALLY SLOW
NORTHWARD MOVEMENT BY TAU 120. WITH REGARD TO INTENSITY, TD 12W IS
EXPECTED TO STEADILY INTENSIFY OVER THE NEXT 72 HOURS DUE TO
UPPER-LEVEL SUPPORT FROM AN EASTWARD MOVING SHORTWAVE TROUGH,
PROVIDING AN ORGANIZED UPPER-LEVEL OUTFLOW MECHANISM WITHIN A
LARGELY MARGINAL TO FAVORABLE ENVIRONMENT. BETWEEN TAU 60 AND TAU
72, AN AREA OF CYCLONIC TURNING ALOFT WILL BEGIN INTRODUCING
ADDITIONAL DRY-AIR AROUND THE CENTRAL VORTEX, LIMITING FURTHER TC
DEVELOPMENT. DURING THE LENGTH OF THE FORECAST AFTER TAU 72, A
SOUTHWEST TO NORTHEAST ORIENTED SWATH OF ENHANCED WINDS IN CLOSE
PROXIMITY TO TD 12W WILL AID IN A SLOW AND STEADY WEAKENING PHASE
UNTIL THE END OF THE FORECAST PERIOD.

[cycloneye]

FORECAST DISCUSSION: TS 12W IS FORECAST TO GENERALLY
NORTH-NORTHEASTWARD, ALONG THE WESTERN PERIPHERY OF A BUILDING
SUBTROPICAL RIDGE TO THE EAST THROUGH TAU 72. AFTER TAU 72, TRACK
SPEEDS ARE EXPECTED TO SLOW AS THE INFLUENCE FROM THE SUBTROPICAL
RIDGE WEAKENS. AROUND TAU 96, A BELT OF ENHANCED SOUTHWESTERLY
WINDS WILL BEGIN INTERACTING WITH THE VORTEX AND WILL PULL THE
VORTEX NORTHEASTWARD THROUGH THE REMAINDER OF THE FORECAST PERIOD.
THERE IS SOME UNCERTAINTY REGARDING THE EXACT TIMING OF THE
NORTHEASTWARD TURN, AS IT DEPENDS ON THE ONSET OF THE ENHANCED
SOUTHWESTERLY FLOW. THE SYSTEM COULD POTENTIALLY DRIFT
NORTHWESTWARD BEFORE MAKING THE NORTHEASTWARD TURN AS A SUBTROPICAL
RIDGE BUILDS IN OVER SOUTHERN JAPAN AND INTERACTS WITH THE VORTEX.
ALL ECMWF AND GEFS ENSEMBLE MEMBERS DO KEEP THE SYSTEM FROM
TRACKING INTO HONSHU THOUGH. REGARDING INTENSITY, 12W IS FORECAST
TO INTENSIFY OVER THE NEXT 48 HOURS AS THE SYSTEM CONTINUES TO
CONSOLIDATE. NORTHERLY VERTICAL WIND SHEAR IS EXPECTED TO REMAIN
NEAR 30 KTS, HOWEVER, INTERACTION WITH AN
UPPER-LEVEL SHORTWAVE TROUGH WILL DRASTICALLY ENHANCE THE EASTWARD
OUTFLOW THROUGH TAU 72. CURRENTLY, A PEAK INTENSITY OF 65 KTS IS
FORECAST TO OCCUR AT TAU 48 AND PERSIST THROUGH TAU 72. NEAR
TAU 72, AN UPPER-LEVEL CYCLONE IS EXPECTED TO BE PLACED OVER THE
VORTEX, WHICH WILL CAUSE INCREASED DRY AIR ENTRAINMENT AND
SIGNIFICANTLY DECREASED OUTFLOW ALOFT. AS A RESULT, 12W IS FORECAST
TO STOP INTENSIFYING AND WEAKEN UNDER THE INFLUENCE OF THE
UPPER-LEVEL CYCLONE.

[cycloneye]

Getting better organized.

