Nov 1st Canterbury Moderate Risk

[Storm Struck]

Moderate risk put out by MS for inland Canterbury today, certainly some mid level instability about.
Just had a few spots of rain too, wind is currently NE and should stay that way for the afternoon.

[03Stormchaser]

Valid to: 00:00am 2 Nov 2009 NZDT

Issued at: 10:54 am 1 Nov 2009 NZDT

In Canterbury during the afternoon northeasterlies are expected to spread over the plains towards warmer northwesterlies inland. Where these winds meet a few showers will probably be triggered and there is a moderate risk that one or two could develop into thunderstorms with localised heavy rain.
No other thunderstorms forecast although a front is expected to bring some heavy
rain to Fiordland in the evening.

- metservice

There thunderstorm forecast has taken me some what by surprise, haven't even had a look at possibilities yet, but with metservice confidence up on this one would have to think its a goer.

[spwill]

I take it nothing developed, looked warm in the upper levels.

[NZ Thunderstorm Soc]

Not interested.

JohnGaul
NZThS

[Myself]

The moisture would have been a struggle today. Cold sea off Canterbury cannot help! And yes...pretty rubbish at upper levels. Perhaps it was more of a low risk? Second guessing their convection forecasters is probably not good practise though as I think they are the most experienced of all the meteorologists there.

[Storm Struck]

Nothing much eventuated apart from some weak CU forms inland, was still good for a practice chase.
The NE wind was reaching quite far inland even blowing NE at Methvan, i just dont think the NWer got up enough further over the plains to allow for that wind convergence.
We did find a water race which was the highlight of the trip .

[spwill]

The moisture would have been a struggle today.

Yes, but the 17C with a 9C dewpoint at Chch airport this afternoon is quite moist for down there.

[NZstorm]

I thought the risk was pretty much nil. That's not meant to be a criticism of the Met Service, just my opinion. While the lapse rates were steep in the upper levels the air was still relatively warm. The 500mb temperature was around the -16C mark which meant there was going to have to be hot moist air on the surface to get enough instability for a parcel to get up to that level, not a condition easily met in early November. The 9C dewpoint would have been juicy for a very cold airmass but not this one.

[03Stormchaser]

I thought the risk was pretty much nil. That's not meant to be a criticism of the Met Service, just my opinion. While the lapse rates were steep in the upper levels the air was still relatively warm. The 500mb temperature was around the -16C mark which meant there was going to have to be hot moist air on the surface to get enough instability for a parcel to get up to that level, not a condition easily met in early November. The 9C dewpoint would have been juicy for a very cold airmass but not this one.

I think the upper air as you put always looked dodgy, just to warm. Dew point in Ashburton was up around 13C at one point although im not 100% of its accuracy, models indicated winds inland winds to tend light so I could see possibly were they were coming from as the NE kicked in, but the NE was very dominate the whole time.

Would be good to hear from Metservice what there thoughts are, esp on what they saw that we didn't that warranted a Mod risk.

[tgsnoopy]

It was a stunner of a day in the afternoon on the Trans Alpine train coming back from Greymouth to Christchurch.

No sign of any storm type activity noticed, but then I was kinda distracted

The Morning in Greymouth & up to Punakaiki Pancake Rocks was lousy low cloud & drizzle, great to leave the West Coast and into good weather.

Really quite warm in Christchurch & I'm pretty sure it was cooler arriving back in Tauranga this morning.

[NZstorm]

great to leave the West Coast and into good weather.

Yes, its a great experience crossing the divide when the flow is W or NW. I've only done it once but went from cold heavy rain to warm sunshine in about 15mins

[NZ Thunderstorm Soc]

I've experienced the opposite when driving the Trans-Alpine, back in the early 1990s.
Total blue-dome day on the West Coast and arriving at ArthursPass out of the Otira Tunnel, grey skies and snow on the ground and grey skies all the way to Christchurch in a cold southerly airflow situation.

JohnGaul
NZThS

[Michael]

IS it that much of a change at either ends of that tunnel?I never been across there excuse my naievety

[tgsnoopy]

Otira tunnel is about 8.5km long, a 33:1 gradient it climbs about 250m from Otira to Arthurs Pass.

When a train travels from Otira to Arthurs Pass doors close behind the train (at Otira) and large extractor fans start to suck the air through from Arthurs Pass end.

I got the impression the doors on the tunnel don't work the other way, obviously they'd need to open to let the train exit the tunnel, but I expect the warm exhaust gasses would rise up the tunnel ok without the fan.

In any case, the tunnel is absolutely covered in soot and they close the outside viewing/photographing carraige and the train stinks of diesel fumes going through the tunnel, it takes about 15 minutes to get through the steep tunnel.

The great divide of the Southern Alps is above the tunnel, hence different weather conditions either side.