SIO: BIANCA - Post-Tropical

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99S INVEST 250217 0600 11.6S 129.6E SHEM 15 1009

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THE AREA OF CONVECTION (INVEST 99S) PREVIOUSLY LOCATED NEAR
11.4S 127.4E IS NOW LOCATED NEAR 13.3S 121.2E, APPROXIMATELY 285 NM
NORTH-NORTHEAST OF ROWLEY SHOALS ATOLL. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) DEPICTS A SLOWLY CONSOLIDATING LOW-LEVEL CIRCULATION
(LLC) AS LOW-LEVEL CLOUD LINES HAVE STARTED TO WRAP BENEATH A
SUPPORTIVE CONVECTIVE STRUCTURE NEAR THE ASSESSED CENTER. THE
ENVIRONMENT IS FAVORABLE FOR DEVELOPMENT WITH LOW VERTICAL WIND SHEAR
(VWS) OF 10-15 KNOTS, MODERATE EQUATORWARD UPPER-LEVEL OUTFLOW, AND
WARM SEA SURFACE TEMPERATURES (SST) OF 29-30 C. DETERMINISTIC AND
ENSEMBLES ARE IN AGREEMENT THAT INVEST 99S WILL CONTINUE TO TRACK
SOUTHWESTWARD TOWARD ROWLEY SHOALS ATOLL AS IT SLOWLY INTENSIFIES OVER
THE NEXT 24 TO 48 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT
23 TO 28 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1005
MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE
WITHIN THE NEXT 24 HOURS IS UPGRADED TO MEDIUM.

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IDW24000
Australian Bureau of Meteorology
Tropical Cyclone Warning Centre

TROPICAL CYCLONE INFORMATION BULLETIN
Issued at 8:44 pm WST on Sunday 23 February 2025

Tropical Cyclone Bianca (Category 1) was located at 8:00 pm AWST near 17.2S
105.8E, that is 1020 km west northwest of Exmouth and 750 km south of Christmas
Island and moving west southwest at 25 kilometres per hour.

Tropical Cyclone Bianca (21U) is located in the Indian Ocean well to the west
northwest of Exmouth and is moving towards the west southwest. It is expected
to strengthen during Monday and Tuesday, and is forecast to move to the south
southwest, remaining well offshore from the Western Australian coast. On
Wednesday it is expected to start weakening.

No direct impacts are expected at any Australian mainland or Island communities.

The next Information Bulletin will be issued at 3:00 am AWST Monday 24 February.

[cycloneye]

Issued by AUSTRALIAN BUREAU OF METEOROLOGY TROPICAL CYCLONE WARNING CENTRE
at: 1857 UTC 23/02/2025
Name: Tropical Cyclone Bianca
Identifier: 21U
Data At: 1800 UTC
Latitude: 17.5S
Longitude: 105.3E
Location Accuracy: within 20nm (35 km)
Movement Towards: west southwest (238 deg)
Speed of Movement: 10 knots (18 km/h)
Maximum 10-Minute Wind: 40 knots (75 km/h)
Maximum 3-Second Wind Gust: 55 knots (100 km/h)
Central Pressure: 997 hPa
Radius of 34-knot winds NE quadrant: 50 nm (95 km)
Radius of 34-knot winds SE quadrant: 70 nm (130 km)
Radius of 34-knot winds SW quadrant: 60 nm (110 km)
Radius of 34-knot winds NW quadrant: 60 nm (110 km)
Radius of 48-knot winds NE quadrant:
Radius of 48-knot winds SE quadrant:
Radius of 48-knot winds SW quadrant:
Radius of 48-knot winds NW quadrant:
Radius of 64-knot winds: nm ( km)
Radius of Maximum Winds: 20 nm (35 km)
Dvorak Intensity Code: T3.0/3.0/D0.5/24HRS STT:S0.0/06HRS
Pressure of outermost isobar: 1008 hPa
Radius of outermost closed isobar: 120 nm (220 km)
FORECAST DATA
Date/Time : Location : Loc. Accuracy: Max Wind : Central Pressure
(UTC) : degrees : nm (km): knots(km/h): hPa
+06: 24/0000: 18.2S 104.7E: 030 (060): 045 (085): 994
+12: 24/0600: 18.9S 104.1E: 040 (075): 050 (095): 990
+18: 24/1200: 19.7S 103.6E: 050 (090): 055 (100): 986
+24: 24/1800: 20.5S 103.2E: 055 (100): 055 (100): 986
+36: 25/0600: 22.1S 102.7E: 070 (130): 060 (110): 981
+48: 25/1800: 23.8S 102.8E: 085 (155): 060 (110): 980
+60: 26/0600: 25.5S 103.4E: 090 (170): 045 (085): 991
+72: 26/1800: 26.3S 104.1E: 110 (205): 030 (055): 999
+96: 27/1800: : : :
+120: 28/1800: : : :
REMARKS:
Tropical Cyclone Bianca (21U) lies well to the northwest of the Western
Australian coast.

