Tropical Cyclone Oma

[Cyclone Tracy]

TC Oma was born overnight. Central pressure is currently 989 hPa, winds around 45 knots moving eastward near Vanuatu at 6 knots.

[Cyclone Tracy]

Oma is doing quite well considering she is in a very unfavourable wind shear environment. 30c sea surface keeping the energy levels up. She will move into a more favourable wind shear environment within 48 hours.

Advanced Dvorak estimate showing a drop in pressure and pick up in wind speed 50 to near 60 knots in the last 5 hours.

[Cyclone Tracy]

The latest (1202 12Z) 168 hour ensemble track from UKMO and ECMWF is showing some fairly good alignment. The combined 85 models having TC Oma steering along the sub-tropical ridge towards the Tasman. Both main UK and EC models very closely aligned at 168 hour mark.

[Cyclone Tracy]

and not surprisingly, TC Oma now elevated to Cat 2 by Fiji Met Service at 2.15pm NZT.

[Cyclone Tracy]

Currently TC Oma is a Cat 2, winds 50 to 60 knots, pressure around 977 hPa near Vanuatu. She has no real steering with the sub-tropical ridge not strong enough yet to take over. She is going to spin around this same area for the next 24 hours and will slowly get stronger as she steer’s into favourable wind shear. This will mean a likely upgrade to a severe tropical cyclone status near New Caledonia. Like any TC, beyond 120 hours is a game of pin the tail on the donkey but the EC and UK ensembles are favouring mid latitude shortwave troughs to weaken the ridge, allowing Oma to move into the Tasman, next week.

[Cyclone Tracy]

Although TC Oma's doesn't look that sexy at the moment, she is a synoptic scale monster. Currently has a diameter of 850 km. As a comparison, Hurricane Katrina (famous for her size) had a diameter of 644km at landfall.

Longer term, tonight's 00Z runs has multiple global models putting her in the Cat 4 to 5 scale mid next week (EC, UK cat 4, GFS borderline Cat 5). Sea surface temps on the projected steering avenue will support a major cyclonic gyre if wind shear is favourable.

[Cyclone Tracy]

Oma is starting to show better structure in the last 2 hours with more of the truck tyre look on the sat pics. She is moving slowly in a south westerly direction as a cat 2. New Caledonia will start to see the ‘Mares Tails’ clouds overhead soon and will have a spectacular sunset over northern areas tonight.

[David]

GFS going for a potential North Island impact

[Cyclone Tracy]

TC Oma is currently travelling SW at 6 knots and has a neutral wind shear environment along with 29c sea surface temp. Over the next 12 to 24 hours, she will gradually show better form as wind shear becomes more favourable. Lightning is being detected in her outer bands but has yet to be seen near her inner core.

Pressure is around 970 hPa, winds around 70 knots, she is a high end cat 2 on the south Pacific scale.

[David]

Interesting the GFS is more or less continuing the scenario where Oma crosses NZ on each model run, this far out from the projected landfall. At close to 960 hPa as it arrives over the North Island it would be one of the worst storms in decades if that scenario were to eventuate. Couple of the latest GFS charts attached...

ECMWF is not taking it over NZ, yet - so still very much uncertain if we'll see much impact at all.

[cbm]

Interesting indeed that both the models are sticking to their completely different paths for which way it goes. I wonder when/where the fork point is, that once it passes, it will be locked into one or the other path.

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[jamie]

I saw a post on Facebook showing the ensemble of the EC and that showed a lot of the runs are having it still over NZ.


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[David]

Interesting indeed that both the models are sticking to their completely different paths for which way it goes. I wonder when/where the fork point is, that once it passes, it will be locked into one or the other path.

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Some good discussion today on Weatherzone forum that you may find interesting. Some mention of a shortwave that Oma is more likely to follow.
http://forum.weatherzone.com.au/ubbthre ... /1490562/7

[cbm]

Some of that discussion over my head but implication I think is could be as late Wednesday/ early Thursday before it's locked into going West, or Southeast towards NZ. I think if I remember correctly the GFS has overestimated the depth of lows crossing NZ in recent past. Countering that though, this one looks like it will have already gone cold core and be strengthing as it crosses. It was being discussed in another thread that summer 1935/36 was a very warm one. There was a very bad storm Feb 1936. Some of the historic reports of that look similar to the GFS predicted scenario now.

