Heavy Rain, Strong Wind and Cold Outbreak July 5th-8th

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Chris W
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Heavy Rain, Strong Wind and Cold Outbreak July 5th-8th

Unread post by Chris W »

Putting this one up as GFS and EC are modelling slightly different variations of a longwave polar trough with embedded deep low pressure centres crossing NZ from Sunday through to Tuesday. It looks like rain is the main severe weather threat as it stands, but there could be strong wind and possibly low snow. Details are best uncovered nearer to the event, but I thought it worth mentioning the possibility.

The tail brings up some cold air and sets up a potential following event, but I'll keep this to the weekend.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Chris W »

The system overall is my now looking less threatening, but the southerly it drags up still looks cold.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by NZstorm »

Keep an eye on the front Tuesday night in the modelling, might come to something interesting.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Bradley »

5-10cm of snow now being predicted down to sea level in Canterbury for next weeks system, could be a rogue run by EC but there is def something there...
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Cyclone Tracy »

It's no longer just a signal, 70+ members from EC & GEFS are locking in a powerful Rossby wave from 5 to 8 July with 2 sharp polar injected upper troughs.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Chris W »

Yes the snow level for midweek looks very low, moisture reasonable too. GFS is modelling a light to moderate intensity of snow for Chch in the early hours of Wednesday morning and again Wednesday night before heavier rain early Thursday as a second front follows up. Some way to go yet of course!

Feel free to rename the thread as appropriate, mods.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Bradley »

I have never seen snow to sea level in Christchurch with 850hpa of only -4C and -5C in a non warm advection scenario yet both EC and GFS are forecasting this for early Wednesday morning. I would be interested to hear ideas on how this could be possible?
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Cyclone Tracy »

That GFS 18z run (EC 12 similar) just output 95% humidity @700 hPa over most of Canterbury on early Wednesday morning. High moisture at this height is 1 crucial piece in the low snow jigsaw puzzle if it verifies. Loads of energy.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by mikestormchaser »

Bradley wrote: Fri 03/07/2020 11:20 I have never seen snow to sea level in Christchurch with 850hpa of only -4C and -5C in a non warm advection scenario yet both EC and GFS are forecasting this for early Wednesday morning. I would be interested to hear ideas on how this could be possible?
I think you will find it could be a sernario simula to a warm advection type event if it accours.
There does seem to be alot of heavy precipitation on Wednesday morning and it can cause what i think is evaporative cooling?? Where such heavy precip drags the freezing level down.Inland areas get it more often then coastal areas. Its still a long way off, id really want to wait 48 hours out to be sure but it does look interesting. 700mb temp -16 over Canterbury wensday morning which suggests pretty cold lower level temps.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by TonyT »

Bradley wrote: Fri 03/07/2020 11:20 I have never seen snow to sea level in Christchurch with 850hpa of only -4C and -5C in a non warm advection scenario yet both EC and GFS are forecasting this for early Wednesday morning. I would be interested to hear ideas on how this could be possible?
The prog seems to be based on a developing wave in the southwest flow. So, its probably close to a warm advection scenario at a sub-synoptic scale (meso-scale I suppose). Thats why the snow is so localised. It will almost certainly disappear from the next model run, appear somewhere else, re-appear, weaken, intensify and go through a range of semi-random changes between now and the event.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Bradley »

TonyT wrote: Fri 03/07/2020 13:09
Bradley wrote: Fri 03/07/2020 11:20 I have never seen snow to sea level in Christchurch with 850hpa of only -4C and -5C in a non warm advection scenario yet both EC and GFS are forecasting this for early Wednesday morning. I would be interested to hear ideas on how this could be possible?
The prog seems to be based on a developing wave in the southwest flow. So, its probably close to a warm advection scenario at a sub-synoptic scale (meso-scale I suppose). Thats why the snow is so localised. It will almost certainly disappear from the next model run, appear somewhere else, re-appear, weaken, intensify and go through a range of semi-random changes between now and the event.
Thanks Tony, based on what you said there is still alot of uncertainty around this scenario, when do you think we may have a better idea when it will occur or not? 72 hours? 48 hours out?
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by TonyT »

