Polar blast & NI low 11-14th July

[LittleTreeHouse]

Friends in rakaia have snow settling on their lawn in the last 10 minutes

[Storm Struck]

Mix of sleet and wet flakes here in Rolleston
Edit.. now its snowing

[Lawrence]

Sleeting here at Rolleston 50m ASL
barometer steady at 1006.9
Temp at 2.4 Celsius
Wind 5.0 kph SW

Expecting rain to turn to snow if temps drop further.

[tigge_nz]

Its snowing here in Weedons for the last 20 minutes and we're at about 65m above sea level. Started much earlier than we were expecting, and the outside thermometer is reading just above 3 degrees

[Lawrence]

Mix of sleet and wet flakes here in Rolleston
Edit.. now its snowing

Confirmed.
it is definitely snowing in Rolleston.
@ 2.1 Celsius

[Storm Struck]

There must be an advection component to this with the upper NW movement peeling over and the S-SW undercutting, its very borderline here goes through phases of more flakes then just sleet so possibly 1C too warm here.
But thats pretty good considering the coldest air arrives into tomorrow

[Richard]

Temps dropping fast here now, just on 3 deg

[mikestormchaser]

Yeah I think there must be an advective little period here as its a couple degrees to warm yet it is snowing

[03Stormchaser]

Winds gone NW in the city, raining still and now 1.6C, May end up with a snow shower at some point

Sent from my VIE-L09 using Tapatalk

[carrot]

Slushy sleety rain falling in ilam. Looks very close to snow. Slush is settling on surfaces

[Richard]

Looks like a low forming over South Westland atm

[tich]

Slushy sleety rain falling in ilam. Looks very close to snow. Slush is settling on surfaces

Same in St Albans.

[Bradley]

Rain turning to sleet in Hanmer Springs, plenty to come...

[Chris W]

Also sleeting in Linwood.

[Razor]

Drove back from Lincoln tonight after successful delivery in town this morning. Mainly rain with the odd bit of sleet. Drove up to top of Worsleys Road for a snowline check just now- still only rain at 223m elevation up there

[Lawrence]

Big flakes here in Rolleston now,starting to accumulate on the ground and pretty intense.

[Storm Struck]

Its certainly a funny ole night innit , good snow there for about 5 minutes enough to settle on the car and sediment on concrete surfaces.
Now its back to rain/sleet, so its more than likely the heavier precipitation is bringing that freezing level down with it to near sea level currently, however this is about 6-7 hours ahead of shedule in terms of really low level white stuff.

[Paul Mallinson]

Soemebody emntioned earlier that it must be hard to forecast the level to which snow will fall - and it is. It can rely on two aspects. The first is how dry the air is. The drier the air (the lower thr RH) the further snow flakes and fall and survive in air above zero. The reason is that loss of mass to the drier air (sublimation) chills the flake and keeps it from melting. Eventually the flake moves into air that is too warm and it melts. So the RH is important. Eventually after precip starts, the RH rises to near 100% with the sublimation going on and then snow melts after the environmental temperature rises above 1.5 to 1.7degC. Once the event has got going (precip over an area), the snow level then becomes determined by how heavy the precip rate is (which is related to upward motion). Often for lighter precip, the rule of thumb is that the snow level penetrates to about 300m below the freezing level, but it is documented that (from memory) a precip rate of 5-6mm/hr can penetrate 600-700m below the freezing level and 10mm/hr about 900 metres! So unless the air at the surface is very cold and it can snow right down to sea level, then getting the snow level correct is a matter of forecasting how heavy and sustained the precip will be! Not easy.

Somebody else mentioned the snow to liquid water ratio. It is often sited as 10:1 meaning 1mm of liquid precip will give you 10mm of snow (or 1cm). However this can vary remarkably during a storm. I have found that commonly the snow ratio will vary something like this: It may start out around 4 to 8:1 (wet snow) then rise to 15 to 20:1 (or more) vary up and down, often around the 8 to 14:1 mark and then as the event ends, it trails off back down to 5 to 8:1. What this says is that the snow starts out wettish or dense because the precip is coming out of warmer lower clouds not ideal for snow, then become lighter and fluffier and perhaps clumpy as the strongest ascent in the storm coincides with the ideal temperature regime aloft (-12 to 18C) to produce dendritic snow. Then as the main synoptic forcing within the storm weakens or moves away, once again lighter denser snow may fall from warmer clouds (say -7 to about -12C) and once this layer becomes too shallow and warm (warmer than -5C) you end up with freezing drizzle down to the freezing level and drizzle below that.
Cheers Paul.

[HeMan]

Well said Paul, so many factors to think about when forecasting snow levels here in NZ, it is a fickle creature indeed!
Think the team has done a pretty good job on this one. Cheers Heath.

[Winter is coming]

North loburn here 150 asl no snow here.

[Talbotm]

There appears to be a pocket of heavy moisture north of Ashburton but its not moving north west as expected. Word is Kirwee has more snow than some areas further west. Certainly not what the models were showing thats for sure.

[NZ Thunderstorm Soc]

Talking about Snow, I see { sorry wrong postings here) Games of Thrones starts next Monday.

[Razor]

Talking about Snow, I see { sorry wrong postings here) Games of Thrones starts next Monday.

You know nothing John Gaul !

[NZ Thunderstorm Soc]

Talking about Snow, I see { sorry wrong postings here) Games of Thrones starts next Monday.

You know nothing John Gaul !

Cheers
Any way congrats

[Razor]

Thanks John