The system location was determined using animated satellite imagery, as well as
an earlier SAR pass. There is only moderate confidence position due to a lack
of good microwave or scatterometry passes in recent hours.

Dvorak analysis: DT is 3.5 based on a curved band of up to 0.8. MET is 3.0
based on a D- trend with no PAT adjustment. FT/CI set to 3.0. Objective aids
(all 1 minute means and at 1800 UTC unless otherwise stated): ADT 35 knots,
AiDT 34 knots, DPRINT 46 knots, DMINT 38 knots (1138 UTC). Development appears
to have stalled in the past 6 hours. Intensity set at 40 knots, which is
consistent with a SAR pass at 1119 UTC.

Vertical wind shear remains low, and is currently estimated at 10 knots. Upper
divergence is strong, with a good outflow channel to the south. Although there
is dry air around the system, it doesn't appear to be significantly impacting
the core. SSTs are currently around 28C.

The expectation is that the environment will continue to be favourable for
Bianca until Tuesday evening, when vertical shear will increase with an
approaching upper trough, and dry air likely to intrude. After this time,
Bianca should rapidly weaken. However, it is worth noting that as Bianca is a
small system, it may weaken earlier if there is any deterioration in
conditions.

There is high confidence that Bianca will be steered by the mid-level high to
the southeast, which means southwest motion today, tending more southerly on
Tuesday. A mid-level trough will steer Bianca towards the southeast on
Wednesday, and then when it weakens the remnant low level centre will likely be
steered back to the west.

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TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION
Issued by AUSTRALIAN BUREAU OF METEOROLOGY TROPICAL CYCLONE WARNING CENTRE
at: 0711 UTC 24/02/2025
Name: Tropical Cyclone Bianca
Identifier: 21U
Data At: 0600 UTC
Latitude: 19.1S
Longitude: 103.8E
Location Accuracy: within 20nm (35 km)
Movement Towards: southwest (222 deg)
Speed of Movement: 11 knots (20 km/h)
Maximum 10-Minute Wind: 55 knots (100 km/h)
Maximum 3-Second Wind Gust: 75 knots (140 km/h)
Central Pressure: 989 hPa
Radius of 34-knot winds NE quadrant: 60 nm (110 km)
Radius of 34-knot winds SE quadrant: 70 nm (130 km)
Radius of 34-knot winds SW quadrant: 60 nm (110 km)
Radius of 34-knot winds NW quadrant: 60 nm (110 km)
Radius of 48-knot winds NE quadrant: 30 nm (55 km)
Radius of 48-knot winds SE quadrant: 30 nm (55 km)
Radius of 48-knot winds SW quadrant: 30 nm (55 km)
Radius of 48-knot winds NW quadrant: 30 nm (55 km)
Radius of 64-knot winds: nm ( km)
Radius of Maximum Winds: 20 nm (35 km)
Dvorak Intensity Code: T4.0/4.0/D1.5/24HRS STT:D0.5/06HRS
Pressure of outermost isobar: 1010 hPa
Radius of outermost closed isobar: 140 nm (260 km)
FORECAST DATA
Date/Time : Location : Loc. Accuracy: Max Wind : Central Pressure
(UTC) : degrees : nm (km): knots(km/h): hPa
+06: 24/1200: 19.8S 103.3E: 030 (060): 060 (110): 984
+12: 24/1800: 20.4S 102.9E: 040 (075): 065 (120): 979
+18: 25/0000: 21.2S 102.5E: 050 (090): 070 (130): 975
+24: 25/0600: 22.1S 102.2E: 055 (100): 065 (120): 979
+36: 25/1800: 23.8S 102.1E: 070 (125): 055 (100): 986
+48: 26/0600: 25.5S 102.5E: 080 (150): 045 (085): 993
+60: 26/1800: 26.1S 103.3E: 095 (170): 030 (055): 1001
+72: 27/0600: 26.1S 102.5E: 110 (200): 025 (045): 1004
+96: 28/0600: : : :
+120: 01/0600: : : :
REMARKS:
Tropical Cyclone Bianca (21U) continues to intensify.

