Rapidly Deepening Low and Possible Snow Event July 20-22nd

[Razor]

Severe weather warning issued! For canterbury however, they mention the rain and snow to 800m? Yet on their plains rural forecast it still says 300m... hmm

The rural ones rely more on the modelling, no? I would take the Warning as their confirmed expectations?

[snowchaser01]

Severe weather warning issued! For canterbury however, they mention the rain and snow to 800m? Yet on their plains rural forecast it still says 300m... hmm

The rural ones rely more on the modelling, no? I would take the Warning as their confirmed expectations?

Rural forecasts are still moderated by forecasts i believe... in saying that they havent been updated today yet. I think 800m seems a bit high, yet 300m seems a tad low... I think 500-600m is the mark.

[melja]

Miss information is a bad thing tho

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[TonyT]

Severe weather warning issued! For canterbury however, they mention the rain and snow to 800m? Yet on their plains rural forecast it still says 300m... hmm

They are all over the place. Their forecasts say 300m, their spokesman quoted on Stuff says "400 to 500m"" and their warning says 800m. There is no consistency at all. And they claim to be the only organisation capable of providing a credible warning service...

[snowchaser01]

Miss information is a bad thing tho

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Yup, gotta go one way or the other.

[snowchaser01]

Severe weather warning issued! For canterbury however, they mention the rain and snow to 800m? Yet on their plains rural forecast it still says 300m... hmm

They are all over the place. Their forecasts say 300m, their spokesman quoted on Stuff says "400 to 500m"" and their warning says 800m. There is no consistency at all. And they claim to be the only organisation capable of providing a credible warning service...

Savage... but so true! Have your thoughts changed Tony?

[mikestormchaser]

Models plotting most of the moisture south Canterbury/eastern Otago now. Chch to rakaia looks a bit shelterd after the rain clears at midday Fri.. still be lighter rain showers around though. But Dunedin looks in firering linr

[TonyT]

Savage... but so true! Have your thoughts changed Tony?

Niwa is no better. In yesterday's Press Brandolino was quoted as 100mm for Christchurch, today he's quoted at 25-35mm. Really? This is not forecasting, its (over)reacting to model output.

My thoughts have not changed much at all. I still expect the worst to be between Dunedin and Christchurch, with North Otago and South Canty mainly in the firing line. I can't see the snow accumulating below 500m, and I'm even starting to wonder if that is too low. Gales in that zone also. The issue will be how local environments cope with 50-100mm of rain. We know from past events that 50-70mm in the Dunedin area causes problems, and that 80-100mm in the South Canty catchments results in flooding. We also know that in the past a rapid 30mm in Chch causes surface flooding, but the CCC seem to be saying they have upgraded their drainage so we are kind of in unknown territory with that.

A big issue with this storm is not just the total rainfall expected, but the intensity. I'm seeing 25-30mm in consecutive six hour periods in some of the data and thats not picking the extreme value but reasonably widespread over South Canterbury and North Otago. So even if totals fall short of 100mm the impact in 12 hours will be substantial.

[einzack]

Yeah, Dunedin and North Otago looks like we'll be in some serious strife.
100-150mm is a huge amount for here, and whenever that's happened recently, its caused big problems.
Hope South Dunedin copes ok. For the rest of us it'll be disruptive, but low potential to be destructive, but unfortunately for those living in South Dunedin, it's far more serious.

[Bradley]

Savage... but so true! Have your thoughts changed Tony?

Niwa is no better. In yesterday's Press Brandolino was quoted as 100mm for Christchurch, today he's quoted at 25-35mm. Really? This is not forecasting, its (over)reacting to model output.

My thoughts have not changed much at all. I still expect the worst to be between Dunedin and Christchurch, with North Otago and South Canty mainly in the firing line. I can't see the snow accumulating below 500m, and I'm even starting to wonder if that is too low. Gales in that zone also. The issue will be how local environments cope with 50-100mm of rain. We know from past events that 50-70mm in the Dunedin area causes problems, and that 80-100mm in the South Canty catchments results in flooding. We also know that in the past a rapid 30mm in Chch causes surface flooding, but the CCC seem to be saying they have upgraded their drainage so we are kind of in unknown territory with that.

A big issue with this storm is not just the total rainfall expected, but the intensity. I'm seeing 25-30mm in consecutive six hour periods in some of the data and thats not picking the extreme value but reasonably widespread over South Canterbury and North Otago. So even if totals fall short of 100mm the impact in 12 hours will be substantial.

