Polar blast & NI low 11-14th July

[Bradley]

EC has increased the moisture even more into Thursday in its latest run this morning, looking more and more likely that this could be a major event for areas over 400m ask around Canterbury and even into some areas above 700m in the north island as well. Maybe a few cms down to 200m as well but nothing too disruptive at the moment...

[snowchaser01]

Metservice now saying 200m for Christchurch and the plains, and I see Tony now mentioning the possibility of sea level on Tuesday night, hes putting out some big numbers too!

[TonyT]

Metservice now saying 200m for Christchurch and the plains, and I see Tony now mentioning the possibility of sea level on Tuesday night, hes putting out some big numbers too!

Not big numbers to sea level though. Actually its an "only just" for sea level. But they do look biggish numbers (by modern day winters standards) for the high Plains and foothills, more so Mid and North Canty than South.

[Bradley]

I don't know what to say about this really

[NZstorm]

The gfs has swung into a stronger warm advection scenario for later in the week for mid Canterbury to Kaikoura with a moisture punch coming in from NE!

[snowchaser01]

Goodness me that is a model run and a half. Going to be an extreme cold week!!

[Richard]

wow we could be talking a metre of snow 400m and higher oh dear, snow raking coming up if that holds up... chuck ya few more of these in

[Richard]

Ive now changed title of the thread to include the development of a North Island low pressure system and extended the duration period to include Friday 14th.

[Talbotm]

Any thoughts?? Mt Hutt expecting over 2m now. http://www.snow-forecast.com/resorts/Mo ... t/6day/mid

[Lawrence]

I've always used the 522 line as a sea level event indicator.
Will be interesting how the low forms to the North east.
Very interesting numbers at this stage but will wait for a few more runs to be confidant of a Coastal Canty snow event.

[Lawrence]

Any thoughts?? Mt Hutt expecting over 2m now. http://www.snow-forecast.com/resorts/Mo ... t/6day/mid

Interesting to see the wind direction on Wednesday night,Thursday morning from the South East onshore flow.
Totally contradicts some weather sites.
Good brisk onshore from that direction has usually given a good chance of a good amount of the white stuff for Canterbury whereas more South West means we get that long shadow from Dunedin area.

[Chris W]

Another swing from the BOM to colder and more moisture Tues evening onwards, with that North Island low looking significant. Must be tough for MetService to call this one.

[Richard]

Another swing from the BOM to colder and more moisture Tues evening onwards, with that North Island low looking significant. Must be tough for MetService to call this one.

Yes would be. Wonder why they dont have a SEVERE WEATHER OUTLOOK yet.

[jamie]

Outlook has been issued. On my app it's just not highlighted to say it's there.

[mikestormchaser]

And this is at midnight fri... surely that will change.. if not, there's going to be trouble

[Nev]

A little late today, but for posterity here's MS's Outlook for the next 4-5 days…

MetService Severe Weather Outlook

Issued: 4:33pm Sunday 9 Jul 2017
Valid from Tuesday 11 July 2017 to Friday 14 July 2017

High impact severe weather is expected to affect many parts of New Zealand from Tuesday to Friday. There may be flooding, disruptions to transport due to snow and stress to livestock and damaging severe gales. It is also worth noting that coastal areas exposed to a strong onshore flow, high waves and heavy rain could be affected by inundation. Further details:

Tuesday: A front is expected to move northeast onto the South Island, followed by cold southerlies. Snow is expected above 100 metres in the far south, but 200 metres in Canterbury. There is low confidence that snow accumulations will reach warning criteria above those levels, in Fiordland, Southland, Otago and much of Canterbury as indicated on the chart.

Wednesday: A low deepens east of the North Island, possibly bringing severe gales, heavy rain and snow to some central New Zealand areas. There is moderate risk of warning amounts of snow above 400 metres in parts of Canterbury and Marlborough as indicated on the chart. There is a moderate risk of severe south or southeast gales over the North Island from Taranaki to Gisborne southwards and over the Marlborough Sounds. Furthermore there is a high confidence that rainfall accumulations will reach warning amounts in Wairarapa, Hawkes Bay and Gisborne and a moderate confidence in rainfall amounts reaching warning criteria from Taranaki to Wellington and in Marlborough and the north of Canterbury. Finally, there is a moderate confidence that snow accumulations above about 600 metres will reach warning amounts in Wairarapa,Hawkes Bay and the east of Taupo.