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[Cyclone Tracy]

Fiji Met has upgraded her to a severe cat 3 cyclone. There is more frequent lightning now in the outer bands in the SW and NW quadrants. Oma is crawling at 2 knots, which means more upwelling from the sea. This will slow down intensification in the short term before the sub-tropical ridge takes over the steering and her forward momentum within 24 hours. Then we might see a strengthening eyewall and possible rapid intensification.

[Cyclone Tracy]

TC Oma almost stationary as she waits for the sub-tropical ridge to take over the steering. Wind shear and upwelling continues to stunt further intensification but this looks like changing if she starts moving SW again in the next 12 hours. Currently a low end Cat 3 on the South Pacific scale, 974 hPa and winds around 65 knots.

I’ve plotted up to 168 hours out on the mean tracks of the 4 main ensembles off the latest 16/02 12 runs. The 4 ensembles are made up of 127 model runs including the main UK,EC, GFS & CMC control runs. The UK ensemble is the best performing in the South Pacific over the last 2 seasons.

[David]

Huge variation in the ensemble members with both GFS and EC... quite fun looking through all the members. Some show extreme hits on northern NZ (<960hPa) and some showing side swipe, some showing a miss altogether...

[Ash]

960hpa would be a major storm-What sort of winds could one expect?

[TonyT]

Huge variation in the ensemble members with both GFS and EC... quite fun looking through all the members. Some show extreme hits on northern NZ (<960hPa) and some showing side swipe, some showing a miss altogether...

Which is a great example of the value of an ensemble system - there is only one conclusion you can draw from these ensemble runs, which is, at the moment, the possible outcomes of the guidance are too wide to be useful in forming a forecast. Its currently unpredictable how this pattern will evolve.

[Cyclone Tracy]

I'm detecting a Von Karman vortex street coming out of her deep convective wrap. Vortex shedding is one sign that the beast is start to turn harder

[David]

Latest EC still favouring Australian landfall. UKMO has it coming down towards NZ.

A couple of the latest GFS ensemble members have Oma going across the North Island less than 950hPa. Scary thought!

I haven't been model watching this intently for a fair while

[Cyclone Tracy]

TC Oma on sunset, showing a ragged eye formation and a more symmetrical structure. Being almost stationary for 24 hours has had a decent effect on Oma with cool water upwelling, dry air intrusions and wind shear hampering further development. She is now a high end cat 2 but has increased her synoptic footprint. She is starting to move SW slowly as the sub-tropical ridge continues to strengthen and assist steering.

Lots of popcorn chewing watching the 00Z runs

[Ash]

[http://www.metvuw.com/forecast/forecast ... ni&tim=162[/]

This would be an intense hit if eventuates?

[Cyclone Tracy]

A huge chunk of dry air has been inhaled by Oma overnight, weakening the core and making her look quite asymmetrical. It will probably take most of the day to spit out that dry air. She is clinging onto her cat 2 status but will have more favourable wind shear and sea surface temps to re-intensify in the next 24 hours.

I'm looking forward to reviewing the 12Z ensemble runs. Some of the individual model runs are epic after the transition to a cold core EX-TC.... but the big outlier is EC, sending her back into the Coral sea to torment OZ.

[http://www.metvuw.com/forecast/forecast ... ni&tim=162[/]

This would be an intense hit if eventuates?

If that GFS run this morning verifies, it would be the most significant storm event in NZ recorded history and break the air pressure record. The 06Z run before it was even worse in some places. In my humble opinion, individual model runs for these types of cyclonic events should only be taken seriously under 100 hours but it sure is entertaining watching them

[Thunder081]

If that GFS run this morning verifies, it would be the most significant storm event in NZ recorded history and break the air pressure record. The 06Z run before it was even worse in some places. In my humble opinion, individual model runs for these types of cyclonic events should only be taken seriously under 100 hours but it sure is entertaining watching them

Yes it is entertaining watching them, I was looking at this run and thinking if this was 24 - 48 hours out then I'd be scoping out spots for a storm chase.