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

[TonyT]

Now we are getting closer the ECMWF ensemble seems to be losing the more extreme high numbers, but the mean remains significant. The 12Z run has a range for Christchurch by midnight Monday of 18-115mm, with the mean around 73mm, and the control run a bit of an outlier at 108mm. The operational ECMWF has the total at 95mm.

A few of the ensemble members want to bring in a second event about a week later with another 50-100mm, but the control run doesn't see this and the mean rises only slightly at that time.

[JP.]

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.

I remember March 3rd to 5th 2014 a south westerly system dumped over 100mm in chch city and it was bedlam. I think this system has the potential to dish out more rain than that. I dug up this old press article stating a "one in one hundred year storm"

http://www.stuff.co.nz/national/9790570 ... year-storm

[Bradley]

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.

I remember March 3rd to 5th 2014 a south westerly system dumped over 100mm in chch city and it was bedlam. I think this system has the potential to dish out more rain than that. I dug up this old press article stating a "one in one hundred year storm"

http://www.stuff.co.nz/national/9790570 ... year-storm

Must be climate change.

[Chris W]

Worth a mention that there's been a lot of work done on drainage since that 2014 series of floods and also the March 3rd event was an intense depression in the Canterbury Bight with S/SW winds that dropped 150mm+ in 24 hours (see the below-linked tweets). While we could get a lot of rain I am not seeing that this is the same type or level of event for Christchurch.

https://twitter.com/MetService/status/4 ... 7757808640
https://twitter.com/MetService/status/4 ... 8417579008

[Razor]

Worth a mention that there's been a lot of work done on drainage since that 2014 series of floods and also the March 3rd event was an intense depression in the Canterbury Bight with S/SW winds that dropped 150mm+ in 24 hours (see the below-linked tweets). While we could get a lot of rain I am not seeing that this is the same type or level of event for Christchurch.

If you are referring to the Heathcote catchment, that work is not yet complete. Coupled with the concrete level soil profile right now it will only take 2 or 3 hours of heavy rain to cause major issues for us I fear

[TonyT]

Worth a mention that there's been a lot of work done on drainage since that 2014 series of floods and also the March 3rd event was an intense depression in the Canterbury Bight with S/SW winds that dropped 150mm+ in 24 hours (see the below-linked tweets). While we could get a lot of rain I am not seeing that this is the same type or level of event for Christchurch.

If you are referring to the Heathcote catchment, that work is not yet complete. Coupled with the concrete level soil profile right now it will only take 2 or 3 hours of heavy rain to cause major issues for us I fear

I'm inclined to agree. The only saving grace for this event is that it looks likely to be strung out over 48+ hours and not concentrated into 24. Other factors are not as helpful (tides, dry subsurface, time of year etc).

I suspect we may look back on this as a rather historic event in the region, remembered for a long time.

[Cyclone Tracy]

Water vapour at 10:30am showing the polar pipe from over the Antarctic continent is injecting at full throttle towards the Tasman region today. 24 hours before it cuts off.

[Chris W]

Fair points both Razor and Tony, though the tides in 2014 certainly caused their share of issues and this one looks less intense time-wise for the city than 2014 was. It does look possible from the Metservice maps that we could get some NE/SE convergence Sunday morning over the city but I wonder about shadowing by Banks Peninsula from the moisture feed.

Inland I wholeheartedly agree Tony, it looks very significant.

[darcyplumb]

Metservice code red heavy rain warnings just issued for Canterbury south of Ashley. 200-300mm for the foothills and 80-120mm for the plains. I feel this will be a significant event.

[Chris W]

That warning text:

Heavy Rain Warning

This rain is expected to cause dangerous river conditions and significant flooding. Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.

Area: Canterbury south of Ashley
Valid: 44 hours from 3:00pm Saturday to 11:00am Monday
Forecast: Expect 200 to 300mm of rain to accumulate about the Canterbury High Country and foothills during this time, with 80 to 120mm of rain farther east about the Plains and coast. Peak hourly rates of 10 to 20mm/h are expected, especially from later Saturday and during Sunday. Also note, the freezing level is forecast to be around 1400 metres on Saturday, then lifting to around 1800 metres on Sunday, before lowering to 1400 metres again on Monday. Heavy Snow is expected above the freezing level throughout the event, but heavy snow may fall as low as 1000 metres in some places at times, especially on Saturday and on Monday.
Changes: Upgraded from a Watch to a Warning (Red)

Stark language. So yes 80-120mm for the city would be significant and I'll backtrack there, but 2-300mm for the high country is huge.

[darcyplumb]

That warning text:

Heavy Rain Warning

This rain is expected to cause dangerous river conditions and significant flooding. Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.

