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

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

Unread post by Cyclone Tracy »

Cyclone Tracy wrote: Thu 27/05/2021 19:18 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.
Access G totally bullseyed the extremes of this event 100 hours out. Special mentions to CMC & Icon as they too kept projecting the upper totals. This is the 3rd extreme rainfall event for the South Island in the last couple of years where the fancied global models failed to project the upper extreme potential. Access G was upgraded by the Aussies in July 2019 with 15 major changes including the all important deterministic model horizontal resolution.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by snowstormwatcher »

NZ Thunderstorm Soc wrote: Mon 31/05/2021 14:18 By the way where is Geraldine forest? I've heard of Peel Forest, Talbot forest and Kakahu forest. around here.
I think there's a sign that says Geraldine forest about 10-15km from Geraldine on the highway heading towards Fairlie. its before you cross the Kakahu river.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Cantab03 »

Still very steady rain falling here, despite the easing on the radar, That very wetting thick drizzle type rain now instead. Up to 36mm here today now and 125mm for the event. Will take a wee drive around soon and see how it is all looking.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Razor »

snowstormwatcher wrote: Mon 31/05/2021 15:06
NZ Thunderstorm Soc wrote: Mon 31/05/2021 14:18 By the way where is Geraldine forest? I've heard of Peel Forest, Talbot forest and Kakahu forest. around here.
I think there's a sign that says Geraldine forest about 10-15km from Geraldine on the highway heading towards Fairlie. its before you cross the Kakahu river.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by snowchaser01 »

This never seems to stop. At work in rolleston it was heavy rain until about 2pm, now moderate with heavy bursts. Crazy event that just wont let go. Enough is enough
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Razor »

Still pouring here in Halswell, 42mm so far since 8am.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Cantab03 »

Serious issues on the roads in Halswell and HoonHay. Just came back home from parents who live in Halswell and the man made lakes are spilling on the roads, Sparks Road same issues. Gutters also failing down near Hendersons Road with water half way up peoples fences and well into garages and property. 140mm here now locally! It still clings on but I think this will ease by 6pm.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by TonyT »

Eased off here now, with another 35mm today added to the 81mm yesterday and the 55mm on Saturday. 171mm over 3 days.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Dean. »

195mm over the few days here
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Razor »

Well we all agreed it would be significant and memorable last week: For posterity from Chris Lynch's Facebook page


More evacuations required, main Canterbury roads cut off
May 31

Pines Beach
Residents of low-lying areas of The Pines Beach are advised to continue to evacuate in advance of the high tide at 9pm tonight.

Canterbury Civil Defence said residents on the higher sections of The Pines Beach can stay if they wish, but may be isolated for some time if Beach Road floods.

Waikuku and Saltwater Creek residents should prepare to evacuate - however, this isn’t mandatory yet.

The Civil Defence Centre at Rangiora Baptist Church is open for people who need somewhere to stay. If you feel unsafe at any point in time please self-evacuate here or stay with family and friends. The Mandeville Centre has now closed.

Arthur’s Pass route to the West Coast
SH73 closure now starts at Springfield and goes through to the Porter River bridge. Waka Kotahi crews from both Canterbury and the West Coast will be working to rebuild the abutments at the Porter River bridge once the river levels have dropped. “We are hoping by Thursday we will have SH73 open to a single lane in a number of places.” Shingle spread over the road near the Porter River bridge also needs to be cleared in coming days. The bridge itself has been assessed and is not damaged apart from the guardrail.

Banks Peninsula
Residents in the Akaroa harbour area are still being asked to conserve water. This is particularly critical around Duvauchelle. Residents should try and keep their water use to a minimum.

Residents in Duvauchelle should only to use water required for essential purposes.

The flooding in Akaroa has greatly receded after our contractors unblocked the major stormwater pipe

However, there is still flooding and slips in other parts of Banks Peninsula so anyone travelling in that area should exercise extreme care.

