NZ Weather Forum

dogmelon wrote:Why on earth do the storms always go out south of Banks Peninsula all the bloody time now? They always used to come straight at chc from Ashburton way about a decade plus ago, then, something happened and they always do this sideways shuffle away from town. Does anyone have an idea why?

Increase in local taxes ? Fear of earthquakes ?

...

OK I go out ->

in Omarama we have large cb.s starting to arrive after a few dry hour

Always? Remember September 4.

tich wrote:Always? Remember September 4.

More of those days please!

Thunderstorm now overhead

Thunder here now!

Thunder and lightning observed to the west now.
Was about to call it a dud! The clouds don't look too special.

Good grief, I thought that those clouds looked like nothing much but oh well. We ended up with a develop in-situ storm instead. That will do.

Very heavy rain and hail 5-10mm in Templeton

Well we cant have three pages to this thread with out at least one photo,taken at 5.30pm to the south

Gosh the thunder is loud.

Yes some development west of the city. Looks like its gonna pass right over the city from the west going by radar. Question is whether it loses energy or not as it passes over. Its quite slow moving. Thunder getting louder and a little more frequent.

Well, the rain is stopping here in Hornby. It was fairly heavy, that dark but otherwise innocuous looking sky sure fooled me. Those couple of hours of sunshine must have been just enough juice to get things running.

House vibrating here. Rain getting heavy.

Convergence associated with the southerly change kicked the storms up in the vicinity of Christchurch. There are still some significant cells moving east across the area but these should weaken soon as the southerly change moves away to the north.

Thunsterstorm now way out to the east. Still can here the thunder as it drifts away.
Sferics are louder than went it developed overhead?
Sun out now. Shame that scrappy low cloud prevented view of the Cb, which obviously developing from the darkening clouds and SW change that came across here and met the stormy cloud from the storm that was moving around BP, that delivered the hail/heavy rain to Ashburton this afternoon?.

Thank you for the explaination. I'm interested as to why the convergence happened here and why it doesn't do it more often? We get NEers ahead of Southerlies a lot and nothing happens, why was today different?

dogmelon wrote:Thank you for the explaination. I'm interested as to why the convergence happened here and why it doesn't do it more often? We get NEers ahead of Southerlies a lot and nothing happens, why was today different?

Thanks for the question. I will have to give you a quick and simple answer but I hope it tells you something. The convergence associated with a southerly change is quite common in Canterbury but a number of things need to come together in order to get significant thunderstorms. The most essential are low temperatures at upper levels and high temperatures and dewpoints near the surface making the atmosphere unstable. The energy associated with the warmth and moisture at low levels can be released if something triggers convection, the air rises and moisture condenses and releases heat.

Things that can help the convection to get going are upper troughs, upper level divergence and of course convergence at the surface. The better these things come together the stronger the storms.

It seems that the storms did not get more intense than they did today due to the lack of a strong upper trough, insufficient cooling aloft, and the fact that there were areas of cloud this afternoon that limited surface heating. By the time the southerly change had reached Christchurch it was late afternoon and not warm enough at the surface to provide energy for strong storms. If the change had reached Christchurch a bit earlier and if there had been less cloud earlier today I believe these storms would have been a great deal more energetic. With a strong upper trough in addition to all that they could have been severe.

Hordur wrote:Convergence associated with the southerly change kicked the storms up in the vicinity of Christchurch. There are still some significant cells moving east across the area but these should weaken soon as the southerly change moves away to the north.

Yeah, true. you normally get that when a cold southerly airmass meets up an onshore NE. There was no NW intrusion as well to hinder the chance of thunderstorm development.
I gave up as nothing happened when the SW, not southerly came through here, but, with the change that developed over here, linked up with the storm around BP that previously delivered hail/heavy rain to Ashburton, was probably the cause of the thunderstorm which initially formed over here, with heavy cloud forms,prior to the SW wind change. Upper development causing it to move out to the east.

Saw a number of ICs and a couple of CGs from near Templeton but only captured a faint IC to the right of the image (sky was too light even stopped down to f/18).

A quick 3mm from that. Some decent long rumbles and a couple of very loud rumbles. SUn has made a return here now.

Thanks for that Hordur. Being a casual observer I found that explaination heplful. Have you taken over from Paul Mallinson as the informal weather forum, metservice liason type person? It was always good to read what Paul had to say.

NZ Thunderstorm Soc wrote: I gave up as nothing happened when the SW, not southerly came through here, but, with the change that developed over here, linked up with the storm around BP that previously delivered hail/heavy rain to Ashburton, was probably the cause of the thunderstorm which initially formed over here, with heavy cloud forms,prior to the SW wind change. Upper development causing it to move out to the east.

Yes, I think you have hit the nail on the head. A line of showers extending northwest from Ashburton moved northeast this afternoon. As it reached the area east of Christchurch a short wave upper tough and warm surface temperatures in the vicinity of Christchurch energized the showers.

dogmelon wrote:Thanks for that Hordur. Being a casual observer I found that explaination heplful. Have you taken over from Paul Mallinson as the informal weather forum, metservice liason type person? It was always good to read what Paul had to say.

No, absolutely not. I hope Paul continues his good work when he has the time. I am glad to be able to help.

Skywatcher wrote:Saw a number of ICs and a couple of CGs from near Templeton but only captured a faint IC to the right of the image (sky was too light even stopped down to f/18).

Good enough, Steven. Same here watching from home. Saw one Cg and a few lightning lines along the cloud, looking ENE from here, leaning on the gatepost looking over the paddocks to the NE.
It was more interesting just listening to the thunder, long rolling sounds as they moved further away to the east.
Reminded me of the old days when I used to record the sound of thunder by having the pause button cocked on the tape recorder, then as I heard the sferic, release the pause button, after counting and then recording the sound.