Cold weather event 17th to 20th August

[Razor]

I've actually got 5 snowfalls in ChCh City this year, counting the one in May which only briefly settled at 3am (I was driving through it to the hospital, most others would have missed it sleeping)

Sure has been a kind winter to us snow bunnies!

Snowing again now

Looking at the projections on Metvuw there may be another on the weekend too

[tich]

Dusting in central city this morning. News mentioned a rare snowfall in Paekakiriki last evening; also Featherston, Wairarapa.

[Razor]

Very heavy snowshower now, fantastic stuff. Metservice still only saying sleet showers, even though its been good nsnowfalls for several hours in the central city

[Weathermad]

No snow here in Northwood/Belfast areas and rain showers off and on

[Razor]

No snow here in Northwood/Belfast areas and rain showers off and on

Isn't it amazing how random and sporadic these falls have been lately? I guess we are all craving a widespread dump to cap off the winter (but not so much it hurts our farming friends)

Another snow shower coming past the office here in Riccarton.

[gllitz]

Ok, it's official...I am now OVER this winter....whilst it has been nice to have the snow we have had, roll on SPRING!!!!

[gopolks]

Wow, colour me suprised, woke up this morning and it was snowing, and its settled on the back lawn, pretty cool if ya ask me.

[TonyT]

Isn't it amazing how random and sporadic these falls have been lately?

Not really - thats how showers are.

[Hypno]

No snow out here in Lincoln, Can see its down to sea level in brighton

[Razor]

Isn't it amazing how random and sporadic these falls have been lately?

Not really - thats how showers are.

You miss my point Tony

The past two events have seen me driving out to the airport in the middle of them. We had snow falling most of the night in St Albans yet a couple of km away either side it was only sleety rain not settling. It was the same 2 weeks ago.

Both events have mirrored each other in this behaviour and it seems there are clear 'channels' where it is tending to snow and others where it falls as rain. I guess this may be the effect of the Port Hills?

[TonyT]

Well, your point is contrary then - if there are clear channels where it tends to snow then its not random and sporadic.

[Manukau heads obs]

the rain radar shows that there are shower lines, aligned SW to NE
so area either side of that line will not be getting as much snow
there is now a new line over banks peninsular instead of over western CHCH

there looks to me like some sort of convergence line, between the airmass over canterbury and that over the water (i,e the boundary where the critical SW shadow effect of otago is occuring)

[Razor]

Well, your point is contrary then - if there are clear channels where it tends to snow then its not random and sporadic.

Humour me Tony- I'm sure you know what I mean, What I am trying to discuss is rationale for why some suburbs get snow all night and others next door only a few km away and at the same altitude get rain instead.

I think Brians response sums up my thinking too.

[Ryan Thomas]

Few cm of snow this morning in my area. Must of been more because the amount of water on the ground was excessive and paths were flooded... Saw a flash of lightning around 11:10 last night and heard a clap of thunder but that was all

[Razor]

Really good hail shower coming through now, activity has certainly intensified again

The Port Hills look majestic!!!!

[spwill]

Christchurch could host the wintry Olympics.

What I am trying to discuss is rationale for why some suburbs get snow all night and others next door only a few km away and at the same altitide get rain instead.

Precipitation differences will really show up when there's snow fall, the heavier precipitation more likely to be snow.

[Inny Binny]

Well, we got about a millimetre at home...

[Paul Mallinson]

the rain radar shows that there are shower lines, aligned SW to NE
so area either side of that line will not be getting as much snow
there is now a new line over banks peninsular instead of over western CHCH

there looks to me like some sort of convergence line, between the airmass over canterbury and that over the water (i,e the boundary where the critical SW shadow effect of otago is occuring)

