Low-level Snow for Canty? - July 18-22

[Bradley]

On a side note mondays cold front is looking even colder based on latest model runs at -7C at 850hpa for a time...seems to be a coastal event though, at this stage inland areas may get around 5cm of the whitestuff...

[NZ Thunderstorm Soc]

Some sleety bits whilst driving to work this morning, but nothing much just cold rain.
7.5mm at the hospital this morning.
Not much wind, if so from the WSW direction.
Grey As cloud layer for much of the day spreading inwards from the SE.

[Storm Struck]

It certainly Is an odd set up this occluded frontal system, I cant say I have seen a set up like this before but also very small too.
Of course with systems like these because the winds are generally lighter there isn't much sheltering going on, and is also the chance for a low to develop on the coast which can sometimes build and lead to developing troughs etc.
Early next week looks good again, here is hoping it stays on target from now on .

[mikestormchaser]

read an article on stuff.co.nz today and the southern annular mode "sam" has gone negative "finally", witch means the polar vortex is alot stronger bringing in the cold air from the antartic.
I remember in 2011 when we had the dump in july and august they talked about SAM and how it works.
Sounds like we are in for a very cold snowy next couple of weeks atleast.

[Talbotm]

She has been a crazy 24hours as far as weather goes. I see hororata had about 5-6cms of snow. I went out there. The models are moving around so much its hard to work out whats going to happen next. Its like the moment theres a forecast the model changes again. I saw on the rain radar there was a huge line of good moisture sitting off the Ashburton coast i think that was what the forecasts were basing on coming inland which would have seen heavy snow like what was predicted. Even the professional forecasters such as M/S have a watch in place for heavy snow now that warning is over. They even say in there forecast they are not sure what this system will do next. Did anyone else see that moisture off the coast after the front cleared??

[Razor]

Ran up Port Hills tonight to around 400m altitude. No sign of snow anywhere, fallen or falling

[Dean.]

South of the Rakaia the trough moved through faster than Metservice models showed.Some rain and sleet here in Ashburton,then cleared from the northwest.Not sure if Banks Peninsula had an affect this far south but seemed to judging by the Rakaia radar.

[spwill]

read an article on stuff.co.nz today and the southern annular mode "sam" has gone negative "finally", witch means the polar vortex is alot stronger bringing in the cold air from the antartic.
I remember in 2011 when we had the dump in july and august they talked about SAM and how it works.
Sounds like we are in for a very cold snowy next couple of weeks atleast.

We will watch the modelling. Currently the GFS and ECMWF point to an end to the below average temps later next week. A strongly negative SAM generally points to increased westerly pattern over NZ, wetter in the west more especially over the South Island and I guess an increased chance of cold air reaching NZ around lows in the westerlies.
The Snowy Mnts in Australian have been snowy in recent weeks under a positive SAM, they are roughly the same latitude as Auckland

[melja]

lol and about 1800m

[spwill]

lol and about 1800m

Snow has fallen to quite low levels at times over there. I don't remember too much snow over the South Island this winter.

[bpo]

Very light snow still falling in Omarama. Began in the early hours, settling before sunrise (~20mm), but is struggling to resist the sun as it breaks in and out of cloud. Lots of happy tourist kids right now.

[NZ Thunderstorm Soc]

I can't see any snow to low levels coming up this week, just the occasional cold wintry rain.

[Bradley]

I think we have more of a chance at getting low level snow then Friday's system - but I guess that's not saying much is it?

[matttbs]

I think there is a chance if getting some snow in the next few hours. Going by the radar there's a lot of moisture moving inland (directly at my house) and it's only 3C here currently and dropping

[Paul Mallinson]

Hi there,

I thought I would offer some background to the recent snowfall event (Friday) in South Canterbury.

The rather sudden change in the situation and forecasts was brought about by the formation of a second cut off upper low directly east of Canterbury and south of the main centre east of the North Island on Thursday morning as I recall. After it had formed, there were two main short wave troughs rotating around the centre. The first moved over Canterbury Thursday evening and brought snow showers to already cold inland places and the second stronger feature was expected to rotate over Canterbury on Friday. Much of the uncertainty originated from the different models having a slightly different wind flow around this upper low (shape of the height contours were a little different). What this does is changes the shape of the short wave trough and the strength and position of the cyclonic vorticity advection (cva). The cva is what generates the upward motion and thereby determines the location of the precipitation (rain near the coast and snow inland).

Wind flows such as I described are tricky to deal with. It is much easier for the models to deal with strong northwest flows associated with the usual travelling fronts. These same problems often occur with features coming down from the north over northern New Zealand. These are usually associated with lighter winds aloft, which, if not represented correctly within the model, will lead to different solutions as to where and how intense the precipitation will be.

The atmosphere is a complicated beast! (but not as complicated as man made economies).

Paul

[melja]

Thanks, and great that M/S post the odd bit of info on here.

[Bradley]

Thanks for explaining the thinking behind that severe weather warning being issued on Thursday night Paul, of course looking back now it wasn't needed (certainly not the 300m asl part anyway) but you certainly explained the reasonings behind the decision to issue it very well..

[Bradley]

Paul just a quick question regarding Monday and Tuesday, will the Metservice be downgrading the forecast maximums for Christchurch down from 9C? It is quite a decent system cold-wise (850hpa temps of between -4C and -6C for that period). 9c just seems a tad high? Maybe more like 6-7C for those 2 days?

[Paul Mallinson]

Paul just a quick question regarding Monday and Tuesday, will the Metservice be downgrading the forecast maximums for Christchurch down from 9C? It is quite a decent system cold-wise (850hpa temps of between -4C and -6C for that period). 9c just seems a tad high? Maybe more like 6-7C for those 2 days?

I don't know Bradley. I am on a couple of days off at the moment. You may be right. It looks like a cold southwest flow gets going. If CHCH gets more sun than cloud then of course the temperature will climb a couple of degrees higher. If there is precip and it gets heavyish at times, then the temps will be lower.

Paul

[Paul Mallinson]

Actually, lets do a simple calculation.
Lets say it is -5C at 850 hPa. In a windy environment (which should be the case in the southwesterly, the temperature lapse rate off the surface will be what we call 'Dry Adiabatic' i.e. the lapse rate will be near 3degrees per thousand feet. The 850 level is located roughly at about 5000 feet. So 5 (thousands of feet) multiplied by 3 = 15 degrees. So a temperature 15 degrees warmer at the surface from -5C is +10C. So you can see how the temperature at CHCH may be near +9C given clear skies/sun and some wind.

Paul

[Dean.]

The forecasted high for Ashburton on Monday is 7 degrees.Seems low considering the main SW surge will be east of Ashburton,more sun likely here than ChCh.

[Richard]

The forecasted high for Ashburton on Monday is 7 degrees.Seems low considering the main SW surge will be east of Ashburton,more sun likely here than ChCh.

I think its looking more of a SSW flow so it should be less coastal

[Bradley]

Yep absolutely Paul, assuming it is a cloudless dry environment those calculations are correct but even the Metservice forecast for tuesday is showers and rain so a lapse rate more like 6C per 1000m would be more appropriate so more like a 6max...anyway it's going to be cold anyway!!

[Bradley]

On a side note the Metservice is now mentioning snow in it's forecast for Christchurch Tuesday morning, very optimistic indeed!!

[Razor]

I see there are warnings all over the shop for tomorrow now. Could get interesting