NZ Weather Forum

First RED alert of the year is on us...warrants a new thread I reckon. From MS:

Significant heavy rain for the west of the South Island during Wednesday, Thursday and Friday


An active front, preceded by a strong and very moist north to northwesterly flow, is forecast to move slowly northwards over the South Island from Wednesday to Friday, bringing significant heavy rain to western areas. The heaviest rain is expected in Westland and Buller, both of which are now covered by a RED WARNING, signalling the likelihood of significant impacts.

Meanwhile, Orange Warnings cover the headwaters of the Otago and Canterbury lakes and rivers, and also Fiordland and Tasman.

Please note, that this is likely to be a significant heavy rain event. Warnings may be extended, added, or upgraded to a Red Warning closer to the time.

In addition, a Watch for possible severe northwest gales has been issued for the Canterbury High Country.

Heavy rain can cause streams and rivers to rise rapidly. Widespread flooding and slips are possible, and driving conditions could be hazardous during the event. People are strongly advised to keep up to date with the latest forecasts.

Heavy Rain Warning - Red

Impact: This rain is expected to cause dangerous river conditions and significant flooding. Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.

Area: Buller
Period: 54hrs from noon Wed, 2 Feb - 6pm Fri, 4 Feb

Forecast: Expect 300 to 400mm of rain to accumulate about the ranges, and 150 to 200mm near the coast. Peak rates of 20 to 35mm/h about the ranges, with thunderstorms possible. Please note, that further heavy rain is likely during Friday night and Saturday, and this Warning could be extended.

Area: Westland
Period: 42hrs from 1am Wed, 2 Feb - 7pm Thu, 3 Feb

Forecast: Expect 550 to 750mm of rain to accumulate about the ranges, and 150 to 250mm near the coast, but possibly 300mm near the Glaciers. Peak rates of 30 to 45mm/h about the ranges, with thunderstorms. Please note, that further rain is likely during Thursday night and Friday, and this Warning could be extended.

Heavy Rain Warning - Orange
Impact: Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.
Area: Tasman west of Motueka

Period: 31hrs from 2pm Wed, 2 Feb - 9pm Thu, 3 Feb

Forecast: Expect 150 to 250mm of rain. Peak rates of 20 to 40mm/h. Please note, that further rain is expected after this period, and the Warning may be extended.

Area: The headwaters of the Canterbury lakes and rivers
Period: 36hrs from 7am Wed, 2 Feb - 7pm Thu, 3 Feb
Forecast: Expect 350 to 450mm of rain about the divide, with 150 to 200mm within 15km east of the divide. Peak rates of 20 to 40mm/h expected about the divide, with thunderstorms likely. Please note, that further rain is likely after this period, and the Warning could be extended.

Area: The headwaters of the Otago lakes and rivers
Period: 24hrs from 3am Wed, 2 Feb - 3am Thu, 3 Feb
Forecast: Expect 200 to 300mm of rain about the divide, and 100 to 150mm within 15km east of the divide. Peak rates of 20 to 35mm/h about the divide.

Area: Fiordland, mainly north of Doubtful Sound
Period: 21hrs from 6am Wed, 2 Feb - 3am Thu, 3 Feb
Forecast: Expect 100 to 130mm of rain. Peak rates of 15 to 25mm/h.

Strong Wind Watch
Area: Canterbury High Country
Period: 13hrs from 9am - 10pm Wed, 2 Feb
Forecast: Northwest winds may approach severe gale at times.

Issued: 10:39am Tue, 1 Feb
Next update: 9pm Tue, 1 Feb

Poor old Buller again.
Should be an interesting event this.
Interesting with MS forecasting lightning from wave cloud across the Alps for this event.

Metservice wrote:
Area: Westland
Period: 42hrs from 1am Wed, 2 Feb - 7pm Thu, 3 Feb

Forecast: Expect 550 to 750mm of rain to accumulate about the ranges,

Lets just take a moment and let that sink in, 0.75 meters of rain!
This is potentially a very bad situ unfolding if that forecast comes true, the West Coast has been abnormally dry so far this summer, the soil being so dry is just going to cause vast amounts of run off, the potential for flooding is very serious.