Thursday: A deep low just to the east of the North Island moves slowly north,and a strong moist southeasterly flow persists over central New Zealand. There remains a moderate risk of severe south or southeast gales over the North Island from Taranaki to Gisborne southwards and over the Marlborough Sounds.Furthermore there is a high confidence that rainfall accumulations will reach warning amounts in Wairarapa, Hawkes Bay and Gisborne and a moderate confidence in rainfall amounts reaching warning criteria from Taranaki to Wellington and in Marlborough and the north of Canterbury. Finally,there is a moderate confidence that snow accumulations above about 600 metres will reach warning amounts in Wairarapa, Hawkes Bay and the east of Taupo.

Friday: The low should move away to the east of the North Island and conditions should slowly ease late in the day.

MetService Severe Weather Outlook - Is. Jul 09.gif

[Bradley]

Snow to sea level on Wednesday is possible if thickness levels around the 520 mark were the only indicator but as we know it isn't ((the cold outbreak last August being a prime example where it sat around the 520 mark for 12 hours in Christchurch with no snow whatsoever)

[NZstorm]

Snow to sea level on Wednesday is possible if thickness levels around the 520 mark were the only indicator but as we know it isn't ((the cold outbreak last August being a prime example where it sat around the 520 mark for 12 hours in Christchurch with no snow whatsoever)

I can't recall that set up but there must have been a wind direction blocking the moist air flow.

[Chris W]

There was sheltering from the peninsula last winter at one point, maybe that was August?

Having seen the Metservice outlook I think they are about there with both the snow and rain threats, but wouldn't be surprised if they both change over the next 48 hours. One thing's for sure, it will be an interesting week.

[Richard]

Snow to sea level on Wednesday is possible if thickness levels around the 520 mark were the only indicator but as we know it isn't ((the cold outbreak last August being a prime example where it sat around the 520 mark for 12 hours in Christchurch with no snow whatsoever)

I dont recall either the timing of that outback but Aug snowfalls are influenced more by slightly extra daytime heating. There's been snows here in the past where it snows at night, come mid morning it often turns to rain only to return back to snow as it gets dark again, doesn't tend to do that as much as June - July snowfalls.

[Storm Struck]

It sure is changing abit each model run, but the consistancy is the main thing which its been at for some days now.
Its actually colder Wednesday night which is when my pick for any reasonable snow chances to near sea level could occur. Snow to 100-200m overnight Tuesday into early Wednesday possibly lowering to near sea level for a small window around 5am to mid morning.
But this would be south and west of the city, with possibly no more than a few centimeters below that 100m mark maybe the city could see a few light flurries at this point.
But as I say the precipitation becomes widespread from Wednesday night again and in conjunction with colder air, could drop back to near sea level with a reasonable dusting.
The 850mb temp is -8C on Wednesday night as apposed to -6C Wednesday morning

[Talbotm]

So much inconsistency with this system. Metvuw is all over the shop. Huge changes from model to model. Any ideas why such big changes? The latest run is significantly different to the earlier run today and now appears no warm advection event for Canterbury at all?

[Bradley]

EC has increased the moisture levels even more with places like Hanmer Springs expected to now receive a metre of snow! For mid Canterbury looks to be around 20-30cm over 300m with lesser amounts to 200m. No snow to sea level at this stage...

[NZstorm]

EC has increased the moisture levels even more with places like Hanmer Springs expected to now receive a metre of snow! For mid Canterbury looks to be around 20-30cm over 300m with lesser amounts to 200m. No snow to sea level at this stage...

looks cold enough for snow showers in Christchurch city Wednesday. The 700mb temps -18C which is a good signal for wintry precip to sea level.

[spwill]

Yes, looking like snow showers in Christchurch and Dunedin on Wednesday, a deep cold system moving across, even a slight chance of thunder. Looked at the last two runs of the GFS.