NZ Weather Forum

Thought this low probably deserves its own thread. It is currently forecast to drop about 25-30hpa in 24 hours, and nearly 50hpa in 48 hours!

Heavy rain in the west, severe gales which could rival those of September 2013, and potentially significant low level snow on the back end are all on the cards in my opinion.

I wonder what Tony thinks of this system and winds for Canterbury, I get the feeling he doesn’t feel like it’s going to be as severe as some people are saying based on a comment from him I saw...

Thunderstorm outlook (i.e heavy downpours at least) has Waikato in the mix and not just Taranaki like yesterday
that was showing up in the Pivotal models way back On monday as Steven pointed out

Bradley wrote: ↑Thu 24/09/2020 08:28 I wonder what Tony thinks of this system and winds for Canterbury, I get the feeling he doesn’t feel like it’s going to be as severe as some people are saying based on a comment from him I saw...

This event has been in both major models with relative consistency for the last 3 days at least. I think 120kmh+ gusts on the plains and coast on Sunday, stronger on the Port Hills. I expect a range of warnings and watches to be issued by Metservice over the weekend.

yes this will be an intresting weekend.. some rain for drying parts of nth island auckland included.

tornado wrote: ↑Thu 24/09/2020 09:47 yes this will be an intresting weekend.. some rain for drying parts of nth island auckland included.

The cold air being dragged up as the low moves away to the SE is some of the coldest air in the last 10 years.

Bradley wrote: ↑Thu 24/09/2020 08:28 I wonder what Tony thinks of this system and winds for Canterbury, I get the feeling he doesn’t feel like it’s going to be as severe as some people are saying based on a comment from him I saw...

Not at all. Hell of a day lining up. Could be worst winds for some years. In fact I don't think enough attention has been focussed on Sunday's gales (or Monday's low level snow) - too much pissing about trying to get oxygen based on model musings in the Southern Ocean. I talked it up in my livestreams on Monday and yesterday.

snowchaser01 wrote: ↑Thu 24/09/2020 09:51

tornado wrote: ↑Thu 24/09/2020 09:47 yes this will be an intresting weekend.. some rain for drying parts of nth island auckland included.

The cold air being dragged up as the low moves away to the SE is some of the coldest air in the last 10 years.

Indeed. And strong cold advection in the airflow should ensure plenty of snow in the showers.

snowchaser01 wrote: ↑Thu 24/09/2020 09:51

tornado wrote: ↑Thu 24/09/2020 09:47 yes this will be an intresting weekend.. some rain for drying parts of nth island auckland included.

The cold air being dragged up as the low moves away to the SE is some of the coldest air in the last 10 years.

For Canterbury the July 2017 event had 850hpa temps of around -7C and -8C which is around what the models are predicting for this event though for southern south island 850hpa temps of -10C wouldn't have been seen since 2011?

TonyT wrote: ↑Thu 24/09/2020 10:18

Bradley wrote: ↑Thu 24/09/2020 08:28 I wonder what Tony thinks of this system and winds for Canterbury, I get the feeling he doesn’t feel like it’s going to be as severe as some people are saying based on a comment from him I saw...

Not at all. Hell of a day lining up. Could be worst winds for some years. In fact I don't think enough attention has been focussed on Sunday's gales (or Monday's low level snow) - too much pissing about trying to get oxygen based on model musings in the Southern Ocean. I talked it up in my livestreams on Monday and yesterday.

Sorry Tony I must have misunderstood your message about the low pressure system - for the plains would you agree this could be the strongest winds since the September 2013 event?

Bradley wrote: ↑Thu 24/09/2020 10:24

snowchaser01 wrote: ↑Thu 24/09/2020 09:51

The cold air being dragged up as the low moves away to the SE is some of the coldest air in the last 10 years.

For Canterbury the July 2017 event had 850hpa temps of around -7C and -8C which is around what the models are predicting for this event though for southern south island 850hpa temps of -10C wouldn't have been seen since 2011?

Yea I think you're right. I was more talking about the surface maps. I can't remember seeing the air truly coming off the ice shelf since the August 2011 event.

Bradley wrote: ↑Thu 24/09/2020 10:26

TonyT wrote: ↑Thu 24/09/2020 10:18

Not at all. Hell of a day lining up. Could be worst winds for some years. In fact I don't think enough attention has been focussed on Sunday's gales (or Monday's low level snow) - too much pissing about trying to get oxygen based on model musings in the Southern Ocean. I talked it up in my livestreams on Monday and yesterday.

Sorry Tony I must have misunderstood your message about the low pressure system - for the plains would you agree this could be the strongest winds since the September 2013 event?

The strength of the winds is largely independent of the rate of deepening of the depression. Its about the jet stream position. Yes, could easily be strongest since 2013. But remember, gales are like snow, they rarely hit everywhere in a region. They touch down in a few places and don't in others. Shot gun type event.

