Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

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talbotmj15
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Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by talbotmj15 »

Models are all locking onto Mid latitude low. Appears to deepen quite quickly into cold core low off Canterbury coast. A cold Southerly feed undercutting also. Advection recipe. Some heavy snow if it verifies. Especially above 600m. Certainly worthy of a mention.
Last edited by talbotmj15 on Fri 28/05/2021 22:29, edited 2 times in total.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by spwill »

Yes, a high country snowstorm looks possible for Canterbury this weekend.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by Chris W »

Possibly very heavy rain/snow for Otago/South Canterbury/Mackenzie/inland Canterbury areas going off this morning's GFS run. Looks steadier at the coast so possibly very welcome in the city. CT flagged this one a couple of days ago so good spot.

Looks like a southerly breaks the current anticyclone down, allowing that low that's currently smashing Coromandel to migrate southwest, bringing the moisture over the cold air. GFS and UKMO on board, haven't checked EC.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by talbotmj15 »

Doing some research the mapping looks similar to some of our previous big snow advection events. I do feel this is a touch warmer but not by much. Need the 850mb level temps around -4/-5 currently gets close at -3 right up into Canterbury on GFS.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by Bradley »

talbotmj15 wrote: Mon 24/05/2021 12:32 Doing some research the mapping looks similar to some of our previous big snow advection events. I do feel this is a touch warmer but not by much. Need the 850mb level temps around -4/-5 currently gets close at -3 right up into Canterbury on GFS.

The June 2015 event which produced 30cm of snow to 300m asl and 20cm to 200m in mid Canterbury only had 850hpa temps of -2C and -3C so definitely possible, the timing needs to be exactly right though...
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by talbotmj15 »

Bradley wrote: Mon 24/05/2021 13:26
talbotmj15 wrote: Mon 24/05/2021 12:32 Doing some research the mapping looks similar to some of our previous big snow advection events. I do feel this is a touch warmer but not by much. Need the 850mb level temps around -4/-5 currently gets close at -3 right up into Canterbury on GFS.

The June 2015 event which produced 30cm of snow to 300m asl and 20cm to 200m in mid Canterbury only had 850hpa temps of -2C and -3C so definitely possible, the timing needs to be exactly right though...
Can anyone find maps for the 2015 June event?
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by Bradley »

talbotmj15 wrote: Mon 24/05/2021 14:59
Bradley wrote: Mon 24/05/2021 13:26


The June 2015 event which produced 30cm of snow to 300m asl and 20cm to 200m in mid Canterbury only had 850hpa temps of -2C and -3C so definitely possible, the timing needs to be exactly right though...
Can anyone find maps for the 2015 June event?
No but there is a couple of different mentions of -3C at 850hpa for the event on the archived 2015 chat so not cold at all really....
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by Cyclone Tracy »

Flurries at -2c, puking at -4c 850 hPa on the evening of 18 June 2015. High levels of moisture at 700 hPa that night.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by snowchaser01 »

Cyclone Tracy wrote: Mon 24/05/2021 15:58 Flurries at -2c, puking at -4c 850 hPa on the evening of 18 June 2015. High levels of moisture at 700 hPa that night.
Yes this was an incredible event, remember it clearly. Very decent flurries in chch city and big accumulations just inland.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by talbotmj15 »

CT What are your thoughts on this weekends event? Are you seeing similar signals? Also what are the key numbers for you in determining snow in Canterbury and to what levels roughly. Like 850mb being -2 snow to say 300m? I know its not exact but just your general guidance as your wisdom in these matters are much appreciated.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by Cyclone Tracy »

talbotmj15 wrote: Mon 24/05/2021 16:27 CT What are your thoughts on this weekends event? Are you seeing similar signals? Also what are the key numbers for you in determining snow in Canterbury and to what levels roughly. Like 850mb being -2 snow to say 300m? I know its not exact but just your general guidance as your wisdom in these matters are much appreciated.
Looking from a synoptic scale level, this is broadly a significant weather event for NZ. On a mesoscale level, I'm sticking with the 100 hour rule for now. Any modelling past 100 hours is a wild guess when you look at individual regions and likely upper levels conditions. I think after the 00z runs tomorrow evening, the picture will becoming clearer. The UK was the first model to pick up cut off low off the east coast of the SI , then Access and GFS jumped onboard today but it's still early days :smile:
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by tich »

