First Major Winter Storm of 2019

[Razor]

Whats wrong with that?

How can you measure how much falls without it accumulating?

Take your point, but I suppose 2cm is little if any

[snowchaser01]

metservice have again updated... CHCH rural mentions snow to 600m on Banks Peninsula.. about 200-300m too high IMO

[snowchaser01]

What are Metservice seeing that I am not... heavy rain low confidence for Canterbury in their outlook, and heavy snow above 800m?!

[Bradley]

I think we may all be missing something here, Metservice just released their severe weather outlook and they are not interested or even entertaining the thought of low level snow. If the experts are only talking about "heavy snow possible above 800m" and not even mentioning any snow below that level then there is obviously a missing ingredient somewhere?

"On Saturday, there is low confidence of significant amounts of southerly rain falling about north Canterbury including Banks Peninsula and the Kaikoura Coast and ranges, with heavy snow possible above about 800 metres"

[snowchaser01]

I think we may all be missing something here, Metservice just released their severe weather outlook and they are not interested or even entertaining the thought of low level snow. If the experts are only talking about "heavy snow possible above 800m" and not even mentioning any snow below that level then there is obviously a missing ingredient somewhere?

"On Saturday, there is low confidence of significant amounts of southerly rain falling about north Canterbury including Banks Peninsula and the Kaikoura Coast and ranges, with heavy snow possible above about 800 metres"

It's not out of the question that they are missing something... but yes it seems odd that they are saying 800m.... even without any of the other factors... the air is cold enough for snow to about 400m?

[Bradley]

It's actually pretty disappointing to see Metservice still forecasting "snow to all levels" in their forecast for Culverden (190m asl) on Saturday but not even mentioning any snow below 800m in their severe weather outlook - which one is it? I just wish they could be a little more consistent in their forecasts...

[snowchaser01]

It's actually pretty disappointing to see Metservice still forecasting "snow to all levels" in their forecast for Culverden (190m asl) on Saturday but not even mentioning any snow below 800m in their severe weather outlook - which one is it? I just wish they could be a little more consistent in their forecasts...

And in their latest facebook post... they say snow to 500-600m over the south island and 800m in the north island?? what is going on...

[Chris W]

MS have added a low risk rain outlook from Chch northward and inland, right call IMO at this stage pending tonight's and tomorrow's modelling.

[talbotmj15]

Dont want to be a knocker of MS but they need to address there communication skills when forecasting events. To have on 3 different platforms 3 different forecasts in relation to snow is very ambiguous. Clearly they are seeing something we are all missing given every model either GFS or EC has some form of snow to 300m with EC near sea level. The models are guidance but when all the most reliable models have snow to as low as 300m surely 1 can assume with some knowledge that snow of 5-10cm is possible to 400m? If so where does MS get there 800m from?

[TonyT]

As promised this morning, here is a peak at some of the ensemble output which is available from the models, to illustrate how wide a range of outcomes are possible from one model being run multiple times with a nearly identical dataset. I dont want to write pages and pages of explanation as to how to read these charts, if you can't work it out then sorry, but its probably not for you.

By way of example, look at the two snow charts, one is ECMWF with 50 individual members ("model runs") and the other GFS with 20. On the ECMWF chart you can see 6 of the 50 members bring snow into Chch, and the rest dont. The amounts are small (all under 0.5cm), and the days they bring the snow vary (most are this coming Saturday or Sunday, but one is next Friday). The GFS chart has 7 of 20 members bringing snow to Chch, but none this weekend, one is next Tuesday, and the rest are next Saturday or later.

ens snow.jpeg

GFS ens snow.jpeg

You can also compare the ECMWF precipitation (QPF) members - which range from 7.5mm to 86mm by Thursday 13th June. The blue control run line is essentially the ECMWF model output which you can see on a number of web sites, but is just one run of the model. The green line is the mean of all the members.

ens qpf.jpeg

You can also compare the precipitation spread across the GFS and ECMWF ensembles (the graphs with the stock market like high/low bars). On these the blue bars show you the range of outcomes for total precipitation for each time step (the range being across 50 members for ECMWF, and 20 members for GFS). The green line is mean of the members, and as you can see its remarkably similar across the two models. However, the black line is the control run (i.e. the one you see when you look this data up on the free-to-use web sites, and which gets often quoted on this forum). Look how different they are from each other, and from the members of the ensembles which they helped generate.

looking at the GFS chart, you can see the control run is consistently lower than the ensemble mean, and in fact falls out of the expected range by the end of 14 days. So we can conclude that perhaps this GFS control run was on the low side for what the GFS might be thinking is going to happen. By the end of the 15 days the ensemble is saying total precipitation could be anywhere between about 25mm and 90mm, and most likely between about 38mm and 70mm. The control run is at 28/29mm.

GFS ens qpf.jpeg

Contrast that with the ECMWF, and see how the black line rises well above the green (mean) line after Wednesday, and in fact is right at the top of the spread from then through to the end of the following week. So we can conclude that the ECMWF control run is in the middle of the ensemble spread till about Wednesday, but the Wednesday event portrayed on the control run is likely at the very top of the possible range. BY the end of the 15 days for this model the total could be anywhere between 7mm and 85mm, and most likely between 35mm and 60mm. The control run is at about 83mm.

ens qpf spread.jpeg

I hope this helps you understand how wide a range of outcomes is possible when only very small changes are introduced into the models run from the same input data. Again, if you can't follow the graphs dont worry, its not easy and requires a bit of statistical experience, just trust me that models produce a wider range of outputs than you might think.

