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

[NZstorm]

I think Ski operators should take an interest in the event, could be some pockets of decent precip with it. but it does not look likely a repeat of the 2015 event to me so far.

[mikestormchaser]

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.

I very well explanation. Im not even going to bother commenting until a day or two out IF at all on these "future events" models love to tease a far out

[Chris W]

MetService on their severe weather outlook are stating a low risk of severe weather associated with this event at least for Friday and Saturday. As for Sunday and Monday they are out of range for that outlook at present.

[Cyclone Tracy]

Latest water vapour image now showing the upper trough is on the move north eastward out of the mothership upper low over the Antarctic continent. It will cut off from the mothership over the Tasman on Saturday morning NZT.

[spwill]

MetService on their severe weather outlook are stating a low risk of severe weather associated with this event at least for Friday and Saturday. As for Sunday and Monday they are out of range for that outlook at present.

They said minimal risk of severe weather Friday/Saturday.

[Chris W]

Yes, that's what I said

[wembley]

That's the thing with these sorts of forecasts, it does not firm up until much closer to the event.

[NZ Thunderstorm Soc]

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.

Thanks for that.
I recorded some sleety showers with a sprinkling of snow along the Port hills on the 22nd but nothing much else to make it a significant event.

[Dean.]

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.

Thanks for that.
I recorded some sleety showers with a sprinkling of snow along the Port hills on the 22nd but nothing much else to make it a significant event.

Had around 10-12cm here in town on that date

[TonyT]

The ensemble range for rainfall for Christchurch by Tuesday is 10mm to 110mm.

0Z ECMWF ensemble has a range of 8 to 170mm, with the control run at about 120mm and ensemble mean at about 60mm.

[Cantab03]

Have been watching the models over the last few days around this one. The last 2 or 3 runs seem to be finding a little more consistency now, with the bulk of the 'heavy rain' looking to be from about Amberley north, especially about the Kaikoura Coast. EC and GFS slowly heading more and more this way. Still time to change and with so many contributing factors with this one, I feel we will have to wait until at least tomorrow evening to get a real picture of what we can expect to see with this one.

One thing for sure though, this will be Christchurch's wettest day of the year and finally we will hit triple figures for our YTD rainfall!

[Chris W]

MS have now gone with moderate confidence of heavy rain for North-mid Canterbury and Marlborough for Sunday.

2021-05-26 15_46_54.png

[talbotmj15]

Man what a day with models ay. Talk about moving around. Colder air coming back in. Advection now looking very likely above about 500m. Huge amounts of moisture. Currently looks about Waimakiriri rive north but theres so many things going on. Rainfalls numbers heading towards 150-200mm for some over just a 48 hour period. Will be some fairly severe flooding for many i suspect.

WW calling snow of 1-5cm to just a few hundred metres. 40cm plus for the passes.

[wembley]

The Met Service 3 day rainfall looks like this:

https://www.metservice.com/maps-radar/r ... ast/3-days

[Orion]

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

[talbotmj15]

Im really interested in Mr CTs thoughts on this incoming setup as it looks about as complex as they can get. We are now on 100hours out.

[TonyT]

Today's 0Z ECMWF ensemble run has a mean of 63mm for Christchurch by Monday night, with a range of 12-140mm, and a control run of 82mm.

[snowchaser01]

Metservice starting to latch on to a major rain event for the plains and eastern coast, from about Ashburton northwards. Snow level will probably be quite high, but hopefully good for the ski fields. Upwards of 150-200mm possible for parts of the plains from Saturday afternoon through to Monday late morning, i expect a range of warnings and watches to be issued by Metservice tomorrow.

[snowchaser01]

Just to add, starting to notice consistency between the ECMWF, GFS and ICON models... all going for truly missive numbers in terms of rainfall for the Canterbury Plains. Also all picking a similar feed zone for the main band of moisture on Sunday, although if I'm honest it looks like most of Canterbury will be impacted one way or another.

[TonyT]

Today's 0Z ECMWF ensemble run has a mean of 63mm for Christchurch by Monday night, with a range of 12-140mm, and a control run of 82mm.

12Z run has a range of 25-170mm, and the model mean has moved sharply upwards to 90mm, with the control run at 98mm. So not only is there a general push towards higher rainfall totals with this guidance, but the increase in the mean suggests more of the 50 ensemble members are tracking towards the high end of the range. Local councils need to be taking this event seriously as flooding will happen somewhere.

[snowchaser01]

Today's 0Z ECMWF ensemble run has a mean of 63mm for Christchurch by Monday night, with a range of 12-140mm, and a control run of 82mm.

12Z run has a range of 25-170mm, and the model mean has moved sharply upwards to 90mm, with the control run at 98mm. So not only is there a general push towards higher rainfall totals with this guidance, but the increase in the mean suggests more of the 50 ensemble members are tracking towards the high end of the range. Local councils need to be taking this event seriously as flooding will happen somewhere.

Thanks Tony. If I am understanding you correctly, the high end of the ensembles being 170mm, the majority of ensemble members are calling for totals in the city close to that? If that happens there will be major flooding problems?

[TonyT]

12Z run has a range of 25-170mm, and the model mean has moved sharply upwards to 90mm, with the control run at 98mm. So not only is there a general push towards higher rainfall totals with this guidance, but the increase in the mean suggests more of the 50 ensemble members are tracking towards the high end of the range. Local councils need to be taking this event seriously as flooding will happen somewhere.

Thanks Tony. If I am understanding you correctly, the high end of the ensembles being 170mm, more than 50 ensemble members are calling for totals in the city close to that? If that happens there will be major flooding problems?

No, there are 50 ensemble members. The lowest of those 50 is 25mm, the highest is 170mm. The mean of them all is 90mm. So you would consider 90mm the most central number (statistically speaking, based on the dataset generated by the ensemble). There is a 50% chance the rainfall total will be under 90mm, and a 50% chance it will exceed that.

My point was that compared to the previous model run where the mean was 63mm, there must be quite a few of the 50 individual values running in the 100mm+ range now, to pull that mean up by so much. 100mm for the city would cause issues with surface flooding, especially at this time of year (high tides, leaves in drainage systems, dry sub-surface so rapid runoff etc etc).

[snowchaser01]

Thanks Tony. If I am understanding you correctly, the high end of the ensembles being 170mm, more than 50 ensemble members are calling for totals in the city close to that? If that happens there will be major flooding problems?

No, there are 50 ensemble members. The lowest of those 50 is 25mm, the highest is 170mm. The mean of them all is 90mm. So you would consider 90mm the most central number (statistically speaking, based on the dataset generated by the ensemble). There is a 50% chance the rainfall total will be under 90mm, and a 50% chance it will exceed that.

My point was that compared to the previous model run where the mean was 63mm, there must be quite a few of the 50 individual values running in the 100mm+ range now, to pull that mean up by so much. 100mm for the city would cause issues with surface flooding, especially at this time of year (high tides, leaves in drainage systems, dry sub-surface so rapid runoff etc etc).

Understood, thanks. Agreed in regards to flooding being an issue with tides and leaves. Do you think the lack of rain so far this year will also play a part in that?

[TonyT]

Do you think the lack of rain so far this year will also play a part in that?

Thats what I meant by dry sub-surface being a factor.

[snowchaser01]

Do you think the lack of rain so far this year will also play a part in that?

Thats what I meant by dry sub-surface being a factor.

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