Active Front 17th Oct

[David]

Looking like a bit of action possible tomorrow with the front - especially over Taranaki where things could be quite intense with MS mentioning the risk of a damaging tornado, but still a moderate risk of squally thunderstorms in Auckland. Maybe something will actually happen this time.

[jamie]

why are the forcasting it to weaken as it moves up and across the country? i would have thought with the timeing of it (afternoon) it would intensify with land heating.

[Manukau heads obs]

certainly the time of year to get a good active front containing active CB's

looking foward to it

[Razor]

There sure is a good lightning show over the alps looking west from ChCh at the moment. Action definitely on the way somewhere!

[David]

Well I did not expect it to be so grey here this morning. Also currently 15C so its warm and all.
Wonder how the thunderstorm outlook for today will change - unfortunately I'll have to wonder to about 3.30pm to see it.

[jamie]

Big patch of blue sky coming over now so hopefully its not grey all day.

Was not expecting it to be this warm and humid already which is good

[Razor]

Another glorious sunny morning here in Canterbury. How long will it last?

[jamie]

MS thunderstorm outlook is looking good today with moderate risk in Canty with hail and high risk in auckland. Still the risk of a damaging tornado in taranaki and wanganui. Should be a good day

[Manukau heads obs]

and metservice have a severe thunderstorm watch out too
waitomo district would be the place to be I think
taranaki getting strikes according to my boltec

[Paul Mallinson]

The front became very active yesterday as expected and there was a slug of lightning strikes all the way up the Southern Alps through to the Nelson ranges and then through the Taranaki Bight this morning from Kapiti and now to Taranaki. Would have been some very heavy rain in the Alps but haven't seen any reginal council rain gauge data yet. 5 to 6mm/hr in a few places like Palmerston North and Wanganui and very strong wind gusts this morning (had a gust to 77knots on the top of the Rimutaka Road). But so far no reports of any twisters. But then most would probably go unnoticed - especially at night.
(We issued the severe thunderstorm watch at about 11:20am yesterday Brian and have kept it going and updated it since).

Paul

[Manukau heads obs]

We issued the severe thunderstorm watch at about 11:20am yesterday Brian and have kept it going and updated it since).

ah, I am a bit slow off the mark, LOL

[TonyT]

But so far no reports of any twisters. But then most would probably go unnoticed - especially at night.Paul

I'd be interested to know your thoughts Paul, on why are tornadoes considered likely in such strong surface airflow? I would have thought (although not having any specialist knowledge in this area) such a turbulent flow would inhibit and destroy any rotation? Or are we talking more about the micro-burst type of wind here?

[Cameo1]

Just wondering if someone could explain to me the difference between positive strikes vs Negative strikes? are one type rarer than the other?
cheers

[gllitz]

MS thunderstorm outlook is looking good today with moderate risk in Canty with hail and high risk in auckland. Still the risk of a damaging tornado in taranaki and wanganui. Should be a good day

....I agree, provided no one gets hurt!

[Paul Mallinson]

But so far no reports of any twisters. But then most would probably go unnoticed - especially at night.Paul

I'd be interested to know your thoughts Paul, on why are tornadoes considered likely in such strong surface airflow? I would have thought (although not having any specialist knowledge in this area) such a turbulent flow would inhibit and destroy any rotation? Or are we talking more about the micro-burst type of wind here?

Hi Tony.
The main ingredients we look for for tornadic potential are:
1. Instability on the front to start with. We try and determine the CAPE which is not an easy matter compared to air mass thunderstorms. Another approach is to look for significant ascent in conjunction with potential instability - this is a fall off of the wet bulb potential temperature with height. When you apply enough lift in a potentially unstable environment, the airmass can become unstable.
2. Significant backing of wind with height in the vicinity of the front (when considering the motion of the thunderstorm cell as well, this is referred to as Storm Relative Helicity or SRH).
3. The wind shear in the lowest levels (surface to 1 or 2kms) has to be significant as does the deeper shear to 6kms.
4. We also look at the Total Totals index and the SWEAT index. (For last evening and this morning, the SWEAT index easily exceeded 300 and even 400 in places).

The land can serve to actually serve to increase the SRH. This most often happens where you get a north to northeast low level flow say along the South Island west coast or perhaps down the Manawatu/Kapiti coast and in other places. Without this land influence, the low level wind might otherwise have been more northwesterly and so result in lower SRH values. This is one of the reasons why weak tornadoes often occur in the west of the country -especially the South Island. The SRH is quite sensitive to the wind over the land and so that is why it is very difficult to account for these variable effects and be very specific or even successful with tornadoes actually occurring. And when they do, they may not even be spotted.

Those are my thoughts for now. It really comes down to instability and windshear.

Paul

[jamie]

looks like hamilton is in the fireing line for thunder...should have gone home today dam it

[spwill]

I see the Taranaki area has had a lot of TS activity last hour or so.

Rather humid 14-15C dewpoints here, cloudy 19C.

[Willoughby]

looks like hamilton is in the fireing line for thunder...should have gone home today dam it

Lol I hope so, I just had to restart NexStorm as it tends to bring the strikes closer to home.. looks to be plotting again properly now.

18.6C/ DP 14 so conditions ripe. But it sure looks as if the lower NI has won today with that stalling front!

[NZstorm]

Yes, looks like northern Taranaki and Waitomo have taken a pasting in the last hour or so from the thunderstorms.
Good that this front is moving into the Waikato during the most unstable part of the day. Will be interesting to see what else fires up on the frontal boundary.

[jamie]

looks like hamilton is in the fireing line for thunder...should have gone home today dam it

Lol I hope so, I just had to restart NexStorm as it tends to bring the strikes closer to home.. looks to be plotting again properly now.

18.6C/ DP 14 so conditions ripe. But it sure looks as if the lower NI has won today with that stalling front!

That would explain why i thought Hamilton was about to get pummiled.

[Weather Watcher]

Just wondering if someone could explain to me the difference between positive strikes vs Negative strikes? are one type rarer than the other?

Try this website...

http://www.srh.noaa.gov/jetstream/light ... htning.htm

Note that NZ gets a much higher percentage of positive CG's than the US (from memory, about 30% here). I think this is mostly due to the higher vertical wind shear environments that storms generally develop within here compared to places like Florida.

I see the Taranaki area has had a lot of TS activity last hour or so.

Note, the MetService lightning strike website was not receiving data for most of this morning, so, as Paul has mentioned earlier, there was also a lot of lightning between the Southern Alps and Taranaki which doesn't show on that site.

[Razor]

Just been speaking to my counterpart in Dunedin...they are apparently getting pummeled down there by the cold snap.

Still sunny, breezy and mild in ChCh but I'm expecting big action later on!

[jamie]

Metservice lightning strike website??
whats the adress for that?

[Willoughby]

Try here Jamie: http://www.geographynetwork.co.nz/websi ... ke/run.htm

[TonyT]

Those are my thoughts for now. It really comes down to instability and windshear.

Paul

Thanks for those comments Paul, I guess what I was driving at was to what extent you think the strength of the surface flow (and hence its tublulence) might negate the ability of the instability and windshear to form tornadoes in the first place, and whether in fact conditions like this might be more prone to produce microburst type destructive winds rather than tightly rotating "twisters"?