Polar blast & NI low 11-14th July

[Chris W]

Doesn't colder air usually end up drier/

Yes but if it's clashing with warmer, more moist air then the warmer air supplies moisture. I am from the UK and this happens with the Atlantic vs continental cold yielding heavy precipitation, often snow. It also looks like this system has picked up a good deal of moisture from the Southern Ocean and with some also feeding in off the Pacific on Thursday, and weren't sea temperatures above average recently? There could be some 'lake effect' moistening the air here as it moves north. I think there could be snow to sea level in Chch, but not convinced of a lot of it given recent years. Definitely a lot of snow for rural parts and the high country. The very cold 500mb temperatures and vigour of the front together with the cold 850s lingering through to Thursday are encouraging however.

[snowchaser01]

It's a very interesting situation. The low that develops later in the week to the north and creates the advection chance is dangerous. it's sitting on a knife edge. Potential big problems in North Cany if that occurs. Across fingers it hangs further north!

[Bradley]

Another factor to consider is the colder then usual May and also average temps in June means the ground is quite cold and the snow has a better chance of settling...

[Richard]

And i see the MetService have greatly increased the amount of precipitation for North Canterbury for the period too

[snowchaser01]

Another factor to consider is the colder then usual May and also average temps in June means the ground is quite cold and the snow has a better chance of settling...

Yup good point! I just have a sneaky wee odd feeling about this. Something we are missing... or just over thinking?

[Bradley]

Also a good amount of snow will potentially fall at night time on Tuesday night and Wednesday morning before the sun rises which will also mean less melting - another positive sign if one wants some decent snow...

[snowchaser01]

Also a good amount of snow will potentially fall at night time on Tuesday night and Wednesday morning before the sun rises which will also mean less melting - another positive sign if one wants some decent snow...

How low will it go is the question... i don't see any reason why it shouldn't fall to sea level with a thk of 520 and very cold temps aloft

[Chris W]

And i see the MetService have greatly increased the amount of precipitation for North Canterbury for the period too

Christchurch too, but you are very likely in for a lot of it on Thursday!

[Razor]

I know we don't look too far ahead, but some will get excited on social media here

[snowchaser01]

I know we don't look too far ahead, but some will get excited on social media here

Lets get through this incoming blast before we look at that

[snowchaser01]

I see WW are sitting on the fence for sea level. They are calling it likely in Southland and Otago, and "perhaps Canterbury" So we shall see i guess. I dont think MS have mentioned sea level once... or anywhere have they?

[NZ Thunderstorm Soc]

Snowy conditions should settle to most eastern areas from tomorrow but should clear away as the week moves on with the high pressure system moving in quickly ahead of the next system moving in from the west.
Could be an interesting low form in the Tasman Sea next week better than this event,which could draw in a stronger ridge to the south to enhance a similar scenario. Have to weight and see.

[Razor]

Metservice has ramped up the moisture for Chch, but its a degree or three too warm which supports the 200m forecast level

[gllitz]

Greetings and salutations from Perth - I *knew* people would be losing their minds over this - happens EVERY year. LOL! Bottom line, some will see snow, some won't. Oh well. Maybe next time! Just go with the flow....Gonna be down right chilly here tomorrow, too - 20C for a high, don't think I can handle it!!

[snowchaser01]

Yes, a big moisture increase. Wouldnt read too much into computer generated temps on MS, their moderated forecasts have a low tonight of 1deg for chch which is interesting for sure.

[Richard]

Can someone tell me what is the equivalent of rainfall amount to snow depth

[Bradley]

Snowlevel looks to have risen a touch overnight, 200m now seems to the snow line with some brief flurries to 100m

[TonyT]

Can someone tell me what is the equivalent of rainfall amount to snow depth

It varies with each storm, but for wet snow the rule of thumb is 1cm of snow yields about 1mm of rain when melted. So if the models are predicting 20mm of rain expect about 20cm of snow, all else being equal, which it never is.

For drier snow its closer to 2cm of snow to yield 1mm of rain (the flakes are smaller and trap less air in the snow pack). So on the ski fields, 20mm of rain might only yield 10cm of powder snow.

