First Major Winter Storm of 2019

Cyclone Tracy · Unread post by **Cyclone Tracy** » Thu 30/05/2019 09:28

UK keeps modelling a sting jet off just off Christchurch at around 970 hPa now. This will launch a wet and damaging 130 to 150 km/h backend southerly attack if it verifies. Given the UK is probably the best individual model for warm and cold core cyclones in the southern hemisphere, MS would be looking at this very closely today.

I can see the Jets moving into position on the water vapour image over the Tasman . Almost time to move into observation mode

spwill · Unread post by **spwill** » Thu 30/05/2019 09:41

Saturday certainly looking wintry for Auckland with a strong cold wind, showers with hail and the odd rumble of thunder likely given how cold the upper level temperatures are.

More snow possibilities showing for the SI next week.

TonyT · Unread post by **TonyT** » Thu 30/05/2019 09:48

snowchaser01 wrote: Thu 30/05/2019 08:35
Tony with all due respect, no one is assuming that the model runs are an attempt at the truth. We are aware they are purely guidance. The question simply was why you believe there is so much difference between EC and GFS.. one of them must be closer to the truth.

If you insist on wanting to guess which one is "closer to the truth" then your only option is to wait till after the event! If you can change your mindset to wanting to try to understand what the models are telling you about the trends as they seem them now, then you might be getting closer to deriving a forecast.

As to why the two are different, well its a bit like asking why Christianity and Islam have different ways of both ending up declaring there is a God. Or why socialism and capitalism both have different ways of trying to improve society. The two models will have different (but similar) inputs, different (but similar) implementations of atmospheric physics, different (but untested) assumptions about certain process and how they work, and different ways of processing and scaling the output data into something meaningful (eg different ways of converting multi-level temperature and moisture data into a guess of how much snow results).

As I said earlier, even multiple runs of the same model with essentially the same input data will produce significant variations in output, so its not surprising that totally different models also vary. In fact it would be quite something if they agreed! What you can take from this, is the extent of agreement is a crude measure of how "predictable" the atmosphere is at any given time. Clearly while on this occasion there is good agreement about the brand pattern, there is considerable disagreement about the details. So maybe the best interpretation is to say "we know its going to be cold and there will be snow about, but the tools we use for predicting where and how much disagree too much at present to allow us to confidently predict the details". If you want to "un-confidently" predict the details then go right ahead, but at least be aware of what you are doing. Sorry to sound like an angry great aunt, but having all these fantastic tools available to us doesnt make us all craftsmen (craftspeople), it just gives us more rope to hang ourselves with if we aren't aware of the limitations of what we are doing.

My impression over the last 6 months is that model accuracy (speaking generally) has dropped a little compared with previous years, and I put this down to solar minimum causing small planetary scale changes in circulation, which no longer reflect the models' underlying assumptions.

tich · Unread post by **tich** » Thu 30/05/2019 09:52

So going by that, does that mean they are expecting snow as low as cheviot? "snow to all levels"

I believe Cheviot is barely above sea-level. (and about 5km form the coast)

TonyT · Unread post by **TonyT** » Thu 30/05/2019 09:57

snowchaser01 wrote: Thu 30/05/2019 08:26
Bradley wrote: Thu 30/05/2019 08:14 Latest run of EC this morning now going for snow on Saturday right down to sea level for Canterbury Only a few centimetres but still a huge upgrade overnight...
This EC run has thrown out 11cm for Chch on Saturday... I don't believe that for a second but EC definitely seems like the more consistent model for this event so far.

Are you sure you have your units correct? My 12Z ECMWF control run data is saying 0.7cm snow depth (so 7mm) for Christchurch. I also dont believe that because a) the forecast 925hpa temps are never predicted to drop below zero, b) the 1000-500 thickness is never predicted to drop below 530dm, and in my experience both need to happen for snow to be on the ground on the eastern Plains, and c) there is 30+mm of rain predicted to fall around the same time, and thats just too much to allow snow to settle.

