Potential Nationwide Snow Event Sunday 8th August 2021

[mikestormchaser]

GFS chance of some heavy showers and hail moving through Canterbury tomorrow evening. Small risk of some lightning coastal fringe

[Bradley]

EC again wants nothing to do with this event for mid-Canterbury whereas GFS is one again forecasting epic snow number right down to 100m asl. Remarkable this divergence in models, GFS has just been so so consistent. Surely GFS starts coming inline with EC in the next 12 hours…

[Chris W]

MS hedging on the difference between GFS and UK/EC with their low risk for heavy snow above 300m down to the Rangitata it seems. Overall it seems to be going north but we can't rule out that the GFS might be right this time.

Definitely cold on Monday though, could be some wintry chain showers through Chch in any case. Before that the forecast wording for Sunday/Monday is currently snow to 200m which makes sense.

[TonyT]

EC again wants nothing to do with this event for mid-Canterbury whereas GFS is one again forecasting epic snow number right down to 100m asl. Remarkable this divergence in models, GFS has just been so so consistent. Surely GFS starts coming inline with EC in the next 12 hours…

Thats just the EC operational. Ec ensemble has 16/50 (32%) for snow on the ground in Chch, about twice what it had this time yesterday.

GFS is still at 85%+

[Bradley]

EC again wants nothing to do with this event for mid-Canterbury whereas GFS is one again forecasting epic snow number right down to 100m asl. Remarkable this divergence in models, GFS has just been so so consistent. Surely GFS starts coming inline with EC in the next 12 hours…

Thats just the EC operational. Ec ensemble has 16/50 (32%) for snow on the ground in Chch, about twice what it had this time yesterday.

GFS is still at 85%+

I notice the 18Z run of GFS (I know it's not as informative as the 00Z run) has the low crossing just off the bottom of the north island whereas EC has it crossing just north of New Plymouth, a difference of approx 320km north to south which is going to make all the difference....Tony you mention that EC ensemble has doubled the chance of snow in Christchurch overnight. In your opinion is that at least a small sign that EC is starting to gravitate towards GFS's forecast?

[Lawrence]

Best looking snow event I've seen in ages. Looking to coming in a bit earlier than I was thinking.
Looking like sea level for a few hours then showers into the early hours of Monday.Still a few days yet, but thinking we usually get a sting in the tail in winter.
What's everyone thinking?

2021080512H072_NZ00_SFC.jpg

[TonyT]

I notice the 18Z run of GFS (I know it's not as informative as the 00Z run) has the low crossing just off the bottom of the north island whereas EC has it crossing just north of New Plymouth, a difference of approx 320km north to south which is going to make all the difference....Tony you mention that EC ensemble has doubled the chance of snow in Christchurch overnight. In your opinion is that at least a small sign that EC is starting to gravitate towards GFS's forecast?

Not really. ECMWF has made it colder later on Monday and wants to bring coastal snow showers over the city. It has backed further away from the heavy inland falls GFS is seeing on Sunday.

There are a lot of runners in this race with a pretty equal chance of winning.

[TonyT]

Best looking snow event I've seen in ages. Looking to coming in a bit earlier than I was thinking.
Looking like sea level for a few hours then showers into the early hours of Monday.Still a few days yet, but thinking we usually get a sting in the tail in winter.
What's everyone thinking?2021080512H072_NZ00_SFC.jpg

There's 9 different models with 9 different outcomes for Christchurch/Canterbury. Given that most models have had a hard time getting the details right in recent months, thats perhaps not surprising. But best not to put all your money on one runner in the race. GFS is either hero or zero for this event. Its consistency is compelling, its output implausible.

[darcyplumb]

Is it common for these types of winter events to still have extreme model disagreement only 48 hours out?

[TonyT]

Is it common for these types of winter events to still have extreme model disagreement only 48 hours out?

Yes. And its not extreme. They all agree on the same pattern. Its their consistency that allows us to use the word "snow" in the forecast for Sunday/Monday. Its the words we need to put around "snow" which are the hard ones to choose!

[Lawrence]

Is it common for these types of winter events to still have extreme model disagreement only 48 hours out?

Good question.

[darcyplumb]

Is it common for these types of winter events to still have extreme model disagreement only 48 hours out?

Yes. And its not extreme. They all agree on the same pattern. Its their consistency that allows us to use the word "snow" in the forecast for Sunday/Monday. Its the words we need to put around "snow" which are the hard ones to choose!

Thanks for your reply Tony. I look forward to seeing how the next few days unfold.

[Lawrence]

Is it common for these types of winter events to still have extreme model disagreement only 48 hours out?

Yes. And its not extreme. They all agree on the same pattern. Its their consistency that allows us to use the word "snow" in the forecast for Sunday/Monday. Its the words we need to put around "snow" which are the hard ones to choose!

And "snow" in its various forms and textures perhaps?

[talbotmj15]

With all thats happening im interested in CTs take on this event?

[Cyclone Tracy]

GFS keeps the cyclonic cold core structure in place near Cook strait. In the sweet spot for a nice wrap around wind veering cold shot into Canterbury, then a chilling miserable backhander on the lower NI as it leaves the scene of the crime.

Jury is out with me until the 7th Aug 00z runs, as there is still misalignment on several models between 500 & 700 hPa.

[TonyT]

I'm starting to think we are now seeing two different questions needing answering - 1) will the snow on Sunday be heavy anywhere?; and 2) will Chrisctchurch and BP see sea-level snow on Monday? Two different potential processes in play at different times.