Contractors are planning to open Lake Forsyth due to the high rainfall over the last couple of days.

The opening of the lake will not result in an immediate reduction in flood levels in Little River but it will ensure the situation does not get any worse.

Akaroa highway, Banks Peninsula

SH75 the Akaroa highway is closed between Barrys Bay and Birdlings Flat.

Selwyn area
Main access routes to Leeston will remain closed tonight.

SH1 is currently closed between Burnham and Dunsandel. Leeston Road at Chamberlains Ford is also closed.

The council is urging residents to make alternative arrangements tonight if they are unable to return home.

For urgent enquiries please call 0800 SELWYN (735 996). Further updates will be available tomorrow when it is safe for contractors to make assessments.

Hurunui District Council
Gore Bay is currently isolated, due to major slips and flooding.

Gore Bay Road is closed at the Cheviot end. Cathedral Rd is closed at Hurunui mouth end. Contractors are looking at options to allow restricted access one way.

Inland Road Route 70 is closed due to flooding, south of Mt Lyford

ASHBURTON DISTRICT
4 Bridges confirmed to be damaged/completely unpassable.

Redcliffs bridge – Double Hill Run Road

Pudding Hill Bridge – Arundel Rakaia Gorge Road

Ballentynes Bridge – Lowerdowns Road

Stour River Bridge – Ashburton Gorge Road

A summary of closed highways in Canterbury as at 5pm

SH73, Springfield to Castle Hill – slips, flooding, bridge approaches washed out in three places, shingle coating the road for 100 metres and one metre thick between Lake Lyndon and Porter River.

SH77, the inland route Darfield, Windwhistle to Mt Hutt/ Winchmore – slips and flooding.

SH75 the Akaroa highway – closed from Barrys Bay to Birdlings Flat.

SH1, south of Christchurch – closed at the Selwyn River/ Waikirikiri, Dunsandel. Water 500 mm deep, should be opened tomorrow/ Tuesday.

SH1 remains closed several places through mid to south Canterbury – Hinds to Temuka, water 600 mm deep north of the Hinds River bridge.

SH77 Methven to Windwhistle and Darfield - the inland scenic route – closed due to flooding several places.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by snowchaser01 »

Looks like Christchurch will finish up with 135mm for the event... crazy numbers, and thankfully spread over a few days.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Awhituobs »

Ashburton Bridge is closed and will be inspected
Ashburton Mayor Neil Brown said people travelling over the Ashburton Bridge in vehicles were feeling some slumping in it.

“We’ve closed it for that reason until we can get engineers to inspect it.”
Ashburton District Council said there was currently no alternate route south as all other road options were closed.
not good.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Orion »

Link to Ashburton District Council May 2021 Weather Event updates page:

https://www.ashburtondc.govt.nz/news/no ... nt-updates
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Chris W »

Worrying about the bridge there.

It's just about stopped drizzling this morning in Lyttelton, yesterday it basically rained all day with some strong gusts now and then. In fact I dare say I see hints of blue through the thinning cloud! 0_o

Also other Hurunui road/bridge closures:
https://hdc.maps.arcgis.com/apps/View/i ... 6034d3bfd4
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Awhituobs »

engineers report cracks big enough to put your hand in on one of the support piles
does this mean its the long way to get south...i,e via west coast further north?
as the news articles reports other roads south are closed?
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

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

Unread post by Razor »

Chris W wrote: Tue 01/06/2021 09:53 Yes, you are correct: https://www.stuff.co.nz/national/300321 ... a-slumping
60 Km diversion up and back to the nearest bridge at Ashburton Forks, when thats open
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Awhituobs »

good that at least an alternative route south should open later today it seems
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Bradley »

Would be interesting to find out if the 150-160mm would be a close to an all-time record for the airport for a 2 day period?
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Orion »

Awhituobs wrote: Tue 01/06/2021 09:18 engineers report cracks big enough to put your hand in on one of the support piles
does this mean its the long way to get south...i,e via west coast further north?
as the news articles reports other roads south are closed?
https://www.tvnz.co.nz/one-news/new-zea ... sive-crack
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Razor »

