Very cold and unstable SW blast, Eastern SI 25th Sept

[Bradley]

Big upgrade in this mornings run of EC with 30-50mm of rain now for Canterbury and 5-10cm of snow down to 300m. These low pressure systems are notoriously hard to predict though so I’m waiting for the weekend before anything can be locked in...

[Chris W]

Which system Bradley?

[Bradley]

Which system Bradley?

The one on Tuesday where a low pressure system comes down from the NW and crosses over North Canterbury, it then dredges up colder air from the south, a possible warm advection scenario is on the cards for the Canterbury high country...

[NZstorm]

Looks like severe gales possible Tuesday over the NI and perhaps squally thunder, though still 5 days out.

[Cyclone Tracy]

Main ensembles have been hinting this in the last 24 hours. After the initial polar jet surge and surface low, GFS 18Z has an upper trough launching from over the Antarctic shelf next Wednesday into the NZ region. Still a while to go but that is a serious cold shot into the SI if it verifies.

Upper trough 2509.jpg

[Storm Struck]

Very cold air next week alright, upper air -36c over most of the south island and -6c on the 850mb.
Suggests snow around 200-300m possibly 100m down Otago Southland area, could potentially be part of the SSW so one to watch as the days get closer.
We had a snowfall on the 19th of september 2005 in chch with up to 10cm recorded in some places, and i remember a few flurries i think it was november 2011 in the city.

[Cyclone Tracy]

Just recorded my highest wind gust of the year of 61.2km/h. Anything over 60km/h is impressive in my location.

Latest EC 18Z run was interesting for parts of Canterbury tomorrow night between 9pm to midnight. 1cm of snow for Christchurch, 2 to 4cm for Ashburton, Rolleston, Methvan etc. 850 hPa projected to be around -4 to -5c with a bit of moisture at 700 hPa. Locals will know better than me if that is just a model dreaming

Projection attached.

[Awhituobs]

the marine layer often means no snow for CHCH though

[snowchaser01]

Just recorded my highest wind gust of the year of 61.2km/h. Anything over 60km/h is impressive in my location.

Latest EC 18Z run was interesting for parts of Canterbury tomorrow night between 9pm to midnight. 1cm of snow for Christchurch, 2 to 4cm for Ashburton, Rolleston, Methvan etc. 850 hPa projected to be around -4 to -5c with a bit of moisture at 700 hPa. Locals will know better than me if that is just a model dreaming

Projection attached.

IMHO, brief period tomorrow night where is it touch and go for snow in the city... maybe in heavier showers, it will drop the snow down. Seems that the models are struggling atm. 1 week out they show decent moisture for events, then for 5 or 6 days they steadily back off. Then right at the last moment they increase again. It happened with the last lot of rain we got down here, and it seems as if it is happening again this time.... big moisture increase in today's runs.

EDIT: Adding to what jase is saying above... thundersnow chances tomorrow?

[TonyT]

Just recorded my highest wind gust of the year of 61.2km/h. Anything over 60km/h is impressive in my location.

Latest EC 18Z run was interesting for parts of Canterbury tomorrow night between 9pm to midnight. 1cm of snow for Christchurch, 2 to 4cm for Ashburton, Rolleston, Methvan etc. 850 hPa projected to be around -4 to -5c with a bit of moisture at 700 hPa. Locals will know better than me if that is just a model dreaming

Projection attached.

Change the word "snow" for "hail" and you might be right!. I can't see snow on the Plains from this pattern, but hail showers have always looked likely.

[Cyclone Tracy]

Some serious cold air south of the SI moving north. Macquarie Island at sea level dropped to -2.6c a few hours ago as the coldest air moved through. That's around 2c less than the latests 12z model run projected. Makes it the coldest air mass since 13 August to pass over Macquarie Island.

http://www.bom.gov.au/products/IDT60801 ... 4998.shtml

[Bradley]

Just recorded my highest wind gust of the year of 61.2km/h. Anything over 60km/h is impressive in my location.

Latest EC 18Z run was interesting for parts of Canterbury tomorrow night between 9pm to midnight. 1cm of snow for Christchurch, 2 to 4cm for Ashburton, Rolleston, Methvan etc. 850 hPa projected to be around -4 to -5c with a bit of moisture at 700 hPa. Locals will know better than me if that is just a model dreaming

Projection attached.

Change the word "snow" for "hail" and you might be right!. I can't see snow on the Plains from this pattern, but hail showers have always looked likely.

MetService now forecasting snow to 100m or lower on the plains overnight Tony, are they believing the models a tad too much as well do you think on this one?

[TonyT]

Just recorded my highest wind gust of the year of 61.2km/h. Anything over 60km/h is impressive in my location.

Latest EC 18Z run was interesting for parts of Canterbury tomorrow night between 9pm to midnight. 1cm of snow for Christchurch, 2 to 4cm for Ashburton, Rolleston, Methvan etc. 850 hPa projected to be around -4 to -5c with a bit of moisture at 700 hPa. Locals will know better than me if that is just a model dreaming

Projection attached.

Change the word "snow" for "hail" and you might be right!. I can't see snow on the Plains from this pattern, but hail showers have always looked likely.

I must admit, I'm starting to change my mind. 3 successive model runs where the control and a good few of the ensemble members have put snow on the Plains this evening is making me wonder. Time to change the forecast I think!

[NZstorm]

Very intense polar trough moving across the country today with snow to 100m in the south and a few thunderstorms/hail about.

Weather highlight for Auckland will be squally conditions possibly gale tonight.

500mb chart for 6pm.

[snowchaser01]

Going to be a very active and wintry evening... hail, thunder/lightning, sleet, snow, blustery, heavy rain and even the kitchen sink... will feel like July rather than nearly October.

[snowchaser01]

MetService have gone nuts this morning. CHCH Rural saying snow to 100m and lower in heavy showers.

And they have issued a Severe Thunderstorm Watch for all of Canterbury including CHCH mentioning snow to sea level, heavy rain and large accumulations of hail.

[Richard]

At least the SE half of Canterbury anyway, I'm just in the STW area,

[Cyclone Tracy]

8am water vapour sat pic showing a significant dual airflow upper polar injection with moisture pluming around the -38c @500 hPa cold pool just off the SI.

[snowchaser01]

8am water vapour sat pic showing a significant dual airflow upper polar injection with moisture pluming around the -38c @500 hPa cold pool just off the SI.

CT, interested to know if you believe this is directly related to the SSW, and what are your thoughts on this afternoon in Canterbury? Appreciated as always.

[NZstorm]

your thoughts on this afternoon in Canterbury?

Isolated thunderstorms appear likely with EU showing convergence over the plains. Current dp 6C at Christchurch airport is a favourable number. Then rain/snow this evening.

[Awhituobs]

we certainly do seem to be getting more cold southerly outbreaks
so could be related to SSW
but also though we are probably over due to get them
and might be more related to the SAM ?

[03Stormchaser]

MS definitely going all out this morning for later today, Severe Thunderstorm Watch issued for canterbury coast. Slightly surprised tbh but I am free this afternoon if it does fire up, sounds like it could be fun:

"Large accumulations of hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as also make driving conditions hazardous"

[TonyT]

Seems to have all the characteristics we have talked about on the SSW thread.

[Powermad]

I'll be hitting the road

[mikestormchaser]

I saw the models this morning and looked good. I would of upgraded to a high risk and low risk severe. So i think MS is on to it. Unfortunately no bloody camera gear on me so cant go straight out!!