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 10:07

TonyT wrote: Thu 30/05/2019 09:57
snowchaser01 wrote: Thu 30/05/2019 08:26

This EC run has thrown out 11cm for Chch on Saturday... I don't believe that for a second but EC definitely seems like the more consistent model for this event so far.
Are you sure you have your units correct? My 12Z ECMWF control run data is saying 0.7cm snow depth (so 7mm) for Christchurch. I also dont believe that because a) the forecast 925hpa temps are never predicted to drop below zero, b) the 1000-500 thickness is never predicted to drop below 530dm, and in my experience both need to happen for snow to be on the ground on the eastern Plains, and c) there is 30+mm of rain predicted to fall around the same time, and thats just too much to allow snow to settle.

The windy model output which has ec and gfs shows 11cm. I know it's not going to happen but it's just interesting to see what it throws out each run.

As always your knowledge and input is appreciated Tony.

TonyT · Unread post by **TonyT** » Thu 30/05/2019 10:12

snowchaser01 wrote: Thu 30/05/2019 10:07
TonyT wrote: Thu 30/05/2019 09:57

Are you sure you have your units correct? My 12Z ECMWF control run data is saying 0.7cm snow depth (so 7mm) for Christchurch.
The windy model output which has ec and gfs shows 11cm.

Thats disturbing that there is such a difference.

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 10:13

TonyT wrote: Thu 30/05/2019 10:12
snowchaser01 wrote: Thu 30/05/2019 10:07

The windy model output which has ec and gfs shows 11cm.
Thats disturbing that there is such a difference.

I know... showing 11cm in central chch on Saturday.

Bradley · Unread post by **Bradley** » Thu 30/05/2019 10:22

An example is with 850hpa temps of only -3C next Wednesday with not much moisture around at all I just can't see the 2-3C maximum temps being put out by EC for Darfield, the upper atmosphere data and ground maximum forecasts are just not lining up. Like Tony says that is why they are just guidance and human intervention is needed to get a more sensible forecast sometimes...

TonyT · Unread post by **TonyT** » Thu 30/05/2019 10:27

Bradley wrote: Thu 30/05/2019 10:22 An example is with 850hpa temps of only -3C next Wednesday with not much moisture around at all I just can't see the 2-3C maximum temps being put out by EC for Darfield, the upper atmosphere data and ground maximum forecasts are just not lining up. Like Tony says that is why they are just guidance and human intervention is needed to get a more sensible forecast sometimes...

Could be because the model is seeing snow on the ground keeping 2m temps close to freezing. Models are like that.

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 10:31

In other news.. the MetService computer generated precip amounts for CHCH on Saturday are pretty insane...

Cyclone Tracy · Unread post by **Cyclone Tracy** » Thu 30/05/2019 11:20

My 2 cents worth on the EC snow estimate for Christchurch. Just had a closer look at the 12Z run and EC seems to be modelling the 'Ocean effect' / over water advection into its projection. North America have a fresh water version called the 'Lake effect' during their winter snow storms. Latent heat flux at the surface is over 300 mw2 offshore, their is plenty of energy involved. 850 hPa will be -2c and the sea surface is around 13c putting it into the sweet spot. Of course its all theory.... we need to see where the low deepens first

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 11:38

Metservice have updated all their forecasts. Still going with rain over the plains... no mention of snow at all.

Bradley · Unread post by **Bradley** » Thu 30/05/2019 11:44

snowchaser01 wrote: Thu 30/05/2019 11:38 Metservice have updated all their forecasts. Still going with rain over the plains... no mention of snow at all.

Yes by the time this event is done and dusted Metservice will either appear to be geniuses with their forecasts of no snow for the plains or they will have serious egg on their faces for not forecasting less then 48 hours out at least a severe weather watch for snow down to 300-400m asl etc. I believe all it requires is 5cm+ to constitute a warning?

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 11:46

Bradley wrote: Thu 30/05/2019 11:44
snowchaser01 wrote: Thu 30/05/2019 11:38 Metservice have updated all their forecasts. Still going with rain over the plains... no mention of snow at all.
Yes by the time this event is done and dusted Metservice will either appear to be geniuses with their forecasts of no snow for the plains or they will have serious egg on their faces for not forecasting less then 48 hours out at least a severe weather watch for snow down to 300-400m asl etc. I believe all it requires is 5cm+ to constitute a warning?

I always thought it was 10cm for a warning? Could be wrong though...