I am not surprised this has justified a RED warning.

Yup those sorts of numbers are the annual rainfall average for many east coast towns, ChCh included

Looking at the models in a bit more detail:

A potent low to mid level atmospheric river (aka Jet Stream) seems to be responsible for the very high rainfall predictions. Very evident tropical moisture feed coming in early Wednesday morning direct to the west coast.

One thing to keep in mind in the models at this stage:
This jet stream is trending to hang around due to subsequent low pressure systems in the Tasman throughout into the weekend, so this could cascade into heavy rainfall later on the week for the west coast of the North Island (in particular New Plymouth). As we progress into the week, we can get bit more confidence in that prognostic.

Where do you get your charts from, Tim?

NZ Thunderstorm Soc wrote: ↑Tue 01/02/2022 12:53 Poor old Buller again.
Should be an interesting event this.
Interesting with MS forecasting lightning from wave cloud across the Alps for this event.

That seems awfully specific of them eh? They've got that feeling in their bones situation going on in their forecasting office.

See there is a lightning flash in the wave cloud inland above the Waitaki River this evening .First feature through this evening at Thornbury gave 3mm .My water tank down to 25% of full. .Was nice to hear rain water dribbling into the tank.

How does wave cloud lightning work? The cloud isn't particularly thick like usual storm clouds given that its already really high up.

dogmelon wrote: ↑Wed 02/02/2022 00:14 How does wave cloud lightning work? The cloud isn't particularly thick like usual storm clouds given that its already really high up.

Normally you dont get CG lightning from wave cloud, that earthing out takes place back in the mountains, wave cloud lightning can be cloud-to-air, Intra-cloud lightning or spider lightning. Makes for safe viewing, little chance of been struck by a bolt.

Two atmospheric rivers as NIWA just pointed out, one comes from due north and the other via Australia:
21C in Lyttelton at 6am today, but dry so not an uncomfortable night.

Reasonbaly heavy rain coming and going here in Halswell. Can only imagine what its like up in the alps today. At least schools are back and most of the extended summer family camping/ tramping season is done, hopefully not too many people in the hills

I just happened to be watching the blitzortung thing when a strike showed up in the south canterbury bight. Weird.

I thought so - thought I'd seen a flash of light a few moments ago, now just heard a long roll of thunder.

Nice thunder and lightning in Ashburton now. Lightning is flashing quite brightly - how exciting!

darcyplumb wrote: ↑Wed 02/02/2022 10:48 Nice thunder and lightning in Ashburton now. Lightning is flashing quite brightly - how exciting!

Yes I see a strike right over you just now

NIWA is modelling over 1000mm in the 48 hours to Friday 1am in the alps and 300-500mm for lower areas. More rain looks likely Friday / Saturday too, this is really worriyng for the Coast

Razor wrote: ↑Wed 02/02/2022 11:36 NIWA is modelling over 1000mm in the 48 hours to Friday 1am in the alps and 300-500mm for lower areas. More rain looks likely Friday / Saturday too, this is really worriyng for the Coast

Oh dear. The only thing that gives me reassurance is that the emergency management on the coast is up there with the best in the country. They have taken this seriously from the moment the red warning was issued. People have already been evacuated from high risk areas. Scary to think that this event could be worse than last year’s flood.

Bands of moderate showers moving through CHCH CBD.
Keeping an eye on the river flows in Westland. https://envirodata.wcrc.govt.nz/dashboa ... levels.php
Hokitika River at the Gorge is up from 1.3m at Midnight to 3.6m at 11am

Might river avulsion be a danger with that much rain?

There's an alert on the Hokitika River now at the gorge. Waiho is up at 7.5m and Haast is going 1700 cumecs:

https://www.ecan.govt.nz/home/flood-warning/

Flood warning from ECAN. Main concern seems to be the Rangitata River, however I do note that they are worried that the high flows to come for the Ashburton River may test the flood repairs from the May/June 2021 event.

Moderate to heavy rain at Thornbury currently.Was 20.3C at 0900 Felt warm and muggy 9.6mm so far today A useful amount for tank and garden

I'm surprised by the lack of lightning showing up on blitzortung this afternoon.

Pouring down in Westport.