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TD02F / TC Yasa

13 December 2020 0900 hrs

A tropical depression has formed between Fiji and Vanuatu. This is expected to form a Tropical Cyclone over the next 24 hours.
Forecast track 202012121950UTC
 
The Cyclone will form in the Fiji Meteorological Service area of responsibility. FMS have the following Tropical Disturbance Advisory in place:
TROPICAL DISTURBANCE ADVISORY NUMBER B2 ISSUED FROM RSMC NADI Dec 121401 UTC. TROPICAL DEPRESSION TD02F CENTRE [998HPA] WAS LOCATED NEAR 12.6S 173.2E AT 121200 UTC. POSITION POOR BASED ON HIMAWARI-8 IR IMAGERY AND PERIPHERY SURFACE REPORTS. TD02F MOVING EAST AT ABOUT 12 KNOTS. MAXIMUM 10-MINUTE AVERAGE WINDS NEAR THE CENTRE ESTIMATED AT ABOUT 30 KNOTS. DEPRESSION INTENSIFYING. CONVECTION REMAINED PERSISTANT OVER LLCC. ORGANISATION HAS IMPROVED WITH PRIMARY BANDS TRYING TO WRAP ONTO LLCC. SYSTEM LIES UNDER AN UPPER RIDGE IN A LOW SHEARED ENVIRONMENT WITH GOOD OUTFLOW TO THE SOUTH AND NORTH. SST IS AROUND 30 DEGREES CELSIUS. DVORAK ANALYSIS BASED ON CURVED PATTERN, WITH 0.50 WRAP ON LOG 10 SPIRAL, YIELDING DT=2.5. MET AND PAT AGREE. FT BASED ON DT THUS, YIELDING T2.5/2.5/D1.0/24HRS
 
The following IR image shows where heat is in the atmosphere. It's been coloured so that blue shows sea level temp and red shows high clouds. TD02F is just above centre on this image, while TD01F (which has begun it's decay pattern and is unlikely to reach cyclone status) is to the right. 
 
HimwariIR202012121900UTC
 
Some models have the ex TC approaching NZ during the week before Christmas, although there remains some variability in this.

15 December 2020 2000 hrs

TC Yasi was formally named by the Fiji Meteorological Service on Monday 14th December. The current track map suggests intensification to a Cat 5 and a path over the main Fiji Islands.
Forecast track 202012142000UTC
 
HURRICANE WARNING 047 ISSUED FROM RSMC NADI Dec 150716 UTC. TROPICAL CYCLONE YASA CENTRE 941HPA CATEGORY 4 WAS LOCATED NEAR 15.0 SOUTH 173.0 EAST AT 150600 UTC. POSITION GOOD. REPEAT POSITION 15.0S 173.0E at 150600 UTC. CYCLONE SLOW MOVING. CYCLONE INTENSIFYING. EXPECT SUSTAINED WINDS OF 100 KNOTS CLOSE TO THE CENTRE INCREASING TO 105 KNOTS BY 151800 UTC.
 
The video below shows the development of the cyclone over the period Dec 13 0830 and Dec 15 2030:

16th December 2130 hrs 

TC Yasa is now a significant Cat 5 storm, bearing down on Fiji's main islands. It has also slowed which will add to the impact on Fiji as it passes over the island of Vanua Levu, tonight around 1am.
ZoomEarth20201216.2110NZDT
 
The Fiji Meteorological Service has published the following bulletin:
HURRICANE WARNING 055 ISSUED FROM RSMC NADI Dec 160639 UTC. TROPICAL CYCLONE YASA CENTRE 913HPA CATEGORY 5 WAS LOCATED NEAR 14.8 SOUTH 174.9 EAST AT 160600 UTC. POSITION GOOD. REPEAT POSITION 14.8S 174.9E at 160600 UTC. CYCLONE MOVING EAST SOUTHEAST AT 8 KNOTS. EXPECT SUSTAINED WINDS OF 125 KNOTS CLOSE TO THE CENTRE. EXPECT WINDS OVER 63 KNOTS WITHIN 45 NAUTICAL MILES IN NE QUADRANT AND WITHIN 30 NAUTICAL MILES IN SE QUADRANT AND WITHIN 30 NAUTICAL MILES IN SW QUADRANT AND WITHIN 45 NAUTICAL MILES IN NW QUADRANT EXPECT WINDS OVER 47 KNOTS WITHIN 90 NAUTICAL MILES IN NE QUADRANT AND WITHIN 60 NAUTICAL MILES IN SE QUADRANT AND WITHIN 60 NAUTICAL MILES IN SW QUADRANT AND WITHIN 60 NAUTICAL MILES IN NW QUADRANT EXPECT WINDS OVER 33 KNOTS WITHIN 180 NAUTICAL MILES IN NE QUADRANT AND WITHIN 150 NAUTICAL MILES IN SE QUADRANT AND WITHIN 150 NAUTICAL MILES IN SW QUADRANT AND WITHIN 150 NAUTICAL MILES IN NW QUADRANT
 