Bradley wrote: ↑Thu 24/09/2020 10:24

snowchaser01 wrote: ↑Thu 24/09/2020 09:51

The cold air being dragged up as the low moves away to the SE is some of the coldest air in the last 10 years.

For Canterbury the July 2017 event had 850hpa temps of around -7C and -8C which is around what the models are predicting for this event though for southern south island 850hpa temps of -10C wouldn't have been seen since 2011?

Its the depth and intensity of the cold advection which is exceptional (well, lets say impressive), not the actual temperature.

Notice that EC has a secondary low developing SE of Stewart Island on Monday morning. if this trend continues, it may just straighten up those isobars slightly, and areas outside of Southland and Otago may be affected by snow. One to watch anyway.

TonyT wrote: ↑Thu 24/09/2020 10:54

Bradley wrote: ↑Thu 24/09/2020 10:24

For Canterbury the July 2017 event had 850hpa temps of around -7C and -8C which is around what the models are predicting for this event though for southern south island 850hpa temps of -10C wouldn't have been seen since 2011?

Its the depth and intensity of the cold advection which is exceptional (well, lets say impressive), not the actual temperature.

Thanks Tony, does this have a direct relation to how rapidly the low will be deepening?

If the 850hpa temps of -11C around Invercargill on Monday night are to be believed I don't see realistically how you could get any colder for very late September, the map below is remarkable with the origins of the air (that region of Antarctica where the winds are pulling from is around -60C at present) - so many factors need to come together for situations like this to happen. Probably at least a one-in 50 year event for late September for the southern part of the South Island...

snowchaser01 wrote: ↑Thu 24/09/2020 10:57

TonyT wrote: ↑Thu 24/09/2020 10:54

Its the depth and intensity of the cold advection which is exceptional (well, lets say impressive), not the actual temperature.

Thanks Tony, does this have a direct relation to how rapidly the low will be deepening?

Possibly, but stop obsessing about a depression. Low air pressure won't hurt anyone! Its interesting, but not important. What it has a direct relation to is how much snow might fall.

Bradley wrote: ↑Thu 24/09/2020 10:59 - so many factors need to come together for situations like this to happen.

Once in a while nature obliges...

Pre-frontal jet 6am Sunday Post-frontal jet midday Monday

TonyT wrote: ↑Thu 24/09/2020 11:30

snowchaser01 wrote: ↑Thu 24/09/2020 10:57
Thanks Tony, does this have a direct relation to how rapidly the low will be deepening?

Possibly, but stop obsessing about a depression. Low air pressure won't hurt anyone! Its interesting, but not important. What it has a direct relation to is how much snow might fall.

Trust me, I have plenty of interest in how much snow might fall!!

Really seems like the super strong vortex has locked in some especially cold air for a few weeks and it is choosing to let it out in our direction. Still a long way to go, but the trend is there.

Saw uppers of -36C over Canterbury on last night's GFS output, crossing over around Sunday/Monday if I recall, can't have been that low for a while.

Edit - MS outlook updated, spotlight on rain and wind but also on snow in the far south:

Tony are you going to do a live stream for this event? Given the potential damage that may be associated with the winds and snow down south? Like a snowman show?

IMO places close to the alps like Arthurspass, Mt Cook Village could see a massive snowfall. Im seeing 150kph winds on the plains which im sure will be problematic. With the snow theres going to be some wicked BLIZZARDS! from this event.

talbotmj15 wrote: ↑Fri 25/09/2020 08:03 Tony are you going to do a live stream for this event? Given the potential damage that may be associated with the winds and snow down south? Like a snowman show?

IMO places close to the alps like Arthurspass, Mt Cook Village could see a massive snowfall. Im seeing 150kph winds on the plains which im sure will be problematic. With the snow theres going to be some wicked BLIZZARDS! from this event.

Hadn't planned to, but maybe thats a good idea. I'm thinking 90% of the snow will be Southland and Otago. But winds stronger Canterbury to Cook Strait.

The complexities of this series of events is starting to get well portrayed by the models now. Strong NW and heavy rain pre-front Saturday, strong W post-front early to mid Sunday, southern snow late Sunday, strong SW and low level snow far south Monday, potentially heavy coastal snow and severe gales far south Tuesday. Plenty of flow on impacts for other areas. Anyone who wants to learn about severe weather systems this is a great opportunity to bury your head in the model data and watch it unfold over the next 4-5 days.

The unified models UKMet and Access G3 12z runs are producing a 508 thickness line over southland and Otago on Tuesday morning, 514 thickness line up to Christchurch. These models have been leading other models on outcomes since they picked up the polar streamer early in the week. This is a top shelf cold shot if they verify.

If only a little moisture for Chch, not too much of course. It's going to be a very interesting few days whatever eventuates.