The system location has been determined with good confidence using animated
visible imagery which shows a developing eye.

Dvorak analysis: DT is 4.0, based on a curved band increasing to 1.1. MET is
3.5, based on a D trend, with a PAT adjustment to 4.0. FT/CI is set to 4.0.
Objective aids (all 1 minute means and at 0530 UTC unless otherwise stated):
ADT 55 knots, AiDT 53 knots, DPRINT 63 knots, SATCON 66 knots (0510 UTC). Deep
convection continues to tighten around the centre of Bianca, and an eye is
developing. Intensity is set at 55 knots, consistent with Dvorak and objective
guidance estimates.

Vertical wind shear remains low, currently estimated at 10 knots or less.
Upper-level divergence is strong, with good outflow to the north and south.
While dry air is present around the system, it does not appear to be
significantly impacting the core. Sea surface temperatures (SSTs) are around
27C.

The environment is expected to remain favourable until Tuesday afternoon, with
Bianca likely to intensify into a severe tropical cyclone. However, from
Tuesday evening onwards, cooler waters, increasing vertical shear from an
approaching upper trough, and dry air intrusion will likely weaken the system.
Given Bianca's small size, it may weaken earlier if environmental conditions
deteriorate sooner than expected.

There is high confidence that Bianca will track south-southwest on Tuesday,
steered by a mid-level high to the southeast. A mid-level trough will then
steer Bianca towards the southeast on Wednesday, and as it weakens, the remnant
low-level centre will likely be steered back to the west.

[Subtrop]

IDW24000
Australian Bureau of Meteorology
Tropical Cyclone Warning Centre

TROPICAL CYCLONE INFORMATION BULLETIN
Issued at 8:48 pm WST on Monday 24 February 2025

Severe Tropical Cyclone Bianca (Category 3) was located at 8:00 pm AWST near
19.7S 103.1E, that is 1170 km west northwest of Exmouth and 1230 km west
northwest of Carnarvon and moving southwest at 18 kilometres per hour.

Severe Tropical Cyclone Bianca (21U) is located in the Indian Ocean, well to
the west-northwest of Exmouth, and is moving towards the southwest. Bianca has
intensified to a category 3 system and is expected to track south-southwest,
remaining well offshore from the Western Australian coast. During Tuesday
conditions become more unfavourable and Bianca should begin to weaken and then
decrease to be below tropical cyclone strength during Wednesday, while still
remaining well west of the state.

No direct impacts are expected at any Australian mainland or Island communities.

The next Information Bulletin will be issued at 3:00 am AWST Tuesday 25
February.

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TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION
Issued by AUSTRALIAN BUREAU OF METEOROLOGY TROPICAL CYCLONE WARNING CENTRE
at: 0730 UTC 25/02/2025
Name: Severe Tropical Cyclone Bianca
Identifier: 21U
Data At: 0600 UTC
Latitude: 21.9S
Longitude: 102.1E
Location Accuracy: within 20nm (35 km)
Movement Towards: south southwest (202 deg)
Speed of Movement: 10 knots (18 km/h)
Maximum 10-Minute Wind: 85 knots (155 km/h)
Maximum 3-Second Wind Gust: 110 knots (205 km/h)
Central Pressure: 967 hPa
Radius of 34-knot winds NE quadrant: 50 nm (95 km)
Radius of 34-knot winds SE quadrant: 50 nm (95 km)
Radius of 34-knot winds SW quadrant: 50 nm (95 km)
Radius of 34-knot winds NW quadrant: 40 nm (75 km)
Radius of 48-knot winds NE quadrant: 25 nm (45 km)
Radius of 48-knot winds SE quadrant: 25 nm (45 km)
Radius of 48-knot winds SW quadrant: 25 nm (45 km)
Radius of 48-knot winds NW quadrant: 25 nm (45 km)
Radius of 64-knot winds: 15 nm (30 km)
Radius of Maximum Winds: 10 nm (20 km)
Dvorak Intensity Code: T5.0/5.5/D1.0/24HRS STT:W0.6/06HRS
Pressure of outermost isobar: 1010 hPa
Radius of outermost closed isobar: 110 nm (205 km)
FORECAST DATA
Date/Time : Location : Loc. Accuracy: Max Wind : Central Pressure
(UTC) : degrees : nm (km): knots(km/h): hPa
+06: 25/1200: 22.8S 102.0E: 030 (060): 070 (130): 974
+12: 25/1800: 23.6S 102.0E: 040 (075): 060 (110): 982
+18: 26/0000: 24.5S 102.2E: 050 (090): 050 (095): 989
+24: 26/0600: 25.3S 102.4E: 055 (100): 040 (075): 996
+36: 26/1800: 26.0S 102.7E: 070 (135): 030 (055): 1001
+48: 27/0600: 25.8S 101.8E: 085 (160): 025 (045): 1005
+60: 27/1800: : : :
+72: 28/0600: : : :
+96: 01/0600: : : :
+120: 02/0600: : : :
REMARKS:
Severe Tropical Cyclone Bianca (21U) is beginning to weaken.