[cycloneye]

FORECAST DISCUSSION: TS 12W IS FORECASTED TO TRACK GENERALLY TO
THE NORTH-NORTHEAST THROUGHOUT THE ENTIRETY OF THE 120 HOURS
FORECAST. AS THE SUBTROPICAL STEERING RIDGE CONTINUES TO BUILD JUST
TO THE EAST, THE SYSTEM WILL MOVE ANTICYCLONICALLY AROUND THE
WESTERN PERIPHERY OF THE DEEP-LAYER RIDGE INTO TAU 120. BETWEEN TAU
72 AND TAU 96, AN ADDITIONAL LOW-LEVEL BELT OF ENHANCED
SOUTHWESTERLY WINDS EXTENDING FROM THE SOUTHWEST TO THE NORTHEAST
JUST SOUTH OF TS KROSAS FORECASTED POSITION WILL INTERACT WITH THE
CYCLONE, AIDING IN THE EXPECTED NORTHEAST TRACK BETWEEN TAU 96 AND
TAU 120. WITH REGARDS TO INTENSITY, 12W IS FORECASTED TO CONTINUE
STEADILY INTENSIFYING TO 65 KTS THROUGHOUT THE NEXT 48 HOURS WITH
ASSISTANCE FROM AN UPPER-LEVEL SHORTWAVE TROUGH, PROVIDING AN
ORGANIZED MECHANISM FOR OUTFLOW ALOFT. STARTING AT TAU 72, TS 12W
IS EXPECTED TO BEGIN SLOWLY WEAKENING TO 60 KTS INTO THE END OF THE
120 FORECAST PERIOD AS DRY AIR ENTRAINS INTO THE CENTRAL VORTEX DUE
TO INFLUENCES FROM AN UPPER-LEVEL CYCLONE. AT THE SAME TIME, VWS IS
FORECASTED TO SHARPLY DECREASE TO LESS THAN 20 KTS, ALLOWING THE
SYSTEM TO SUSTAIN 60 KTS FROM TAU 72 TO TAU 120.

[Hayabusa]

It looks like the rainy weather over the Philippines is done but the monsoon gyre that is developing northeast of Taiwan will be interesting to watch and how it would affect the monsoon over the western Philippines and its possible interaction with Krosa...

[cycloneye]

FORECAST DISCUSSION: TS 12W IS FORECASTED TO TRACK TO THE NORTH AND
NORTH NORTH-EAST OVER THE INITIAL 72 HOURS OF THE FORECAST PERIOD
AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE LOCATED
DIRECTLY TO THE EAST. AFTER TAU 72, THE WEAKENING RIDGE WILL BEGIN
ALLOWING THE SYSTEM TO BEGIN ITS GENERALLY NORTHEASTWARD TRACK
UNTIL THE END OF THE FORECAST PERIOD. DURING THAT SAME TIME, A
SOUTHWEST TO NORTHEAST ORIENTED BELT OF ENHANCED SOUTHWESTERLY
WINDS WILL AID IN THE INITIAL APPROACH NORTHEASTWARD UNTIL TAU 120.
WITH REGARD TO INTENSITY, TS 12W IS FORECASTED TO STEADILY
INTENSIFY TO 60 KTS OVER THE NEXT 48 HOURS AS VWS DECREASES TO LESS
THAN 20 KTS. AFTER TAU 48, A SLIGHT WEAKENING PHASE TO 55 KTS IS
EXPECTED AS DRY AIR WRAPS CYCLONICALLY AROUND THE CENTRAL VORTEX
FROM THE WEST UNTIL TAU 96. FOLLOWING TAU 96, TS KROSA IS FORECAST
TO RE-MOISTEN WHILE ALSO GAINING A VIGOROUS POLEWARD OUTFLOW
CHANNEL ALOFT TO THE NORTHEAST, ALLOWING FOR REINTENSIFICATION TO
65 KTS BY TAU 120. WHILE THIS IS HAPPENING, VWS IS EXPECTED TO
SHARPLY DECLINE TO BELOW 10 KTS, AS THE SYSTEM BECOMES COMPLETELY
VERTICALLY ALIGNED.

