NZ Weather Forum

tich wrote:

Snow in Lower and Upper Hutt...

Trademe Forum reports of the white stuff in Masterton, Wainuiomata and Wellington hill suburbs too - funnily, news item had just said that Wellington had escaped the stormy weather that was plaguing other parts of North Island.
Only 7C at Awhitu at 4pm - were you getting hail Brian?

We had 2 "flicks" today - the afternoon one came in around 4.30-5.00 but I saw only sleety showers in the downtown area. Clear again by 6.30.

Photo I took to day from a passing cb about lunchtime.

Photo I took to day from a passing cb about lunchtime.

Much of the cloud over Chch this afternoon appeared to be virga. Certainly had the appearance of a very cold airflow.

Kaikoura (by the look of radar shots) must've had quite alot of shower activity. (rain, hail, or snow, or all 3)

I see it has been gusting to 50 knots on the coastal here...would have blown a few cobwebs away ( been a while since a half decent SW wind (have seen it much stronger than today, today was only just worth mentioning))

Looks like the wind is here for a week...

Wow at those Auckland thunderstorms!!

Think we all know who's got dibs so far with this thunderstorm season so far!!

Max gust today was 66 km/h here

Off to Whakapapa or Turoa today, outlook is windy and mainly fine.
10cm of fresh powder at Turoa, it better be open!!

While it was a cold airstream anyway, am I correct in claiming that the upper atmospheric airflow was extremely cold, and this contributed to the hail up north and the freak snow reported around Wellington? The high clouds visible over Chch yesterday had a very icy, glaciated look about them.

Yes, the upper air temps were very cold -38C at 500 hPa over Invercargill and about -35C over central New Zealand. But the temperature profiles were capped just above this level and so there was good bouyancy (CAPE) up through to about 20,000ft and then warm temps above. This limited the depth of the convection and the amount of lightning. (I've always used as a rough rule that CB tops need to go above about 20,000ft to get lightning when doing a parcel lift on a sounding).

But with the good bouyancy in the low and mid levels, this produced the sort of hail seen in Auckland (and elsewhere) (very informative hail picture NZStorm). At the same time, temperatures around 10 to 12C and dew points around 2 to -2C were seen from central parts down to Canterbury - and about 15-17/6 upper North Island). These sfc values along with along with dry layers aloft and near the surface on the Paraparaumu and Whenuapai soundings brought strong outflow winds to the surface, cooled temps off by about 5 to 7C as squall lines passed, and also snow pellets to virtually sea level in the Wellington area yest afternoon.

There were several areas of wind convergence at the surface that passed over the North Island. A couple of notable ones apart from what happened in Wellington was a squall line that passed through Taupo which gave a gust of 55kts from the gust front and the line of thunderstorms that went through Auckland at midday stayed intact and moved over the Whagarei area about 3pm giving a gust there to about 60knots.

So a very interesting weather day all in all. It just shows you that instability is a relative thing. Even if the surface dewpoints are quite low, you can still get good convection if the upper air is sufficiently cold.

Paul

Thanks Paul. I'm no expert in technical meteorology, but that was an interesting explanation. The air over NZ was certainly very cold and unstable - the squall lines sending the temperatures plummeting and snow/snow pellets falling to below the 800m forecast by MetService. (Taupo only 4C at 1pm yesterday - must've been snow or sleet nearby)

I've heard about simiar situations in past springs, eg late September 1977, when hail fell in many northern areas. The NZ Herald then reported snowflakes even in parts of central Auckland and Waiheke Island, plus a blanketing in Matamata. This could easily have been in the form of snow pellets, like what the Wellington area got yesterday.

I've always used as a rough rule that CB tops need to go above about 20,000ft to get lightning when doing a parcel lift on a sounding).

Yes, I use 7000m as the level for thunder. But recently I read that lightning had been observed in cb tops of only 5000m in a study in Japan.
But that was in a polar air mass.

I think this was the coldest airmass to strike NZ this year but I'll need to check that.

NZstorm wrote: I think this was the coldest airmass to strike NZ this year but I'll need to check that.

Have a pretty good feeling it was.. LAPS was progging 518dm thickness over Dunedin at 4am yesterday.

Blotches of snow on the northern-lower part of SH 47 from yesterday's fall.. snow was melting quite quickly today at the top of the Bruce in the sunshine. The freezing rain we had around lunch was quite painful I might add!

. But recently I read that lightning had been observed in cb tops of only 5000m in a study in Japan.
But that was in a polar air mass.

I disagree, seen numerous times here with Cb tops to 4000m with 10+ CG discharges. (29 Sep for example)
Also a few times here with weak IC and cumulus mediocris in the right (usually humid) conditions.

seen numerous times here with Cb tops to 4000m with 10+ CG discharges. (29 Sep for example)

Have a look at your cloud photo's foggy from sep 29, clouds are much higher than 4000m.

Yes, I think they were higher. I took a look at the Whenuapai sounding for noon Sept 29 and using Temp=20 and Dewoint (Td) = 12 for Hamilton, and this pointed to CB tops to 25000-30000ft and max tops to about 350000 (T= -52C). There was a convergence line to the east of Hamilton and a concentration of lightning strokes there.

In the mid 70's, I spent 18 months on Campbell Island and during that time we saw CBs with classic anvil shapes moving through all the time, yet I only ever observed one flash of lightning. I was peering into a Stevenson screen at midnight on New Years Eve and the next second - poof! and that was it!).

I think the comment about the Japanese study with tops to 5000m is reasonable, but in my experience, with tops much lower than that, lightning is quite rare.

Paul

i remember when living in the waikato observing some spring style storms with very low tops and the odd flicker of lightning, so I geuss in a way I can back up (to a point) what the fogster is saying.

Its funny too cos i have seen on many occasions down there, C-C in non-glaciated Cb's? anyone else ever seen this?

remember when living in the waikato observing some spring style storms with very low tops and the odd flicker of lightning

Lightning will mean the Cb clouds are high topped.
Sometimes Cb clouds can appear to be small but if they produce Lightning that's the clue they are high topped.

Sorry but I still disagree..
http://www.weatherforum.org.nz/forum/vi ... 7093#37093
That afternoon Cb shot had heights of 8-9500m

The sunset Cb (T/Cu there in it's early stage..) was 20-30km away, and looking at it again it probably had heights of 5-6000m. Pileus cap there with several CG's. We also get the rare occurance of dry lightning (IC).

You might not experience this in the maritime-effected meso-climate of Auckland, but we certainly do here!

Your sunset cloud foggy is an active Cb ( photo below from sep 29)
You can see fresh active Cb top blowing out of old Cb anvil cloud and I'm not suprised your saw Lightning from it.

Yes, that cb top is about 10,000m, at the tropopause. The give away is the pileus cap which shows the cloud has reached the stable layer.