KEY POINTS:
Winter decided to come back across the entire country over the weekend as a low just east of New Zealand deepened and pulled bitterly cold air up from south of us.
The snow came as a surprise to many and so too did the frosts - with one
reader at weatherwatch.co.nz saying it was the heaviest frost she's seen in the Bay of Islands so far this year - that's right, big frosts as far north as the "Winterless North".
Frosts were also recorded in Auckland as the mercury dropped below zero - and for many Aucklanders the sudden drop on Saturday night/Sunday morning would've come as a shock - it's been about 12 to 14 degrees warmer at night for the past few weeks. Snow fell in Christchurch and on the Rimutaka Ranges - but failed to really make an impact anywhere else.
Snow forecasting in New Zealand can be quite a tricky business. Being two mountainous islands surrounded by relatively mild waters makes for a challenging set up for any forecaster.
In America and Canada cold fronts usually march southwards across the plains from Arctic Canada and what happens 'up the road' is almost always what happens 'down the road'. It makes predicting very easy and forecasters there can accurately predict snow falls a week in advance!
Here in New Zealand it's much harder to see what's on the horizon. Big southern storms that come out of Antarctica are the easiest to forecast. These are the ones that on a time-looped satellite map appear to simply "explode" from the frozen continent and move directly towards southern Australia and New Zealand.
But lows that form over or near New Zealand and then bring localised snow are the ones that are harder to predict. That's what happened over the weekend. Many weather enthusiasts couldn't make up their minds last week if this system was going to bring snow to Christchurch or not.
Some computer models said yes, some said no. Some said the air would be too dry, others said it would be just right. Some predicted the freezing level wouldn't quite be low enough, some said it would be near sea level. It was a 50/50 gamble to some observers.
To snow in places like Dunedin or Christchurch we need the freezing level to be near sea level and moisture levels need to be fairly high. Westerlies and sou'westers usually dry by the time they reach these cities (When air moves over land it loses it's moisture so a southerly or sou'easterly that comes straight off the ocean and into eastern areas of the South Island is most likely to bring snow to low levels).
The air can't be too cold either - snow doesn't form if the air is too cold, because for the air to get really cold in the first places it needs to lose it's moisture and become dry! Ever wondered why Antarctica is the driest continent on earth? Bingo! In fact, in one part of Antarctica, known as the Dry Valleys in the Ross Dependency, it hasn't rained or snowed for over 2 million years - making it the driest place on Earth - and that's simply because the air is too cold to hold moisture.
Of course if the temperature is just a couple of degrees too warm the snow will melt before it reaches sea level. You can start to see why it's hard to predict small snow events like this!! Also, because New Zealand has so many micro-climates, snow may fall in one suburb but not another.
So why didn't the snow hit the rest of the North Island? Well much like how the Southern Alps keep Canterbury much drier than the West Coast, the Tararua Ranges this time kept much of the snow at bay over the Central North Island - that coupled with the fact that this low was actually pretty weak and didn't have too much moisture in it - limiting the advancement of those snow clouds.
And we have another low forming in the Tasman Sea this week - and it's set to bring warmer weather to many areas - but the moisture moving in from this system mixing with the polar air still over much of the country may see some further snow dumps over the ski fields - a silver lining on a what will be yet another damp week for many.
Philip Duncan
For the latest weather news keep up to date with The Radio Network's new Weather Watch Centre or the NZ Herald weather section.
Pictured above: Andy Wilson clears snow from the windscreens after an over night snow fall in Christchurch. Temperatures fell to below zero with hail and sleet falling during the day. Herald on Sunday photo / John McCombe