[cycloneye]

FORECAST DISCUSSION: TROPICAL STORM (TS) 12W IS FORECAST TO TRACK
NORTHWARD THROUGH TAU 72 ALONG THE WESTERN PERIPHERY OF THE STR.
AFTER TAU 72, THE SYSTEM WILL TURN NORTHEASTWARD AS THE STR
REORIENTS. THIS IS NOT A RECURVATURE SCENARIO WITH EXTRA-TROPICAL
TRANSITION. THE SYSTEM IS NOT EXPECTED TO ACCELERATE AND WILL NOT
ENCOUNTER THE MIDLATITUDE WESTERLIES UNTIL AFTER THE FORECAST
PERIOD. INSTEAD, THE SYSTEM WILL TRACK ALONG THE NORTHWESTERN
PERIPHERY OF THE STR WITHIN A REVERSE TROUGH FORMATION, WITH THE
SURFACE TROUGH EXTENDING FROM TAIWAN NORTHEASTWARD TO THE SYSTEM.
ENVIRONMENTAL CONDITIONS ARE MARGINALLY FAVORABLE, WITH THE HIGH
VWS OFFSET BY VIGOROUS EQUATORWARD OUTFLOW, WHICH IS FUELING THE
RAPIDLY CYCLING DEEP CONVECTIVE BURSTS. THEREFORE, THE INTENSITY IS
EXPECTED TO REMAIN RELATIVELY FLAT THROUGH TAU 72. AFTER TAU 48,
THE VWS SHOULD RELAX, WITH IMPROVED EASTWARD OUTFLOW. HOWEVER, THE
SYSTEM IS EXPECTED TO ENTRAIN DRY AIR ASSOCIATED WITH THE DEEP
MIDLATITUDE TROUGH. AFTER TAU 72, ENVIRONMENTAL CONDITIONS WILL
IMPROVE, WITH A RE-MOISTENING PHASE, ENHANCED POLEWARD OUTFLOW AND
INTENSIFICATION TO TYPHOON STRENGTH BY TAU 120.

[cycloneye]

[cycloneye]

JMA upgrades to STS.

T2509(Krosa)
Issued at 2025/07/26 01:00 UTC
Analysis at 07/26 00 UTC
Grade STS
Scale -
Intensity -
Center position N17°40′ (17.7°)
E143°30′ (143.5°)
Direction and speed of movement Almost stationary
Central pressure 990 hPa
Maximum sustained wind speed near center 25 m/s (50 kt)
Maximum wind gust speed 35 m/s (70 kt)
Radius of 30-kt wind area 440 km (240 NM)

[cycloneye]

Looks like it will be around for many days adding some needed ACE for the basin.

[cycloneye]

FORECAST DISCUSSION: TROPICAL STORM (TS) 12W IS FORECAST TO TRACK
NORTHWARD THROUGH TAU 72 ALONG THE WESTERN PERIPHERY OF THE STR.
AFTER TAU 72, THE SYSTEM WILL TURN NORTHEASTWARD AS THE STR REORIENTS.
THIS IS NOT A RECURVATURE SCENARIO WITH EXTRA-TROPICAL TRANSITION. THE
SYSTEM IS NOT EXPECTED TO ACCELERATE AND WILL NOT ENCOUNTER THE
MIDLATITUDE WESTERLIES UNTIL AFTER THE FORECAST PERIOD. INSTEAD, THE
SYSTEM WILL TRACK ALONG THE NORTHWESTERN PERIPHERY OF THE STR WITHIN A
REVERSE TROUGH FORMATION, WITH THE SURFACE TROUGH EXTENDING FROM TAIWAN
NORTHEASTWARD TO THE SYSTEM. ENVIRONMENTAL CONDITIONS ARE MARGINALLY
FAVORABLE, WITH THE HIGH VWS OFFSET BY VIGOROUS EQUATORWARD OUTFLOW,
WHICH IS FUELING THE RAPIDLY CYCLING DEEP CONVECTIVE BURSTS. THEREFORE,
THE INTENSITY IS EXPECTED TO REMAIN RELATIVELY FLAT THROUGH TAU 72.
AFTER TAU 48, THE VWS SHOULD RELAX, WITH IMPROVED EASTWARD OUTFLOW.
HOWEVER, THE SYSTEM IS EXPECTED TO ENTRAIN DRY AIR ASSOCIATED WITH THE
DEEP MIDLATITUDE TROUGH. AFTER TAU 72, ENVIRONMENTAL CONDITIONS WILL
IMPROVE, WITH A RE-MOISTENING PHASE, ENHANCED POLEWARD OUTFLOW AND
INTENSIFICATION TO TYPHOON STRENGTH BY TAU 120.