[snowchaser01]

You're a living legend Tony. Thanks for all of that.

[snowchaser01]

Also, for the first time in about a week... EC and GFS are in agreement!!

[Blair]

Hey Guy's, slightly off topic here, but we are heading out to have a look at this weather event on Saturday morning for the day. Not totally sure if we head to Tekapo, then on up to the Macaulay Hut or Arthurs Pass?? I'm bassed around South Canterbury area. Any advice on to which area to head to would be fantastic. Thanks Blair

[TonyT]

Hey Guy's, slightly off topic here, but we are heading out to have a look at this weather event on Saturday morning for the day. Not totally sure if we head to Tekapo, then on up to the Macaulay Hut or Arthurs Pass?? I'm bassed around South Canterbury area. Any advice on to which area to head to would be fantastic. Thanks Blair

I guess it depends whether you want to get snowed in till Sunday or to be able to drive home again on Saturday. Eastern foothills will give the maximum chance of getting stuck, if thats what you want. Wherever you go make sure it has a pub with plenty of firewood and take a sleeping bag in the car.

[talbotmj15]

Nice Tony. Feeling more confident now for snow to 300m IMO. Both GFS and EC as well as others all picking between 1-10cm so reading between the lines with other data and my brain moderating it all i can see 5-7cm to 300m where the heaviest moisture falls and providing the Low holds it track.

What im curious about Tony is the speed of the Lows Deepening out East which i believe CT touched on. Is there some kind of Advection element to this or am i way off? Doing some old research i found there are some similarities to the 2006 snow event which everyone seemed to miss to some degree as the low deepened so quickly which ramped up the moisture in a coveyerbelt like manner that many didnt see happening.

Given you and CT have argubably the best knowledge you guys may be able to shed some light on this?

[Razor]

Bravo Tony, let's sticky that post for future read backs when the models are all over the shop

[NZ Thunderstorm Soc]

I'd say, stay at home. Have lots of cups of tea, with the feet up watching Games of Thrones repeats on TV.

[Cyclone Tracy]

7.40pm water vapour image showing the synoptics coming together

[Blair]

Thanks Tony, We have a pretty well set up 4x4, plus a bucket load of experience. Famous last words I know, but yeah we'll be ok.

Going by the looks of all the different reports etc, looks like we will be heading north of here and on to the Arthurs Pass. But if the forecasts changes we will reevaluate to suit where the rain/snow is happening.

[snowchaser01]

Models still seem to be holding true, maybe just slightly less duration. Main bulk of the moisture is gone by midday Saturday with showers handing around after that (based off models). Could very well be different.
Still going for snow level around 300m, and possibly lower tomorrow night in beefy showers.

[snowchaser01]

Just awaiting warnings and watches now...

[Razor]

Models still seem to be holding true, maybe just slightly less duration. Main bulk of the moisture is gone by midday Saturday with showers handing around after that (based off models). Could very well be different.
Still going for snow level around 300m, and possibly lower tomorrow night in beefy showers.

Metservice less bullish, going for down to 700m tomorrow in Canty High Country.

Their models suggesting Mt Hutt will see well over a metre base by Sunday

[snowchaser01]

Models still seem to be holding true, maybe just slightly less duration. Main bulk of the moisture is gone by midday Saturday with showers handing around after that (based off models). Could very well be different.
Still going for snow level around 300m, and possibly lower tomorrow night in beefy showers.

Metservice less bullish, going for down to 700m tomorrow in Canty High Country.

Their models suggesting Mt Hutt will see well over a metre base by Sunday

Yes they seem very shy with snow levels, maybe because there is so much uncertainty? Oh well, I've always been bullish with that sorta thing haha! Still picking 300-400m on the plains...

[snowchaser01]

HEAVY SNOW WATCH... FINALLY!
The Kaikoura Ranges
Valid: 13 hours from 8:00am to 9:00pm Saturday
Forecast: Snow is expected to fall to 300 to 400 metres during Saturday morning, then ease during Saturday evening. Snow amounts may approach warning criteria above 400 metres, and more than half a metre of snow may accumulate above 800 metres. The amount of snow expected is likely to disrupt travel and may damage infrastructure such as power lines.

Area: Canterbury High Country and foothills, mainly north Mt Cook
Valid: 13 hours from 1:00am to 2:00pm Saturday
Forecast: Snow is expected overnight tonight (Friday) and to fall to 300 to 400 metres early Saturday morning, then ease during Saturday afternoon. Significant snow could accumulate above 400 metres, more than half a metre of snow may accumulate above 800 metres in some areas. The amount of snow expected is likely to disrupt travel and may damage infrastructure such as power lines.

[snow101]

Quite a failure really on MetService part.

People with no weather knowledge who follow MetService forecasts would not have seen this coming, with only rain forecast in there Canterbury Plains forecast, then all of a sudden warning of nearly half a metre of snow to 800m?!

Well done to everyone on here, and other (non govt forecasters) for correctly forecasting heavy snow to lower levels.