[mikestormchaser]

I'm starting to think we are now seeing two different questions needing answering - 1) will the snow on Sunday be heavy anywhere?; and 2) will Chrisctchurch and BP see sea-level snow on Monday? Two different potential processes in play at different times.

Hi Tony, my thoughts are if it's going to snow to ground level in the city and surrounding areas I'm picking early hours of Monday morning but that's just how I'm reading the models.
Accumulations though if any is another question.

As for tomorrow evenings change and showers, the models are looking abit suspicious with hints of coastal thunderstorm/ hail risk. Not a big risk but a low risk

[Bradley]

EC now has the low crossing around 100km further south then the previous run…

[Northsouth]

My numbers if anyone interested.

My numbers. 6-12 methven, 12-20 Springfield, 12-18
CH, 4-8PH, 8-15BP, 1-2cm CHCH (Monday morning before dawn), 20-30 hanmer, 15-20 culverden above 300m. 30-50 about hanmer/lyford. 20-25 Porters pass.

Mt Hutt 20-30, CB range 25-35, Hanmer/lyford/Rainbow 30-50.

[talbotmj15]

I have changed the name of the thread as I believe it seems more relevant now given the current modelling data. Access moisture is showing up as much or more than GFS. Access has absolutely nailed the moisture for big events this year. They are also aligned with GFS in terms of the big moisture from Rakaia river North. UKMO also seems to like this scenario and is moving towards it too. Ikon is close and EC has begun to move towards GFS now.
At this stage GFS is winning this. Which is great but very concerning for farmers across the nation as many are into lambing.

[Dean.]

I expect to see Metservice expand their watches and warnings this morning to include all areas from Timaru north.EC moving towards GFS with every run.

[Razor]

Certainly looks like the models are aligning and pointing towards the more drastic end of the spectrum. Interesting couple of days or so ahead

[Bradley]

From Blue Skies:

Snow Forecast for Canterbury High Country and Plains:
A significant cold snap is now locked-in for Canterbury tomorrow and Monday. Guidance has come into a closer alignment suggesting that heavy snow is now likely inland, and possibly on the western Plains, with light to moderate falls further east and some light snow likely for Christchurch city. Snow will already be falling inland at dawn tomorrow and heavy falls are likely during the morning, so there will be no time to react tomorrow – all preparation for this event needs to be completed today.

For farmers:
For Mid and North Canterbury above 200m, expect snow to begin falling before midnight tonight and continue through the night. Heaviest falls will be in the morning tomorrow with snow slowly easing during the afternoon and evening. Snow should clear slowly from the south early Monday morning. Expect 10-20cm accumulations by dawn on Monday, with 20-30cm possible and falls of 30-50cm likely above 400m. Below 200m expect snow to push slowly across the Plains tomorrow night, reaching Christchurch and the coast around 3-4am. 1-5cm accumulations are likely on the Plains by dawn on Monday, with further brief light snow flurries till Monday afternoon.

For South Canterbury above 200m expect snow to begin this evening and continue through tomorrow, with heaviest falls around dawn, and snow breaking to showers during the afternoon. Showers will clear early tomorrow night. Accumulations of 5-10cm are expected, with 10-20cm possible. Accumulations of 1-5cm are expected to very low levels through tomorrow.

Banks Peninsula may stay snow free through tomorrow with just rain during the day, but snow will lower to sea level later tomorrow night and frequent snow showers are likely through Monday till around 6pm. Expect 5-15cm above 200m, and 1-5cm to sea level by late Monday. Southerly winds will be strong on Monday with severe windchills

From tonight through Monday night will be a hazardous 48 hours for all stock, but newborn lambs and pregnant ewes especially. All stock will need extra care and attention, especially in areas with substantial accumulations.

For travellers:
Expect alpine passes to be affected by snow from late this evening . The quantity of snow expected may mean passes stay closed through Sunday. Roads in inland Canterbury above 200m (this includes the western Plains) are likely to experience snow accumulations. Roads on Banks Peninsula will also be affected from Sunday afternoon. For most of the Plains snow will be lighter and may not settle for long periods, but roads may be treacherous or briefly impassable at times. Christchurch city looks likely to experience light snow falls and accumulations of 1-5cm by early Monday morning, then further brief snow flurries till about 4-5pm Monday. This could cause significant localised disruption to traffic at times.

Queenstown airport may be affected by snow tonight. Christchurch airport may be affected by snow later tomorrow night. Cook Strait ferries may be affected by heavy swells and strong winds Sunday night and Monday. If you have plans to travel on Sunday or Monday be flexible and allow for possible disruption.

Elsewhere
Light to moderate snowfalls to 200m look likely in Southland and Otago later today and tonight, with light falls to sea level tomorrow. Expect heavy snow above 3-400m on the ranges of Nelson and Marlborough tomorrow night and Monday morning, sufficient to close higher roads and inland passes. Moderate snowfalls are looking likely in the central North Island tomorrow night and early Monday, and light falls to 2-300m on hills in the southern half of the island during Monday.

(C) Blue Skies Weather & Climate Services Ltd 2021

[mikestormchaser]

Yes it looks more and more likely this event will happen.
Snow looks most likely from Sunday night into Monday morning for chch. The risk of heavy showers and hail i mentioned later today still looks a chance

[TonyT]

Certainly looks like the models are aligning and pointing towards the more drastic end of the spectrum. Interesting couple of days or so ahead

I trust Mrs Razor is safe somewhere?