[cycloneye]

FORECAST DISCUSSION: TROPICAL STORM (TS) 12W IS FORECAST TO TRACK
NORTHWARD FOR THE NEXT 36 HOURS ON THE WESTERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE TO THE EAST. BETWEEN TAU 36-48, THE STEERING
ENVIRONMENT WILL BECOME COMPLEX BETWEEN THE NEAR EQUATORIAL RIDGE
TO THE EAST AND THE BUILDING SUBTROPICAL RIDGE TO THE NORTHWEST.
AFTER TAU 72, THE RIDGE TO THE SOUTHEAST WILL REORIENT ON AT
SOUTHWEST-NORTHEAST AXIS, DRIVING TS 12W NORTHEASTWARD FOR THE
REMAINDER OF THE FORECAST PERIOD. THE SYSTEM WILL REMAIN IN A
HIGHLY SHEARED ENVIRONMENT FOR THE FIRST 24 HOURS OF THE FORECAST
PERIOD, STAGNATING THE RATE OF INTENSIFICATION. THE SYSTEM WILL
SLOWLY INTENSIFY FOR THE REMAINDER OF THE FORECAST AS THE SHEAR
GRADUALLY DECREASES AND THE UPPER-LEVEL ENVIRONMENT REMAINS
STRONGLY DIVERGENT. ULTIMATELY, THE SYSTEM WILL PEAK AT 70KTS AT THE
END OF THE FORECAST PERIOD.

[Hayabusa]

Up to typhoon

T2509(Krosa)
Issued at 2025/07/26 18:50 UTC
Analysis at 07/26 18 UTC
Grade TY
Scale Large
Intensity -
Center position N19°30′ (19.5°)
E145°10′ (145.2°)
Direction and speed of movement NNE 20 km/h (10 kt)
Central pressure 970 hPa
Maximum sustained wind speed near center 35 m/s (70 kt)
Maximum wind gust speed 50 m/s (100 kt)
Radius of 50-kt wind area 130 km (70 NM)
Radius of 30-kt wind area SW650 km (350 NM)
NE440 km (240 NM)

While JTWC

12W KROSA 250726 1800 19.5N 145.0E WPAC 55 987

[cycloneye]

[cycloneye]

FORECAST DISCUSSION: TROPICAL STORM (TS) 12W IS FORECAST TO TRACK
NORTHWARD FOR THE NEXT 36 HOURS ON THE WESTERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE TO THE EAST. BETWEEN TAU 36-48, THE STEERING
ENVIRONMENT WILL BECOME COMPLEX BETWEEN THE NEAR EQUATORIAL RIDGE
TO THE EAST AND THE BUILDING SUBTROPICAL RIDGE TO THE NORTHWEST.
AFTER TAU 72, THE RIDGE TO THE SOUTHEAST WILL REORIENT ON AT
SOUTHWEST-NORTHEAST AXIS, DRIVING TS 12W NORTHEASTWARD FOR THE
REMAINDER OF THE FORECAST PERIOD. THE SYSTEM WILL REMAIN IN A
HIGHLY SHEARED ENVIRONMENT FOR THE FIRST 24 HOURS OF THE FORECAST
PERIOD, STAGNATING THE RATE OF INTENSIFICATION. THE SYSTEM WILL
SLOWLY INTENSIFY FOR THE REMAINDER OF THE FORECAST AS THE SHEAR
GRADUALLY DECREASES AND THE UPPER-LEVEL ENVIRONMENT REMAINS
STRONGLY DIVERGENT. ULTIMATELY, THE SYSTEM WILL PEAK AT 70KTS AT THE
END OF THE FORECAST PERIOD.