17th December 2020 2130hrs

TC Yasa made landfall today at 7pm NZDT at Bua Province, on the island of Vanua Levu, as a Cat5 storm

ZoomEarth20201217

 The inner core now looks to have collapsed - certainly it is not as well defined as it was yesterday. While still packing powerful winds, the TC is expected to now weaken, before it makes a sharp turn towards the Tasman Sea
Heavy rainfall has been experienced in parts of Viti Levu, including slippages and road closures. A national state of emergency has already been declared. stuff.co.nz
In it's most recent report on TC Yasa, the Fiji Meteorological Service has provided the following information:
HURRICANE WARNING 063 ISSUED FROM RSMC NADI Dec 170713 UTC. TROPICAL CYCLONE YASA CENTRE 907HPA CATEGORY 5 WAS LOCATED NEAR 16.6 SOUTH 178.8 EAST AT 170600 UTC. POSITION GOOD. REPEAT POSITION 16.6S 178.8E at 170600 UTC. CYCLONE MOVING EAST SOUTHEAST AT 11 KNOTS. EXPECT SUSTAINED WINDS OF 130 KNOTS CLOSE TO THE CENTRE.
It remains a possibility that an ex TC Yasa will reach New Zealand over the next few days, but this still remains uncertain at this point

19 December 0800 hrs

TC Yasa has now passed over the South Eastern islands of Fiji, and as it moves south commencing transition into a subtropical low. The latest information from the Joint Typhoon Warning Centre follows (Last Modified: 0400 NZDT:

Yasa is located 428 km (231 nautical miles) east-southeast of Suva, Fiji, and has tracked south-southeastward at 15 km/h (8 knots) over the past 6 hours.

Enhanced infrared satellite imagery shows deep convection obscuring a low level circulation center with poleward outflow feeding into an upper level jet.

The initial position is placed with high certainty based on spiral banding wrapping into the low level circulation in the infrared imagery loop and an AMSR2 image.

Environmental analysis reveals a marginal environment characterized by warm (26-27°C) sea surface temperatures and robust poleward outflow aloft, offset by moderate (15-25 knots) vertical wind shear, and convergent upper level flow along the northern periphery of the system.

The system is tracking along the periphery of a subtropical ridge positioned to the northeast. In the near term, the steering ridge will reposition due to interaction with an amplifying midlatitude trough positioned to the southwest.

This interaction, coupled with the building of a strong transitory ridge north of New Zealand, will cause the storm track to become southward after 12 hours and then southwestward after 24 hours.

Environmental conditions will continue to degrade throughout the remainder of the forecast period with wind shear increasing to 30+ knots in 24 hours and cool (< 26°C) sea surface temperatures in 36 hours.

Wind shear is then forecast to further increase as the system tracks further poleward and underneath strong westerly flow aloft. As a result, Yasa will weaken and begin subtropical transition in 12 hours.

Thereafter, the system will continue to weaken as the wind field expands and becomes asymmetric and in 2 days, Yasa will complete subtropical transition.

Numerical model guidance has converged over the past 6 hours and is good agreement throughout the forecast period. This lends overall fair confidence in the JTWC forecast track which is placed slightly east of the multi-model consensus.

Maximum significant wave height at 18/12:00 UTC is 23 feet (7 meters).

The latest satellite image shows a rather disorganised picture

ZoomEarth20201908

20th Dec 2000 hrs

TC Yasa continues to weaken as it heads south into cooler waters. The latest bulletin from the Joint Typhoon Warning Centre and Zoom Earth is as follows:

Yasa is located 683 km (369 nautical miles) south-southeast of Suva, Fiji, and has tracked south-southwestward at 11 km/h (6 knots) over the past 6 hours.

The current position is situated within an exposed low-level circulation evident in recent visible satellite imagery. The intensity of 95 km/h (50 knots) is supported by a 19/19:30 UTC SCATSAT pass and recent automated intensity estimates from CIMSS and cira.

Yasa is tracking south-southwestward under the steering influence subtropical ridging to the south and east. The system is rapidly transitioned into a subtropical cyclone as it passes over cooler water and interacts with upper-level troughing.

Favorable baroclinic interaction between the surface low and upper-troughing have enabled Yasa to maintain intensity over the past 6 hours despite strong vertical wind shear (over 55 km/h (30 knots)) and passage over cooler water (25-26°C).

Full transition into a subtropical cyclone is imminent. The system will continue to follow the subtropical steering flow southwestward for the next 12 hours, and will likely maintain intensity or weaken only slightly with persistence of the environmental factors discussed above.

Numerical models are in excellent agreement over the next 12 hours, lending high confidence to the track forecast. This is the final warning on this system by JTWC.

The system will be closely monitored for signs of regeneration.

Maximum significant wave height at 20/00:00 UTC is 20 feet (6.1 meters).

Information provided by the Joint Typhoon Warning Center (JTWC).

 Ex-TC Yasa is now unlikely to impact New Zealand directly
Forecast track 202012202000NZST
 

News Snippets

19 December 2020
21 December 2020

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Maungaraki, Lower Hutt, New Zealand
TC Zazu
 

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