Rapidly Deepening Low and Possible Snow Event July 20-22nd

[spwill]

My thoughts are the inland high Canterbury plains will only see rain, the snow will be where the air is drier, the likes of the Mackenzie Basin and Naseby.

[TonyT]

Nothing too worrisome in this morning's data for Christchurch or North Canterbury, a bit wet and blowy. But from the Rakaia south to Dunedin still looking like heavy rain, gales or near gales, and heavy snow above about 500m. I could see Central Otago picking up a snowfall out of this.

[Bradley]

Metservice still confident of heavy snow above 400m for Canterbury, very interesting!! WeatherWatch and Metervice on one side with low level snow and the rest on the other side with 500m or above. Fascinating to see how it pans out!

SEVERE WEATHER WATCH FOR NORTHLAND, AUCKLAND, GREAT BARRIER ISLAND,COROMANDEL PENINSULA, BAY OF PLENTY, GISBORNE, MARLBOROUGH,CHRISTCHURCH, CANTERBURY PLAINS, CANTERBURY HIGH COUNTRY, NORTH OTAGO,CENTRAL OTAGO, DUNEDIN, CLUTHA, SOUTHERN LAKES ISSUED BY METSERVICE AT 0937hrs 19-Jul-2017

SIGNIFICANT WINTER STORM FOR MOST PARTS OF NEW ZEALAND

A rapidly deepening low pressure system is forecast to approach New Zealand from the Tasman Sea on Thursday and move slowly east across the central districts on Friday and Saturday.

The low is expected to bring strong winds and a period of heavy rain to the upper North Island on Thursday and Friday.

This WATCH is for the possibility that northeast gales will become severe in the following areas...

NORTHLAND and AUCKLAND: From Thursday afternoon to Thursday evening. COROMANDEL PENINSULA and BAY OF PLENTY: From late Thursday afternoon to Thursday night.

GISBORNE: From Thursday evening to Friday morning.

This WATCH is also for the possibility that rainfall accumulations may reach warning criteria in the following area...

BAY OF PLENTY, especially in the east: From Thursday afternoon to Thursday night.

In addition, this low is expected to bring heavy rain and snow to the eastern and southern parts of the South Island on Friday and Saturday as it slowly moves to the east of the country.

Snow is likely to fall to low levels over inland southern and eastern parts of South Island during Friday and Saturday with significant accumulations above 400 metres. This will affect many higher roads in the region, and could cause problems for livestock.

This WATCH is also for the possibility that rainfall accumulations may reach warning criteria and fall as snow above 400 metres in the following areas...

CANTERBURY and OTAGO: Starting Friday morning and may continue into Saturday.

[Andy]

Nothing too worrisome in this morning's data for Christchurch or North Canterbury, a bit wet and blowy. But from the Rakaia south to Dunedin still looking like heavy rain, gales or near gales, and heavy snow above about 500m. I could see Central Otago picking up a snowfall out of this.

Just the higher parts of Central above 500m? How much rain? won't take much to cause problems any more than 20mm may cause issues.

[TonyT]

Nothing too worrisome in this morning's data for Christchurch or North Canterbury, a bit wet and blowy. But from the Rakaia south to Dunedin still looking like heavy rain, gales or near gales, and heavy snow above about 500m. I could see Central Otago picking up a snowfall out of this.

Just the higher parts of Central above 500m? How much rain? won't take much to cause problems any more than 20mm may cause issues.

You could be looking at 20-40mm in Central, snow down to perhaps 3-400m.

My main concern for flooding would be North Otago and South Canterbury. 100mm in these areas usually results in floods. Gales about the coast also. Dunedin and Christchurch will be on the edges of it, but its the areas in the middle who will get the worst.

[spwill]

South Dunedin dodging a bullet ?

[Bradley]

Latest update for Canterbury plains still says snow down to 300m, that's with all the new model data from yesterday which keeps the cold air to the south too - yet they are still sticking to their guns...

[Bradley]

Nothing too worrisome in this morning's data for Christchurch or North Canterbury, a bit wet and blowy. But from the Rakaia south to Dunedin still looking like heavy rain, gales or near gales, and heavy snow above about 500m. I could see Central Otago picking up a snowfall out of this.