The eye is becoming less defined and has appeared irregular at times over the
past few hours. Confidence in the system's centre location remains moderate.
The vertical structure of Bianca is under increasing pressure, showing signs of
tilting due to strengthening wind shear, as indicated by a recent microwave
pass. Cloud tops are warming, and dry air is beginning to intrude into the
system.

Dvorak analysis places the data T-number at around 5.0, primarily based on a
WMG eye surrounded by LG bands. However, the eye has recently become irregular.
The weakening trend results in a MET of 5.0, with a PAT adjustment to 4.5. The
final T-number and CI are set at 5.0/5.5, based on the DT. Objective intensity
estimates (all 1-minute means and at 0600 UTC unless otherwise stated) include
ADT at 109 knots, AiDT at 106 knots, DPRINT at 94 knots, and SATCON at 104
knots (0500 UTC). The intensity is set at 85 knots, consistent with subjective
Dvorak analysis and slightly biased higher to align with objective guidance.

Vertical wind shear has increased to around 15-20 knots. Upper-level divergence
remains strong to the poleward outflow, while sea surface temperatures are
around 27C. Bianca is moving into an increasingly unfavourable environment.
Cooler waters, strengthening vertical wind shear from an approaching upper
trough, and ongoing dry air intrusion are expected to drive a rapid weakening
trend. Given its small size, Bianca will be particularly vulnerable to these
deteriorating conditions.

An approaching mid-level trough will influence Bianca's movement, steering it
south or southeast through the remainder of Tuesday and into Wednesday. As
Bianca weakens, the remnant low-level centre is expected to drift back westward
during Thursday.

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REMARKS:
Severe Tropical Cyclone Bianca (21U) is weakening over the ocean.

Bianca has been located using animated visible satellite imagery and recent
ASCAT passes with with moderate confidence. The low level centre has started to
become elongated.

Dvorak analysis using a shear pattern (<45nm from tight gradient) yields a DT
of 2.5. The recent trend is W+ which gives a MET of 3.5, with PAT at 3.0.
FT=3.0, based on PAT. CI set at 4.0 due to constraints. Objective intensity
estimates (all 1-minute means and at 0530 UTC unless otherwise stated) include
ADT at 61 knots, AiDT at 56 knots, DPRINT at 39 knots. Raw T numbers in the ADT
method continue to fall and show 1.5 but FT is held up at 3.3 by weakening flag
being used. ASCAT-B pass at 0232 UTC shows 50 kn in the north east and
southwest quadrants. Intensity set at 50 kn based on dvorak and scat passes.

Vertical wind shear has increased to at least 35 knots from the west and dry
air is apparent to the west and north of the system. Upper-level divergence has
weakened to the south but some poleward outflow is still evident in imagery.
Sea surface temperatures have decreased to around 27C. Bianca is weakening
under this increasingly unfavourable environment. Cooler waters, strengthening
vertical wind shear from an approaching upper trough, and ongoing dry air
intrusion are expected to drive a rapid weakening trend over the next 6 to 12
hours. Given its small size, Bianca will be continue to weaken quickly due to
these deteriorating conditions.

As Bianca weakens, it will loose the steering influence of the approaching
mid-level trough. The system, while still move to the southeast recently, is
starting to slow down and as it weakens further on Thursday, it will drift to
the west under the influence of a lower level ridge .

[cycloneye]

Bye.

TROPICAL CYCLONE INFORMATION BULLETIN
Issued at 2:22 am WST on Thursday 27 February 2025

Ex-Tropical Cyclone Bianca was located at 2:00 am AWST near 26.6S 104.2E, that
is 970 km west southwest of Carnarvon and 1130 km west southwest of Exmouth and
moving east southeast at 8 kilometres per hour.

Ex-Tropical Cyclone Bianca (21U) has weakened below tropical cyclone intensity,
well to the west of Carnarvon. It will continue to weaken and is forecast to
dissipate later today (Thursday).

No direct impacts are expected at any Australian mainland or Island communities.

No further Information Bulletins will be issued.