Deepening low prediction/ cold outbreak Mid June

[RWood]

My experience from lengthy periods in both towns was that ground conditions were considerably drier in Dunedin than in Invercargill at this time of year. Snow did seem to settle more easily in the former - but it also seemed to be the case that hail and sleet were more likely in Invercargill, snow more likely in Dunedin. Just an impression.

[southernthrash]

With reference to what part of Dunedin would that be RWood? It needs to be remembered, that suburbs of Dunedin extend to elevations of 300 m asl +, and the hill suburbs get snow pretty frequently through the year. Seeing actual snow settle to sea level in Dunedin is much rarer, and probably occurs about as frequently as in Invercargill (where the entire city is below 40 m asl). The relief in Dunedin is STEEP too, I have had several experiences of near blizzard conditions on Highgate, only to encounter rain 2 minutes later down toward the Octagon.

[RWood]

Probably mid-level and upwards, but I can recall a few occasions (don't know dates) when it was down at low-lying parts - university zone etc. Perhaps no commoner at those levels than at Invercargill - probably few stats. around to verify I suppose.

[southernthrash]

Seems about right!

[Manukau heads obs]

cold air has arrived up this way now
2.5C with a light SSW wind still blowing

[Janos]

Probably mid-level and upwards, but I can recall a few occasions (don't know dates) when it was down at low-lying parts - university zone etc. Perhaps no commoner at those levels than at Invercargill - probably few stats. around to verify I suppose.

Ive seen snow at a friends house 3 times so far this year ...hes at 280m asl.

[NZstorm]

Interesting to note that such effects are not seen for Dunedin under the same conditions, when topographic barriers are larger and more numerous.

Topography does have an impact, there is a critical SW for snow showers into Dunedin. The wind flow really has to get into true SW for Dunedin to get the decent showery activity.

Seeing actual snow settle to sea level in Dunedin is much rarer, and probably occurs about as frequently as in Invercargill (where the entire city is below 40 m asl).

Dunedin is historically more exposed to heavy snow dumps at sea level than Invercargill and obviously geography is the big factor. Along the Otago coast there is some orographic enhancement and mesoscale cooling effects that Invercargill doesn't get. I think that is a big difference. But the Stewart Island blocking effect certainly is a player as well for Invercargill when the flow is SSW.

edited for typos

[southernthrash]

Wind and air flow are not the same...

There may be some orographic enhancement during a SE flow, but not so sure about a SW flow, as the air is coming in over Southland/South Otago - this would enhance cooling though.

Still not entirely convinced about Stewart Is, of all the topographic barriers in this part of the world, it is probably the least significant, yet what you are saying would imply that it has the most significant effects. Essentially, there is one small 980 m peak, the rest of the island is of fairly limited relief. The flow direction for Stewart Island to have any impact would be very very critical.

I'm thinking it was more due to the influence of the high - we only copped the W edge of the fronts/trough, while Dunedin was in the thick of it.

But there are a multitude of factors which influence snowfall and accumulation - from antecedent ground conditions, localised inversions, intensity...

[Razor]

How on earth are the forecasters still seeing another wintry flick on Saturday? Looks to me like that High is going to dominate the South Island for at least a week now.

[southernthrash]

2 degrees here and heavy showers with sleet, all pretty coastal though I think.

[Philip Duncan]

How on earth are the forecasters still seeing another wintry flick on Saturday? Looks to me like that High is going to dominate the South Island for at least a week now.

Agree. Unless that high isn't quite strong enough in which case that southerly flow will remain up the east coast. But all the current models we use show cloudy periods, 10% POP in the morning with SSW winds turning SW in arvo for Chch on Saturday.

[gopolks]

They have changed it now, to fine frosty weather!

[spwill]

Southernthrash wrote,

Saturated soil has a greater thermal capacity (due to increased water content), this changes the thermal inertia of the soil, and slows down radiative heat loss. Subsequently, the lag between cooling of soil and air increases, meaning that the surface onto which snow falls is relatively warm, causing melting of snowflakes on impact, therefore snow can't settle.