Tony it looks like there will be some colder air (around -2C at 850hpa) in Mid Canterbury from around 3am on Saturday morning which will still be fairly and squarely in the middle of the heavy rain coming down - perhaps this might be the best chance for heavy snow down 300m?

[TonyT]

Nothing too worrisome in this morning's data for Christchurch or North Canterbury, a bit wet and blowy. But from the Rakaia south to Dunedin still looking like heavy rain, gales or near gales, and heavy snow above about 500m. I could see Central Otago picking up a snowfall out of this.

Tony it looks like there will be some colder air (around -2C at 850hpa) in Mid Canterbury from around 3am on Saturday morning which will still be fairly and squarely in the middle of the heavy rain coming down - perhaps this might be the best chance for heavy snow down 300m?

I see the heaviest rain earlier in the day, and easing (although by no means gone) after midnight. By then we are into the cold advection. I guess it all depends on micro factors which are naturally inconsistent from model to model and model run to model run. Which is why its best to remember that models are guidance, not a forecast.

[mikestormchaser]

Story on stuff with MS saying 100+ mm for chch city now

Sent from my Redmi Note 4 using Tapatalk

Looks like moisture totals have backed away again. And putting it more south Canterbury event then rain returning for the city sat morning.

[wembley]

I wish this low could rocket over us real fast, limiting the impact of severe weather

[NZstorm]

Latest gfs precip chart for this event. Dunedin looking very much in the flood risk zone.

[NZ Thunderstorm Soc]

The Waihi through Geraldine could be bank to bank
Cleared some soak pits around the house today, to make sure the place doesn't flood and the water sinks into the clay underneath !
I'm expecting about 80mm here from this event.
Less than that would be nice

[Richard]

Looking likely the center of this low will pass right over here, shame its not a summer low pressure system, would have been a great chance for an electrical storm.

[Bradley]

Latest EC update has ramped up the moisture levels with 60mm now for Christchurch and between 80-100mm for inland towns south of the Waimak - still stubbornly keeping the snow level at 500-600m though!!

[RichyDunedin]

Wouldn't want to be in South Dunedin this weekend.

[wembley]

The met service is saying exactly the same thing for Central Otago today that it did yesterday depsite it being uodated.

[Bradley]

See below, just saw a really detailed explanation from the Metservice of how snow could be falling to 300m despite the lack of cold air (850hpa temps above 0C) intrusion from this event, I guess this is why these guys went to University lol

Here is a tephigram over Fairlie at 6pm tomorrow. Isotherms run diagonally, the blue line is 0C and the red -15C. Despite the warmer air aloft (as you say), it is plenty saturated in the 'dendritic zone' - upper set of light blue lines either side of -15C. This is a great snow making set-up, producing enough snow for plenty to fall through the above-zero layer without melting, and able to reach the surface.

[NZstorm]

I think the more people that can use those charts the better.

The skew-t (Tephigram) is the primary forecast tool for storm chasers.

[Talbotm]

Yes agree Bradley. They have been very consistent with snow to low level through out. I did notice that as the low rapidly deepens it appears to be dragging cold air down from higher in the atmosphere outside of the cold air that is coming in from south. I guess time will tell with what actually happens. Certainly got people talking this system.

[spwill]

Not looking good for the Dunedin area. Looks like the lower Taieri Plains will flood and have they sorted out the drainage issues in South Dunedin.

[Orion]

Very strong gusts of wind here around 5am that woke me up then.

[Bradley]

I think the more people that can use those charts the better.

The skew-t (Tephigram) is the primary forecast tool for storm chasers.

So in saying that do you see snow to 300-400m like the Metservice for Canterbury?

[Richard]

See below, just saw a really detailed explanation from the Metservice of how snow could be falling to 300m despite the lack of cold air (850hpa temps above 0C) intrusion from this event, I guess this is why these guys went to University lol

Here is a tephigram over Fairlie at 6pm tomorrow. Isotherms run diagonally, the blue line is 0C and the red -15C. Despite the warmer air aloft (as you say), it is plenty saturated in the 'dendritic zone' - upper set of light blue lines either side of -15C. This is a great snow making set-up, producing enough snow for plenty to fall through the above-zero layer without melting, and able to reach the surface.

So what are the horizontal lines that have 150 and higher as you head down the chart

[snowchaser01]

Severe weather warning issued! For canterbury however, they mention the rain and snow to 800m? Yet on their plains rural forecast it still says 300m... hmm