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 11:55

Bradley wrote: Thu 30/05/2019 11:44
snowchaser01 wrote: Thu 30/05/2019 11:38 Metservice have updated all their forecasts. Still going with rain over the plains... no mention of snow at all.
Yes by the time this event is done and dusted Metservice will either appear to be geniuses with their forecasts of no snow for the plains or they will have serious egg on their faces for not forecasting less then 48 hours out at least a severe weather watch for snow down to 300-400m asl etc. I believe all it requires is 5cm+ to constitute a warning?

Could be a 2006 repeat... forecast nothing major at all, then get made to look like complete numpties when heavy snow starts falling in Timaru!

Bradley · Unread post by **Bradley** » Thu 30/05/2019 12:00

snowchaser01 wrote: Thu 30/05/2019 11:55
Bradley wrote: Thu 30/05/2019 11:44

Yes by the time this event is done and dusted Metservice will either appear to be geniuses with their forecasts of no snow for the plains or they will have serious egg on their faces for not forecasting less then 48 hours out at least a severe weather watch for snow down to 300-400m asl etc. I believe all it requires is 5cm+ to constitute a warning?
Could be a 2006 repeat... forecast nothing major at all, then get made to look like complete numpties when heavy snow starts falling in Timaru!

Sorry snowchaser I wasn;t actually living in New Zealand at the time of the 2006 event, was no snow forecast by the Metservice for places like Timaru and Ashburton 48 hours out?

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 12:09

Bradley wrote: Thu 30/05/2019 12:00
snowchaser01 wrote: Thu 30/05/2019 11:55

Could be a 2006 repeat... forecast nothing major at all, then get made to look like complete numpties when heavy snow starts falling in Timaru!
Sorry snowchaser I wasn;t actually living in New Zealand at the time of the 2006 event, was no snow forecast by the Metservice for places like Timaru and Ashburton 48 hours out?

Yea, they forecast it above 500m.. i think, (someone correct me if I'm wrong) Then it started snowing in Timaru.. and all the way down to sea level up the entire east coast... then they changed their forecast to say sea level. Looked like complete numpties.

Bradley · Unread post by **Bradley** » Thu 30/05/2019 12:10

The air in this event is certainly cold enough compared to the 2006 system but it's all about the timing of it 's arrival and how it interacts with the warm, moisture-laden air from the low pressure system, here are the 2 meteograms for both events:

Razor · Unread post by **Razor** » Thu 30/05/2019 12:12

Bradley wrote: Thu 30/05/2019 12:00
snowchaser01 wrote: Thu 30/05/2019 11:55

Could be a 2006 repeat... forecast nothing major at all, then get made to look like complete numpties when heavy snow starts falling in Timaru!
Sorry snowchaser I wasn;t actually living in New Zealand at the time of the 2006 event, was no snow forecast by the Metservice for places like Timaru and Ashburton 48 hours out?

It is all in this thread here, the major snow warnings came basically as the event was underway. The real fun kicks in about page 4 but the preamble before is still entertaining to look back on

https://www.weatherforum.nz/phpBB3/view ... 5&start=75

Chris W · Unread post by **Chris W** » Thu 30/05/2019 12:18

Not sure we're going to be all that certain until late tomorrow, there is so much variation going on. MS have tweaked Saturdays forecast wording to go heavier in terms of rain, but little else has changed. Kind of hoping the current computer generated rain totals and even wind barbs are way off for Chch.

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 12:35

So yea... 2006 they updated their warnings to snow to 200m the night of the event... by which time Ashburton already had 12cm...

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 12:37

MetService have a road snow warning out for Milford road... would have to be the worst worded warning I've ever seen

"2-3cm of snow expected to FALL but little if any expected to accumulate"

Razor · Unread post by **Razor** » Thu 30/05/2019 12:48

snowchaser01 wrote: Thu 30/05/2019 12:37 MetService have a road snow warning out for Milford road... would have to be the worst worded warning I've ever seen

"2-3cm of snow expected to FALL but little if any expected to accumulate"

Whats wrong with that?

snowchaser01 · Unread post by **snowchaser01** » Thu 30/05/2019 12:50

Razor wrote: Thu 30/05/2019 12:48
snowchaser01 wrote: Thu 30/05/2019 12:37 MetService have a road snow warning out for Milford road... would have to be the worst worded warning I've ever seen

"2-3cm of snow expected to FALL but little if any expected to accumulate"
Whats wrong with that?

How can you measure how much falls without it accumulating?

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