Have a look at the ground frost stats for Invercargill (plenty of frost)and it has a generally cooler climate than that of Dunedin. The waterlogged Taieri Plains (West Dunedin) have no problem collecting snow during low level snow events. There is plenty of pasture land around Invercargill, grass cover will reduce heat loss from the soil and allow for more rapid cooling off the surface.

Rwood wrote,

but it also seemed to be the case that hail and sleet were more likely in Invercargill, snow more likely in Dunedin. Just an impression

Invercargill is more exposed to cold unstable airflows so will get a lot more days with wintry showers. My memory tells me Invercargill is a little warmer on the very cold Southerly flow days than Dunedin, probably due to lack of high ground and cold air flows would very seldom have much of a land track like it can do with Dunedin eg S/SW flow.

[Razor]

They have changed it now, to fine frosty weather!

Hmmm well Metservice still clinging to "few showers" for Satruday. I just can't see it- bit like Phil says, unless the High isn't strong enough...

[sthguy]

Stewart Island has a major sheltering effect on Invercargill in SW flows and I'm sure thats part of the reason why the city gets less snowfall than eastern areas.

This is correct. There is a critical direction of wind flow 220-200 deg. where the convective process gets broken down in the lee of the Island and there is a path free of showers stretching to the NNE.
Sometimes this is very marked on weather radar images and was visible at times during this episode, especially yesterday afternoon and evening. Shower bands keep driving up inland from Riverton - Nightcaps towards and west of Lumsden, and also up and to the east of the Mataura Valley.
Any direction more westerly or southerly bring showers back over all coastal areas.

[spwill]

This satellite pic shows that sheltering quite well.
http://satellite.landcareresearch.co.nz ... n17069.txt

[sthguy]

This satellite pic shows that sheltering quite well.

Thanks. By that time - 1:44am - the flow was back to 230-240 and showers were back onto the Invercargill area, weather radar show this. But gthe satpic it shows a cloud-free shadow in the lee of the island.

[Myself]

I think the major issue with Invercargill missing out on this event was that the coldest upper air was out to the east, so there was always going to be more intensity out that way.

[spwill]

I think the major issue with Invercargill missing out on this event was that the coldest upper air was out to the east, so there was always going to be more intensity out that way.

West Otago/ Southland got atleast light snow fall.

[Myself]

I think the major issue with Invercargill missing out on this event was that the coldest upper air was out to the east, so there was always going to be more intensity out that way.

West Otago/ Southland got atleast light snow fall.

I don't get your point. The country as a whole missed the pool of 500hPa cold air. The further east you went the better it got, hence the coastal Clutha area and Dunedin.
There's also the issue of freezing levels gradually (very gradually) rising as you headed west.

[sthguy]

I think if you look carefully at total snow depths the Catlins area east of Mataura and Gore got the heaviest falls. Less widely reported was several cm's in Western Southland. Northern Southland only had a small fall as showers were only being driven into that area for a short time. Queenstown only had a cm or so.
Snow depth was more about the length of time showers were affecting particular areas. Invercargill was hovering around 1-2C with a 15kt SSW breeze for much of Monday night and early Tuesday morning, even with the wind coming straight off the sea, but Stewart Island was sheltering. So at the time of lowest freezing levels there was little if any precipitation around the city.

[CHCH Weather Chaser]

I see that snow showers have returned to some parts of Banks Peninsular in the last hour or so as showers have been pushed back towards the Peninsular.

[Storm Struck]

There was a definate coating of snow on Mt Herbert which can only really be seen from the outskirts of the city, and a wee dusting on the Port Hills.
Just a few snaps from yesterday when the brief showers came through late morning with the first trough.

Cold Snap no5 006.JPG

Cold Snap no5 007.JPG

[Chris Raine]

I was at my house at Thornbury yesterday and we had 1-2cm of snow which froze which fell between 0430-0600. Temp at 0900 was 0.2 degrees in the screen . The roads were like a skating rink with one car the wrong way round in a ditch on SH 99 by the Otautau turnoff at 10 am

[Manukau heads obs]

wintry heavy showers overnight here , with cold temperatures for here! (again!)