[Richard]

oh ok, so the ski fields would receive whats termed cold snow, temperatures around zero would be called a warm snow event.

[shovelopikis]

Can someone tell me what is the equivalent of rainfall amount to snow depth

It varies with each storm, but for wet snow the rule of thumb is 1cm of snow yields about 1mm of rain when melted. So if the models are predicting 20mm of rain expect about 20cm of snow, all else being equal, which it never is.

For drier snow its closer to 2cm of snow to yield 1mm of rain (the flakes are smaller and trap less air in the snow pack). So on the ski fields, 20mm of rain might only yield 10cm of powder snow.

So you mean 1cm of snow equals 2mm of rain in a drier snow event?

[TonyT]

So you mean 1cm of snow equals 2mm of rain in a drier snow event?

No. 1cm of snow yields 0.5mm of rain, because the powdery snow crystals are smaller, and pack closer together than the wet flakes.

[snowchaser01]

New severe weather watch out! They are down to 100m now.
SEVERE WEATHER WATCH FOR SOUTHLAND, FIORDLAND, CENTRAL OTAGO, NORTHOTAGO, SOUTHERN LAKES, DUNEDIN, CANTERBURY PLAINS, CHRISTCHURCH,CANTERBURY HIGH COUNTRY, MARLBOROUGH, WELLINGTON, KAPITI HOROWHENUA,WAIRARAPA, HAWKES BAY, TAIHAPE, NELSON, STEWART ISLAND ISSUED BY METSERVICE AT 1015hrs 11-Jul-2017

SNOW FOR PARTS OF THE SOUTH ISLAND AND THE LOWER NORTH ISLAND, ALSO HEAVY RAIN AND STRONG WINDS ACROSS CENTRAL NEW ZEALAND

A cold front, followed by strong cold southerlies, will sweep northwards across the South Island today and the North Island on Wednesday. In the wake of this front, snow is forecast to fall to low-levels in southern and eastern districts of both islands.

Snow is likely to fall as low as 100 or 200 metres across the southern and eastern parts of South Island at times until Thursday with significant accumulations at higher elevations. 20 to 30cm, possibly even more, could accumulate on Canterbury High Country stations. Snow is likely to fall as low as 400 metres across central and eastern parts of the North Island with 15 to 20cm acumulations expected at higher elevations. This will affect many higher roads,and could cause problems for livestock.

This Watch is for the likelihood of significant snow accumulations below 500 metres in the following areas...

Southland and Fiordland: During today through to Wednesday afternoon.

Otago: Until around midday today, and again from tonight until Wednesday evening.

Canterbury and Marlborough: From this evening until Thursday afternoon.

This Watch is for the likelihood of significant snow accumulations below 1000 metres in the following areas...

Taihape and Hawkes Bay ranges: From Wednesday afternoon until Thursday morning.

Additionally, a low is forecast to develop east of central New Zealand during Wednesday and deepen rapidly on Thursday, bringing heavy rain and gale south to southeast winds to the upper South Island and the lower North Island.

This Watch is for the possibility that south to southeast gales will become severe in the following areas....

Northern Canterbury, Marlborough, Nelson, Wellington and Horowhenua Kapiti Coast: From Wednesday afternoon till overnight Thursday.

This Watch is also for the possibility that rainfall accumulations may reach warning criteria in the following areas...

Wellington and Coastal Marlborough: During Thursday.

Wairarapa: From Thursday morning until Friday morning.

People in these areas are strongly advised to stay up to date with the latest forecasts and Warnings, as this event unfolds in case more areas are added to this WATCH or parts of this WATCH are upgraded to a full WARNING. Road Snowfall Warnings are currently in effect for many high level roads.

This Watch will be reviewed by 9pm Tuesday 11th July

[Bradley]

I would have thought snow settling to 100m would be a stretch if that's what they are referring to - settling

[melja]

I'm just not seeing the amounts forecast.

Sent from my Redmi Note 4 using Tapatalk

[Bradley]

Yep this is primarily an event for places above 200m, anywhere below that where people are expecting big amounts or any amount settling at all will be dissapointed I'm thinking...