Brian is on the right track here. In cold unstable southerly/southwesterly flows we see parralel lines of convergence set up over the water (near the coast). These are caused by a more southwesterly flow over the land meeting a more southerly flow over thge water. At the point of convergence, there is vertical motion (ascent) and so the convection is fired up. When it is cold enough the precipitation can be snow rather than rain showers. These lines tend to wobble around in the flow due to ripples travelling along them.
There are two main trigger points I can think of in the east of the South Island -
1. The Otago Peninsula. This one often feeds lines of heavy showers over Banks Peninsula or over Christchurch.
2. Banks Peninsula. This one feeds a line(s) of heavy showers north along the Kaikoura Coast (aided by an offshore flow, often quite westerly at Kaikoura) and onto the lower North Island. It is usually these lines of convergence that dump snow on the Rimutaka Road.
Usually this convection is occurring in a fairly dry airmass (dewpoints down around 2 to -2C for eg). The drier the environment is, the greater the evaporative cooling from the precip and the lower the snow will come. The heavier the precip is, the lower the snow will come also. This is why you get snow sometimes and mixed rain and snow at other times.
In the case of the convergence off Kaikoura, we often see this feature set up over night helped by the cold overnight temperatures over the land leading to a cold westerly drainage flow meeting the offshore southerly. As the temperatures rise over the land during the morning with day time heating, this westerly flow decays and often turns more southwest or southerly. The convergence line falls apart and the showers often clear from the lower North Island.

Paul

[Razor]

Thanks Paul just what I was after!
Legendary work

[RWood]

Regarding the Kaikouras, I can recall an article by Ereck Brenstrum explaining how several hours after the last showers appeared to have moved eastwards in a decaying Wellington southerly, some interaction with the mountains (a barrier flow involved?) can produce another line of showers that reaches Wellington. Can't now find any reference online.

[gllitz]

Brian is on the right track here. In cold unstable southerly/southwesterly flows we see parralel lines of convergence set up over the water (near the coast). These are caused by a more southwesterly flow over the land meeting a more southerly flow over thge water. At the point of convergence, there is vertical motion (ascent) and so the convection is fired up. When it is cold enough the precipitation can be snow rather than rain showers. These lines tend to wobble around in the flow due to ripples travelling along them.
There are two main trigger points I can think of in the east of the South Island -
1. The Otago Peninsula. This one often feeds lines of heavy showers over Banks Peninsula or over Christchurch.
2. Banks Peninsula. This one feeds a line(s) of heavy showers north along the Kaikoura Coast (aided by an offshore flow, often quite westerly at Kaikoura) and onto the lower North Island. It is usually these lines of convergence that dump snow on the Rimutaka Road.
Usually this convection is occurring in a fairly dry airmass (dewpoints down around 2 to -2C for eg). The drier the environment is, the greater the evaporative cooling from the precip and the lower the snow will come. The heavier the precip is, the lower the snow will come also. This is why you get snow sometimes and mixed rain and snow at other times.
In the case of the convergence off Kaikoura, we often see this feature set up over night helped by the cold overnight temperatures over the land leading to a cold westerly drainage flow meeting the offshore southerly. As the temperatures rise over the land during the morning with day time heating, this westerly flow decays and often turns more southwest or southerly. The convergence line falls apart and the showers often clear from the lower North Island.

Paul

I think we need to frame this somewhere.......

EDIT:......wait a minute....just noticed that it DOES get framed...what I mean is, print it out and frame it...put it in the pool room, whatever...absolutely golden!

[Storm Struck]

Sounds like another city and close to BP event this one, a very much simular repeat of a few weekends ago.
Nothing out here not even on the grass when i checked at 7:30am, the showers seem to loose strenth as they head northwards which is abit of a bugger but oh well.
What interests me is that the showers are still pummeling through now and then as i thought they might have eased off by now and if they keep going then there might still be some snow showers later on in the city.
Bring on the next event i say .
Cheers
Jason.

[Ryan Thomas]

Its fine in my area of town but has looked cloudy to the west most of the day with clouds with tops that look very high (from what I can see) hopefully the clouds come more to the east

[Namytnap]

Thankyou Paul Mallinson...a superb explanation, often wondered if those two features had any impact on cimatic conditions further afield.

[CHCH Weather Chaser]

Check out my pics that got posted on Weather Watch . One of them not so clear with the snow on port hill view from around midday but the others were great fun half way up the hill