Front with multiple lows April 27 to April 30

[Cyclone Tracy]

I think this one needs a thread. There is a lot going on that will affect north and south islands. An initial front moves onto the west coast of the SI which is followed in by an upper polar trough. In parallel, a trough originating from the tropics connects with the southern front and polar uppers, spawning a surface low that deepens around the upper NI. This surface low cuts off forcing the front moving across NZ to stall. From there it's a guessing game at this stage as there are differences with the models on where the cut off low will travel. Snow and rain for the SI, rain, wind and possibly storms for the NI.

[NZ Thunderstorm Soc]

Yet again for the North Island?

[wembley]

Darling Danny Corbett thinks the south will get off lightly from this

[NZstorm]

For the upper North Island I think the heavy rain most likely in areas exposed to the east. Also a risk of severe gales for Auckland/Waikato, gfs showing a low level jet at 850mbs, so the risk this could affect exposed areas at least.

[NZ Thunderstorm Soc]

Darling Danny Corbett thinks the south will get off lightly from this

Could do, but later, much colder weather coming in to start off May.
Whether wet or not, I don't know at the moment

[Razor]

Latest GFS is pretty much a non-event for most of the South Island. Bit of a flip flop run?

[Cyclone Tracy]

Models really struggling to pin point the timing and placement of the upper trough driving this event. Plenty of change on each run across all of them. We might need another 24 hours before we know how this might unfold.....

[NZstorm]

GFS showing a fairly robust negative tilt trough crossing upper North Island Saturday. This type of system can bring volatile weather, strong winds/heavy rain. Its moving quickly so brief impacts.

[jamie]

GFS showing a fairly robust negative tilt trough crossing upper North Island Saturday. This type of system can bring volatile weather, strong winds/heavy rain. Its moving quickly so brief impacts.

What’s a negative tilt trough?

[NZstorm]

GFS showing a fairly robust negative tilt trough crossing upper North Island Saturday. This type of system can bring volatile weather, strong winds/heavy rain. Its moving quickly so brief impacts.

What’s a negative tilt trough?

It relates to the orientation of the trough. You get a better overlap of the dynamics and stronger forcing when the trough is negative tilt, that is axis orientated into the northeast for NZ. Referred to a lot in USA storm chasing. http://www.theweatherprediction.com/habyhints/127/.

Successive runs of the gfs are showing variations but we can say we are in for a wet weekend with some instability on the cards as well for northern areas.

[spwill]

There will be moist air over the upper North Island over the weekend with dps 17-18C. Rain/showers and the chance of thunder for Auckland over the weekend but a few sunny spells in the mix by Sunday. I've used the GFS.

[Cyclone Tracy]

UKMet and EC increasing rainfall intensity on the 12z runs to around 100mm+ over multiple days around Auckland. Atmospheric river running at nearly 50mm tomorrow night into the upper NI. Access R showing damaging NE wind gusts tomorrow evening across large areas of the NI. MS warnings and watches will be looking quite busy soon off those runs.

[David]

Rain warning issued for Coromandel and western BOP, watches issued for multiple other regions.

[Cyclone Tracy]

MS publicly saying they think the UKMet modelled scenario for the North Island is the most favoured, which is why they have the direct heavy rain warnings for the Coromandel and western BOP at the moment. I think that will extend to other areas tomorrow and Sunday based on that current UK modelling. UkMet model also has heavier falls around Dunedin.

Access R on the 18z run continues to funnel nasty NE 120 km/h gusts tomorrow afternoon and evening across large areas of the NI. It was the only model to pick up the extreme level of the westerlies on 10th April that smashed Auckland.

[NZstorm]

MS publicly saying they think the UKMet modelled scenario for the North Island is the most favoured, which is why they have the direct heavy rain warnings for the Coromandel and western BOP at the moment.

I think the ukmet is the best model generally. The Brits are very good with IT.

[TonyT]

The Brits are very good with IT.

Indeed.

https://www.youtube.com/watch?v=RlmzwZXa-Ww

[David]

Access R on the 18z run continues to funnel nasty NE 120 km/h gusts tomorrow afternoon and evening across large areas of the NI. It was the only model to pick up the extreme level of the westerlies on 10th April that smashed Auckland.

EC has some big winds as well for later tomorrow. Also, watches for both wind and rain in Akl now upgraded to warnings by the metservice.

[Cyclone Tracy]

@NZstorm, yes ..agree on Ukmet currently the most accurate. Proved especially so during the cyclone season and picking the strength of the ridge.

@David, yes good spotting. EC 00z has now jumped on board with Access R with regards to wind strength levels around Auckland. Not surprising MS upgraded.


Just checked the latest Access R run 06z and it really has locked onto a brutual North Easterly tomorrow evening across the upper NI. Both Access R and EC have aggressive sustained 120 km/h @850 hPa moving down the NI during the night. EC has modelled 150 km/h gusts in parts of the Hauraki gulf. Candles and torch are ready, batteries will be charged by 3pm tomorrow

[David]

12Z EC has pulled back a little on the wind speed.

[Nev]

MS now forecasting gusts to 120 km/h for the Gulf and the Waitemata this arvo/evening - same as for Apr 10 except they're NE'ers this time…

[spwill]

Queenstown is on 3C, Wanaka 4.5C with rain, the Mnts getting some reasonable snow at ski field level.

[snowchaser01]

Holy heck... Big moisture increase in the models for later tonight for Canterbury. Metsevice have come to the party with a rain warning for Canterbruy south of the Rakaia river and a watch for north of the Rakaia... hmm

[Cyclone Tracy]

Complex set up this one Whoever from MS who is drawing up the surface chart is earning their money today.
Latest GFS wind map https://earth.nullschool.net/#2018/04/2 ... 36,-36.839 at 850 hPa showing the Fujiwhara effect now in progress and one slight movement of any low making a big difference to impacts. The northern Tasman low will swallow the others based on latest guidance. Some areas in Canterbury went from a 20mm event to 150mm in 12 hours on EC

Wind picking up here, took a gust of 70 km/h on Mt Victoria using a hand held about an hour ago. Pressure 1006.5 and falling, 18c with a 17c DP

[spwill]

Up to 10am Wanaka has had 29.4 mm of cold rain, still raining there. That will likely be a good fall of snow at ski field level.

[Skywatcher]

Up to 10am Wanaka has had 29.4 mm of cold rain, still raining there. That will likely be a good fall of snow at ski field level.

Snowing at the top of the Crown Range Road

crown-range.jpg

Source: http://www.metservice.com/national/traf ... queenstown