General October Weather

[NZstorm]

A number of ingredients have been there? Answers to learn from would be great. Cheers

The 500mb shortwave trough crossed Canterbury in the morning and was well departed by 00Z (1pm). That was the biggest issue. The short wave creates divergence aloft which lifts the air allowing thunderstorms to develop.

[NZstorm]

Forecast sounding for 4pm from GFS for Christchurch. Even if there was good lifting mechanisms, the instability is low topped in the afternoon.


Just add that the rule of thumb for lightning possibility in convection is that the cloud tops reach -20C and the cloud depth is at least 3km. It is possible to get lightning off low topped cb if those 2 requirements are met.

[Storm Struck]

Completely unaware of what happened today as it was fairly calm and sunny in Hornby, just shows how isolated it was.
Quite an interesting week ahead by the looks, a strong front due to sweep the country mid week followed by a mid Tasman low set to cross central NZ at this stage.
If it does this could bring much needed rain to the likes of north canterbury etc.
Saturday and Sunday look like potential isolated thunder and hail set up days in Canterbury.
El Nino well and truly kicking into gear

[Razor]

Yes this low looks interesting. Driving to the Coast on Friday evening, right when its due to hit. Ross Goldfields 150th celebrations over there for labour weekend plus a wee daytrip to Okarito to fix up a geocache I own there, hopefully it pushes through quickly Saturday morning for the opening of the Hokitika to Ross section of the new Rail Trail

[Richard]

Winds fair cranking into it now and this not even the peak of the wind yet

[Rogue]

Quick question about the sounding - is the cloud top the point at which the grey line intersects the temperature line?

[spwill]

Lifting dewpoints over northern NZ from tomorrow should set up the incoming low at the weekend for some meaningful rain, looking like a drier air mass moving in for Sunday.

[mikestormchaser]

Quick question about the sounding - is the cloud top the point at which the grey line intersects the temperature line?

Good question. . I'm not familia exactly with reading soundings.. be good for nzstorm or paul malinson to explain it. I know he did to some degree the other day but it's hard to understand reading it. I dont fully understand it but I'd love to keep learning

[Willoughby]

Quick question about the sounding - is the cloud top the point at which the grey line intersects the temperature line?

Theoretically, subject to interpretations of the lifting parcel start. There is a soft inversion at 550 hPa (15k ft), and an absolute inversion (the tropopause) at 300 hPa (29 k ft). Due to the low surface dew points this day it couldn't get past the first inversion.

I've modified Steven's sounding slightly to reflect 4pm observations at Christchurch Airport and using surface-based obs 15C, dewpoint 6.5C. NZstorm used the mixed-layer method which is perfectly fine in this instance.

gfs.stormcast-sounding.bsch.init-2015101900z.fcst-2015101903.lat_-43.5_lon_172.9_custom-t15dp6.5_hodo-off[1].png

Shows cloud-top height of about 18k feet.

Using satellite is a good way to check out cloud-top temperatures and heights using a sounding.

chchcloudtoptemps.png

Shows about -30C over the Canterbury region, and using the forecast sounding from above (follow the -30 isotherm) indicates cloud top heights to about 20,000 feet / 6 km.

[NZstorm]

Re sounding interpretation

I think 100mb mixed layer dewpoints are the way to go for the east of the South Island when assessing convection chances. Its usually quite a dry atmosphere down there.

[NZstorm]

I dont fully understand it but I'd love to keep learning

http://www.theweatherprediction.com/thermo/skewt/

A good starting link here.

[mikestormchaser]

I dont fully understand it but I'd love to keep learning

http://www.theweatherprediction.com/thermo/skewt/

A good starting link here.

Thanks steven ☺ Yes very dry atmosphere down here witch does kill a lot of chances. We have the ingredients and can get a hell of a storm set up as you know on the day but just never seems to happen often. Speaking of witch the October 27th 2002 Labour weekend supercell storm anniversary is coming up! I'll have a drink to that.

[Rogue]

Cheers Will for that further info

[spwill]

Hopefully some decent rain for the Auckland gardens tomorrow. Dewpoint up to 17C over northern NZ tomorrow

[David]

Very warm today with a max temp of 22.4C for Howick. My October record is 22.6C.

[Nev]

Last night MS have a moderate risk of T-storms in Northland and Waikato area this arvo - low risk elsewhere for the upper North Island...

MetService - Thunderstorm Outlook

Valid to: Midnight Saturday 24 Oct 2015
Issued at: 9:30pm Friday 23 Oct 2015

The frontal rainband is expected to move across northern New Zealand during the afternoon and evening. In Northland, Auckland, Coromandel Peninsula and the western Bay of Plenty during the afternoon, and the remainder of Bay of Plenty and northern Gisborne in the afternoon and evening, there is an associated low risk of localised downpours with possible hourly intensities of 25 to 35mm/hr in one or two places. At this stage, it is difficult to say exactly where these localised very heavy falls will occur, but rainfall of this intensity can bring localised surface flooding and slips. Additionally there is a moderate risk of thunderstorms in Northland during the afternoon. In Waitomo, Waikato and western Bay of Plenty following the rainband, there is a moderate risk of isolated thunderstorms developing for a time. These thunderstorms have the poterntial to produce localised heavy rain of 10 to 25mm/hr. Also, there is also a low risk of thunderstorms in the eastern Bay of Plenty and northern Gisborne in the evening.

Elsewhere, no thunderstorms or significant convection expected.

MetService T-storm Outlook map for Oct 24 pm.png

[treetop]

Cant see much happening. Going through too quick, maybe East BOP ?

[David]

Not looking very significant on the radar. Only 21mm this month and really need some good rain!

[Nev]

Interesting that Akld Aero has also had 21mm, while NIWA's nearby Mangere site only has about 12mm, as has Whangaparaoa. And just 13mm here so far.

Looks like the moderate risk for T-storms this arvo is now confined to the BoP, dependent on daytime heating and sea breeze convergence...

[spwill]

It looks like high cloudiness will hinder the chance for daytime heating Bay of Plenty today, cloud clears tonight.
Monday looks a chance for thunder in the Bay of Plenty depending on the timing of an upper trough.

A few very welcome showers here this morning

[NZstorm]

It looks like high cloudiness will hinder the chance for daytime heating Bay of Plenty today, cloud clears tonight.
Monday looks a chance for thunder in the Bay of Plenty depending on the timing of an upper trough.

A few very welcome showers here this morning

I tend to agree with these comments.

GFS has a SE change pushing into BOP/SE Waikato this afternoon but there would need to be some decent heating to generate instability.

[NZstorm]

The weatherzone satellite looking great now that they have improved it to 10min updates.

[Storm Struck]

Still remaining very dry here probably the most dry and warm ive seen for October, Port hills starting to turn brown normally they stay green until late November.
We had a little tease of drizzle the other morning barely 1mm, I think maybe theres abit of a strong el nino peak happening at the moment and will rebalance for awhile come November - December.
Perhaps abit more unsettled around xmas new years then becoming more settled from January.

You can see a more SW phase kicking in from next week, whether it brings much needed rain and storms im not sure of.
Tomorrow looks like there could be a few isolated heavy showers about the North Canterbury foothills and High Country late afternoon or evening.

[David]

6mm here in the last hour

[Richard]

Tomorrow looks like there could be a few isolated heavy showers about the North Canterbury foothills and High Country late afternoon or evening.

Yes its looking like a possibility, would be great if it does.

The NW wind returned again about 3.00 pm today, though not strong thankfully.