Bradley wrote: Tue 01/06/2021 11:14 Would be interesting to find out if the 150-160mm would be a close to an all-time record for the airport for a 2 day period?
I don't have the database access but I can think of at least 3 examples that are most likely larger events for ChCh

March 2014
October 2000
Wahine storm 1968

I'm sure there would be others
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by TonyT »

Razor wrote: Tue 01/06/2021 11:53
Bradley wrote: Tue 01/06/2021 11:14 Would be interesting to find out if the 150-160mm would be a close to an all-time record for the airport for a 2 day period?
I don't have the database access but I can think of at least 3 examples that are most likely larger events for ChCh

March 2014
October 2000
Wahine storm 1968

I'm sure there would be others
There were several 100mm+ events over several days in the mid 1970s also. I doubt its a record 2 day fall, but its certainly getting up there.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by TonyT »

Bradley wrote: Tue 01/06/2021 11:14 Would be interesting to find out if the 150-160mm would be a close to an all-time record for the airport for a 2 day period?
As far as I can tell (and may be out of date), the 50 year return period rainfall for Christchurch is calculated at 153mm, the highest 48 hour total was 121mm in January 1980.

BUT - We need to be very careful when comparing numbers. These two values would be 9am to 9am totals, not a "highest of all time" consecutive 48 hour period. We would need to know the 24hr rainfall up to 9am Sunday ("Saturday's rainfall"), and the 24hr rainfall up to 9am Monday ("Sunday's rainfall") this time around to make a direct comparison. In fact we probably need to know the value measured at 9am today and make that a 72 hour total to fully capture the event. We cant take a 48 hour period centred around the heaviest period and then make a comparison to daily data. Apples vs oranges and all that.

I expect NIWA will crunch the numbers eventually.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by Nev »

TonyT wrote: Tue 01/06/2021 13:06
Bradley wrote: Tue 01/06/2021 11:14 Would be interesting to find out if the 150-160mm would be a close to an all-time record for the airport for a 2 day period?
As far as I can tell (and may be out of date), the 50 year return period rainfall for Christchurch is calculated at 153mm, the highest 48 hour total was 121mm in January 1980.

BUT - We need to be very careful when comparing numbers. These two values would be 9am to 9am totals, not a "highest of all time" consecutive 48 hour period. We would need to know the 24hr rainfall up to 9am Sunday ("Saturday's rainfall"), and the 24hr rainfall up to 9am Monday ("Sunday's rainfall") this time around to make a direct comparison. In fact we probably need to know the value measured at 9am today and make that a 72 hour total to fully capture the event. We cant take a 48 hour period centred around the heaviest period and then make a comparison to daily data. Apples vs oranges and all that.

I expect NIWA will crunch the numbers eventually.
Chch Aero recorded 138mm in the 72 hrs to 9am this morning (or 136mm for the 72 hrs to midnight).

I think Chch Aero's previous 3 day record was probably during ex-TC Giselle (Wahine Disaster) in April 1968, when it recorded 135mm for the 72 hrs to 9am on the 12th (or 138mm for the 72 hrs to 7am) - from records since 1955.
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Re: Large Rain Event Canterbury and high altitude snow. 29-31st May 2021

Unread post by tich »

Razor wrote: Tue 01/06/2021 11:53
I don't have the database access but I can think of at least 3 examples that are most likely larger events for ChCh

March 2014
October 2000
Wahine storm 1968

I'm sure there would be others
Looking at Jeff's Hoon Hay station archive, 56mm was recorded in 24 hours in October 2000 storm, which I recall brought heavier rain to southern parts of the city nearer the Port Hills than the airport. In March 2014, 44mm was recorded on the 4th and 58mm the next day, so even wetter than the latest event. (with more extensive and severe flooding throughout the city) Don't know the totals from the Wahine Storm.
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