29-31st May.
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Re: Mid latitude Low cold air collision. 29-31st June

Unread post by talbotmj15 »

tich wrote: Mon 24/05/2021 16:5929-31st May.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by NZ Thunderstorm Soc »

I'm getting confused about the dates of this 2015 event?
According to my diaries, the only event that eventuated at that time (late May/early June),from this, was flooding in Dunedin on the 3rd of June?
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by talbotmj15 »

Refer back through the Archive John. We were referring to the event on 18-20th June 2015. Just comparing the data from that event to this event this weekend and into Monday.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by NZstorm »

a quick look back of 18th of June 2015 shows a warm advection snow event due strong divergence over the South Island within a moist upper-level flow and undercutting cold air.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by snowstormwatcher »

Yes I remember the June 2015 event around 5cm of snow on the ground here at 100m asl in South Canterbury and 30cm in Fairlie.

Probably a bit to far out but any idea how bad the Mackenzie will be effected by this event? I have trip planned to Twizel on Monday the 31st and wondering if Brukes Pass will be closed?
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by spwill »

Chilled air sitting in inland valleys( from previous cold nights) can bring snow levels down locally especially where precipitation is heavy.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by TonyT »

snowstormwatcher wrote: Tue 25/05/2021 08:06 Probably a bit to far out but any idea how bad the Mackenzie will be effected by this event? I have trip planned to Twizel on Monday the 31st and wondering if Brukes Pass will be closed?
Would be wise to assume you won't make it.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by talbotmj15 »

Tony T. What is your take on this current system with Advection and the like?
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by darcyplumb »

Certainly an exciting few days ahead for snow loving cantabrians. These warm advection events, I find, over perform in Canterbury with the snow often falling lower than forecast.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by TonyT »

talbotmj15 wrote: Tue 25/05/2021 09:05 Tony T. What is your take on this current system with Advection and the like?
Anything could happen. (and I'm not trying to be smart). The range of possible outcomes from where we are today is very wide in my opinion. Prepare for the worst, hope for the best, whatever that means in your own context.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by talbotmj15 »

TonyT wrote: Tue 25/05/2021 09:16
talbotmj15 wrote: Tue 25/05/2021 09:05 Tony T. What is your take on this current system with Advection and the like?
Anything could happen. (and I'm not trying to be smart). The range of possible outcomes from where we are today is very wide in my opinion. Prepare for the worst, hope for the best, whatever that means in your own context.
Lol. You should a politican Tony. 🤣🤣 i won't hold ya too it. I think most of us are wondering what you look at for key drivers in advection events and things we all might be missing when looking at the data.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by TonyT »

talbotmj15 wrote: Tue 25/05/2021 09:45 Lol. You should a politican Tony. 🤣🤣 i won't hold ya too it. I think most of us are wondering what you look at for key drivers in advection events and things we all might be missing when looking at the data.
I look for consistency. And for now that isn't present. Take 50 pieces of paper with a number on them, throw them on the floor and pick up the one behind you. Thats the forecast as of today. I would suggest that whatever the models show today is unlikely to be what actually happens. The ensemble range for rainfall for Christchurch by Tuesday is 10mm to 110mm. And has been for quite some time. Based on Razor's precipitation-pessimism we might lean towards the 10mm. But thinking about how rapidly that rubber band will ping back when it goes, the 110mm is worth consideration.

You know all the key drivers and numbers for snow, they get re-hashed on here every year. As for your last comment, we never know what we are missing till after the event. But odds are that the models WILL miss something.
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Re: Mid latitude Low cold air collision. 29-31st May

Unread post by talbotmj15 »

Thanks Tony.
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