Bradley wrote: Fri 03/07/2020 14:53
TonyT wrote: Fri 03/07/2020 13:09

The prog seems to be based on a developing wave in the southwest flow. So, its probably close to a warm advection scenario at a sub-synoptic scale (meso-scale I suppose). Thats why the snow is so localised. It will almost certainly disappear from the next model run, appear somewhere else, re-appear, weaken, intensify and go through a range of semi-random changes between now and the event.
Thanks Tony, based on what you said there is still alot of uncertainty around this scenario, when do you think we may have a better idea when it will occur or not? 72 hours? 48 hours out?
We may never get a solid lock on what it turns out to be. Hindsight is a wonderful thing. Butterfly's wings and all that.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by NZstorm »

mikestormchaser wrote: Fri 03/07/2020 11:33 I think you will find it could be a sernario simula to a warm advection type event if it accours.
Yes, same process as warm advection snow, dynamic cooling from melting snow aloft, lowers the freezng level.

Many of low level snow events in New Zealand involve this process.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by TonyT »

Thanks to Paul Gorman for pointing out that this event seems to look a little like what we had last year on September 25th, which brought snow around Ashburton but not Christchurch.
viewtopic.php?f=34&t=6030
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by tich »

TonyT wrote: Fri 03/07/2020 18:34 Thanks to Paul Gorman for pointing out that this event seems to look a little like what we had last year on September 25th, which brought snow around Ashburton but not Christchurch.
viewtopic.php?f=34&t=6030
Not in central Chch, but flurries were reported at the airport and western outskirts of city.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Razor »

Certainly looks to be brewing up an interesting week. Hoping for a more settled second week of the school hols though
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Chris W »

GFS has pushed the cold air back slightly today, but Wednesday into Thursday is going to be very interesting to watch for both rain and snow to low levels. I’ll avoid trying to watch every run though!
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-9th

Unread post by Cyclone Tracy »

The polar vortex loosening its belt into the mid latitudes, 2 upper troughs showing up nicely on tonight's water vapour image.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Cyclone Tracy »

Locally, the average of 10 global and regional models are projecting 20 to 25mm in the next 24 hours with the first system. Occluded front should be in the area later tonight.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by NZstorm »

A cold trough crossing the upper half of the North Island tomorrow with chance of thunder and possible hail. Northland looks to be most in the firing line, particularly in the morning. Hopefully not just cold rainy skies in Auckland which can happen in this type of set up.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by tgsnoopy »

It'll be a nervous night for me tonight. This is the kind of setup that can see rainfall rates high enough that this place becomes a flood plain. Every year more in fill housing goes in above us and the heavy rain events cause more flow down here. Oh well the doorways downstairs will be sand bagged again.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Storm Struck »

If I'm reading it right correct me if not 🤔, 850mb temperature at this stage for Wednesday morning suggests - 6C with a 700mb of - 13C and 500mb of - 28C.
Would suggest snow to 100-200m? For the Eastern South island.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Cantab03 »

Models have defintely pulled back the idea of sea level snow in the last 3 or so runs, both GFS and EU. Moisture also pulled back, looking very SW. Thinking snow flurries to 300m at this stage, possibly 200m in the heavier showers.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by tornado »

rain is developing in auckland now. the front has been lightning active at times. perhaps a rumble somewhere tonight? tommorow thunder/hail possible. maybe a low risk of another tornado somewhere too.
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Re: Heavy Rain, Strong Wind and Cold Outbreak July 5th-7th

Unread post by Cyclone Tracy »

Wind now starting to squeeze between a north easterly to northerly locally. Whangarei Heads just had a gust around 104 km/h
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