Area: Canterbury south of Ashley
Valid: 44 hours from 3:00pm Saturday to 11:00am Monday
Forecast: Expect 200 to 300mm of rain to accumulate about the Canterbury High Country and foothills during this time, with 80 to 120mm of rain farther east about the Plains and coast. Peak hourly rates of 10 to 20mm/h are expected, especially from later Saturday and during Sunday. Also note, the freezing level is forecast to be around 1400 metres on Saturday, then lifting to around 1800 metres on Sunday, before lowering to 1400 metres again on Monday. Heavy Snow is expected above the freezing level throughout the event, but heavy snow may fall as low as 1000 metres in some places at times, especially on Saturday and on Monday.
Changes: Upgraded from a Watch to a Warning (Red)

Stark language. So yes 80-120mm for the city would be significant and I'll backtrack there, but 2-300mm for the high country is huge.

I should have added the warning text but I’m too lazy for that. You’re right about the language used. Certainly very serious considering we are still a good 24-30 hours out. There’s certainly a lot of complacency happening. I feel people aren’t taking this event seriously at all.

[talbotmj15]

It now appears the air temps with this event are alot lower than originally noted. GFS has thickness levels of around 530hpa for Sunday and with advection thats surely snow to lower levels than 800m??

[mikestormchaser]

Certainly a much needed rain event for Canterbury coming in from tomorrow evening! Very nice to see

[TonyT]

Certainly a much needed rain event for Canterbury coming in from tomorrow evening! Very nice to see

The rain is needed, but I'm not sure anyone should be looking forward to this particular event.

[JP.]

Certainly a much needed rain event for Canterbury coming in from tomorrow evening! Very nice to see

The rain is needed, but I'm not sure anyone should be looking forward to this particular event.

Indeed Tony, I'm worried for lower flood prone and river areas of Christchurch as well. I can't recall the terminology/wording but when we have been so dry for so long, really heavy rain events don't soak in and can cause more intense flooding

[TonyT]

Looking more and more like a Mid to South Canterbury event Saturday into Sunday, then shifting to a North Canterbury focus for Sunday into Monday. Christchurch sandwiched in the middle means either not really much affected by either extreme, or being the pivot point and under the heavy weather for longer. Hmmm.

[Razor]

Fun fact its only the second Code Red the Metservice has issued

[darcyplumb]

Fun fact its only the second Code Red the Metservice has issued

The other one was for the fox glacier area I believe?

[Razor]

Fun fact its only the second Code Red the Metservice has issued

The other one was for the fox glacier area I believe?

Feb 2020 Fiordland- the storm that destroyed the Milford Track, Milford Road and obliterated the Hollyford Camp

https://www.nzherald.co.nz/nz/state-of- ... PYTW3MJJM/

[darcyplumb]

The other one was for the fox glacier area I believe?

Feb 2020 Fiordland- the storm that destroyed the Milford Track, Milford Road and obliterated the Hollyford Camp

https://www.nzherald.co.nz/nz/state-of- ... PYTW3MJJM/

Ah, I forgot about this event. It seems anything that happened from Jan-Mar 2020 has be lost in the pre covid blur.

[Cantab03]

Latest GFS and ICON models not long updated are still showing very similar numbers to this morning, will be interesting to see what EC shows when it updates soon. If we are still seeing these sorts of numbers tomorrow morning then it is going to be a very interesting 48 hours to see how the Heathcote River in particular copes. As mentioned, there has been some work done on this, but it still struggles with more than 30-50mm in a day.

[Razor]

MS appear to be scaling back their rain amounts for ChCh. Good news if that holds

[snowchaser01]

MS appear to be scaling back their rain amounts for ChCh. Good news if that holds

In my opinion it won't hold.

[TonyT]

Now we are getting closer the ECMWF ensemble seems to be losing the more extreme high numbers, but the mean remains significant. The 12Z run has a range for Christchurch by midnight Monday of 18-115mm, with the mean around 73mm, and the control run a bit of an outlier at 108mm. The operational ECMWF has the total at 95mm.

A few of the ensemble members want to bring in a second event about a week later with another 50-100mm, but the control run doesn't see this and the mean rises only slightly at that time.

OZ ensemble is not a lot changed on mean and control, but the low end boundary has moved sharply higher, seemingly removing the chance that this could be only a small value event. Range (for Christchurch by midnight Monday) 42mm to 130mm, ensemble mean 79mm, control run 102mm.

The week later event seems to have evaporated.

Still looking at 150mm plus for almost of Canterbury above about 150m altitude, with 200mm plus for some.

[Cyclone Tracy]

Looks like a mid latitude cyclone induced nocturnal low-level jet is developing along the occluded front during early Sunday morning. It will be running at about 30 to 40km/h faster than at 700 hPa over the South Pacific. The potential of this system is wild if it stalls.