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

[wembley]

I don't want to jinx it but it looks like the lower South Island will be getting off fairly lightly if the MS website's 3 Day Rain forecast is to be believed.

[Orion]

Am supposed to travel to Christchurch for the day on Sunday: should I abandon the idea?

Acting on information received, I have abandoned the idea.

Now to await developments

[TonyT]

Thanks for the insight Tony! Regression to the mean was bound to occur at some point...

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[Cyclone Tracy]

If this GFS 2605 12Z scenario comes off, a fully bona fide cold core vortex will develop near Cook strait. Water bomb for Canterbury.

[snowchaser01]

If this GFS 2605 12Z scenario comes off, a fully bona fide cold core vortex will develop near Cook strait. Water bomb for Canterbury.

CT, as always your knowledge is legendary. Do you think the main models are picking up on this currently? In terms of the rainfall amounts being thrown around...

[talbotmj15]

Before i go completely mad what surface temps do we need for advection to allow snow? I remember snow fall heavily and settled at 3deg at 300m. Castle hill is showing 3deg for most of Sunday yet temps at surface are only 3deg.

[Chris W]

Severe weather watch for heavy rain issued from 11am Saturday through 11am Monday, currently covering all of Canterbury except the Kaikoura district north of Clarence:

https://www.metservice.com/warnings/home

[snowchaser01]

Severe weather watch for heavy rain issued from 11am Saturday through 11am Monday, currently covering all of Canterbury except the Kaikoura district north of Clarence:

https://www.metservice.com/warnings/home

And mentioning heavy high country snow... I don't think they would mention this unless it was going to impact at least some population or roads...

[NZstorm]

Before i go completely mad what surface temps do we need for advection to allow snow? I remember snowfall heavily and settled at 3deg at 300m. Castle hill is showing 3deg for most of Sunday yet temps at surface are only 3deg.

Keep an eye on the 850mb temperatures. The 850mb temperature is typically around 1300m level in the atmosphere. Anything close to 0C at that level may indicate snow potential to below 1000m. But every set-up has its own dynamics so no hard rules.

[wembley]

The Central Otago forecast has a passing showers icon for Saturday and yet says "Occasional Rain".

[Flutterbye]

The Central Otago forecast has a passing showers icon for Saturday and yet says "Occasional Rain".

Metservice are inconsistent when it comes to the use of the icons. But, you can almost guarantee that the worst case will be used for some areas but not for others when the forecast has the exact same wording!

[dogmelon]

So we will get "snow rain" in the city at least. Yay

[TonyT]

So we will get "snow rain" in the city at least. Yay

Unless you live in the tropics all rain starts life as snow...

[Cyclone Tracy]

If this GFS 2605 12Z scenario comes off, a fully bona fide cold core vortex will develop near Cook strait. Water bomb for Canterbury.

CT, as always your knowledge is legendary. Do you think the main models are picking up on this currently? In terms of the rainfall amounts being thrown around...

It's looking like an intense moisture flux convergence event associated with the topography. If the cold core cut off low stalls between Pegasus Bay and Cook strait, this is a red warning for quite a few areas of Canterbury and Marlborough. I don't think MS would pull that trigger until after they see model runs of 28 May 12z.

[Cyclone Tracy]

Interesting to watch the other global models that have well outperformed EC, UK & GFS over last 2 years for extreme rainfall events on the west coast. Icon, CMC and Access showing half a metre bullseye's on the 00z runs. Quite a bit of snow in that as well.

[talbotmj15]

Thats some serious rain CT. You mention quite alot of snow. But to what levels do you see snow fall? Also how much of this event do you see as advection?

[wembley]

Also what do you make of the hourly observations here? It doesn't show (at least) Alexandra getting rain until later Saturday or Sunday.

https://www.metservice.com/towns-cities ... /alexandra

[tich]

Looking for a historical comparison, one could go back to August 2000, when a very damp southeasterly flow affected Canterbury for about 3 days. I remember persistent but not heavy rain in Chch, but inland 24 hour falls over 150mm were recorded near the foothills. (240mm in Lees Valley) Snow fell heavily above about 500m in South Canterbury, but 1000m+ further north.

[tgsnoopy]

Thats some serious rain CT. You mention quite alot of snow. But to what levels do you see snow fall? Also how much of this event do you see as advection?

Interesting.I know squat about meteorology compared to most in here. Whilst it might start out as rain, transition to snow, resume to rain (which melts the snow), there is potentially still a risk a freezing air temperature may result in ice. A low risk I'd expect, that this may result in very dangerous driving conditions on roads etc. Some of the precipitation figures are huge & considering with an advection event that equates to 10 times that in snow, it looks like it may be an interesting weekend. I'm now considering heading down for this. Hope the boss is in a good mood tomorrow morning.

[Chris W]

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MS zeroing in on the warning area. Looks to start midday Saturday and really crank through the night and Sunday.

The rivers will be all be going some.

[NZ Thunderstorm Soc]

Looking for a historical comparison, one could go back to August 2000, when a very damp southeasterly flow affected Canterbury for about 3 days. I remember persistent but not heavy rain in Chch, but inland 24 hour falls over 150mm were recorded near the foothills. (240mm in Lees Valley) Snow fell heavily above about 500m in South Canterbury, but 1000m+ further north.

I remember that event, Ben. 17th to the 20th of August. I recorded 46mm on the Friday. Flooding from the Selwyn and Ashley Rivers. The town of Irwell near Leeston was under flood threat level.

[snowchaser01]

EC, GFS and ICON all pretty much on the same page in this morning's runs. If anything ICON has the most moisture overall, EC least and GFS right down the middle. Who is going to win this one?

[Chris W]

1C and a little cold rain on the way to Amberley this morning, but otherwise not too bad a morning. MS rain chart (UKMO) and last night's GFS not looking amazing for Mackenzie but trending towards a bit less rain for Kaikoura.

[snowchaser01]

1C and a little cold rain on the way to Amberley this morning, but otherwise not too bad a morning. MS rain chart (UKMO) and last night's GFS not looking amazing for Mackenzie but trending towards a bit less rain for Kaikoura.

Yes the general consensus seems to be a river of moisture between about Culverden and and Methven, with the main feed dropping slightly south for a time on Sunday. Inland mid Canterbury could easily see 250mm+

[einzack]

If it's as wet in Chch as the hourly forecast is currently showing up until Sunday 8am (that's as far as it goes currently) we could be in for a lot of trouble.