And yet for Canterbury high country updated only 20 minutes ago (see below) they mention snow down to 400m - very confusing!

Rain with possible heavy falls, and snow lowering to 400 metres overnight.

[melja]

And there warning only say Canterbury with no mention of what parts- very long geographical area.

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[Bradley]

Latest Canterbury Plains update now has no snow forecast for Friday at all but now some for Saturday:

Rain, possibly heavy at first, gradually clearing from the south in the evening. Snow down to about 300 metres for a time in the afternoon.Strong southwesterlies, gale in exposed places, easing at night.

[melja]

8 minutes after I post they change there warning, bit embarrassing taking ques from social media.

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[moylanr]

Savage... but so true! Have your thoughts changed Tony?

Niwa is no better. In yesterday's Press Brandolino was quoted as 100mm for Christchurch, today he's quoted at 25-35mm. Really? This is not forecasting, its (over)reacting to model output.

1) Media quotes don't necessarily represent what was said (I know some of you know that)

2) The "Links v" dropdown menu for forecasts at the top of this page goes to MetService & Niwa (labeled Blue Skies, but still linked to Farmlands which shows Niwa).

It would be so much better to have a useful link, eg one that went to Blue Skies.

[RichyDunedin]

The warning for Dunedin is expecting 150mm to 200mm in an 18 hour period. That is the highest I have seen there before and will cause huge issues in South Dunedin.

[harleyb]

See below, just saw a really detailed explanation from the Metservice of how snow could be falling to 300m despite the lack of cold air (850hpa temps above 0C) intrusion from this event, I guess this is why these guys went to University lol

Here is a tephigram over Fairlie at 6pm tomorrow. Isotherms run diagonally, the blue line is 0C and the red -15C. Despite the warmer air aloft (as you say), it is plenty saturated in the 'dendritic zone' - upper set of light blue lines either side of -15C. This is a great snow making set-up, producing enough snow for plenty to fall through the above-zero layer without melting, and able to reach the surface.

So what are the horizontal lines that have 150 and higher as you head down the chart

Atmospheric pressure, in hPa

[David]

Pressure sure is dropping fast here today:

[Storm Struck]

The models this morning were going for 20-25mm for Christchurch surrounding and northwards, the latest has up the totals to around 80-90mm thats surface GFS I mean.
Majority falling between 3am and midday tomorrow, of course there will be a sheltering for the city so my pick is around 50mm max for the city but just inland and south could see alot more.

[Bradley]

Bit of a worry when they are now forecasting the low to deepen to 968hpa on Friday afternoon - I can't remember a low that deep of the east coast before?

[petem8nz]

2) The "Links v" dropdown menu for forecasts at the top of this page goes to MetService & Niwa (labeled Blue Skies, but still linked to Farmlands which shows Niwa).

It would be so much better to have a useful link, eg one that went to Blue Skies.

I'm pretty sure Blue Skies is a primarily a paid service and now Farmland are using Niwa. I think we're lucky to get Tonys input here for zip.. The Blue Skies facebook page would be your best bet https://www.facebook.com/BlueSkiesWeather/

[moylanr]

I'm pretty sure Blue Skies is a primarily a paid service and now Farmland are using Niwa. I think we're lucky to get Tonys input here for zip.. The Blue Skies facebook page would be your best bet https://www.facebook.com/BlueSkiesWeather/

Agreed!

Can the link labeled 'Blue Skies' be changed to go to http://www.facebook.com/BlueSkiesWeather?

[Nev]

Can the link labeled 'Blue Skies' be changed to go to http://www.facebook.com/BlueSkiesWeather?

Something only Admin can do unfortunately.
So best to discuss this sort of stuff in the 'Forum Talk' topic in the 'Off Topic' section.

[snowchaser01]

I think its safe to say that for anyone below 500m this is primarily a rain/wind event. With some big totals promising. SNow above 500m, could cause problems through much of the Makenzie District. Tekapo in the firing line i feel, and all skifields should receive upwards of 80cm!!

[Bradley]

The latest run of GFS is putting a good portion of the rain over Canterbury on Saturday morning and afternoon, Coming as almost a second wave. This is is interesting as it also coincides with the arrival of the colder air, if it develops as a warm advection scenario then these colder 850hpa temps of around -3C will certainly lay drop the snow level down to 200-300m...

[harleyb]

Auckland has dropped from 1012 hPa to 988 hPa in under 24 hours -- guess that makes it "bomb" territory but don't tell the media...