Snow in New Zealand

Aug 16, 2011; 12:23 PM ET

MEDIA HAVE CALLED IT “ONCE IN A LIFETIME”

Being a mountainous, mid-latitude nation, New Zealand is not at all unfamiliar with snow. Indeed, mighty glaciers are fed by snowfields on the Southern Alps. Even the North Island has snow-clad volcanoes and a ski season.

Nevertheless, the envelope of relatively warm sea water means that snow is fairly unusual at low elevation, where most of its people call home. It is an occasional visitor to South Island cities such as Christchurch, Queenstown, Invercargill and Dunedin.

However, the North Island, with its main cities of Auckland and Wellington, only rarely sees significant lowland/seaside snowfall. Before this week’s falls, the last significant snow fell in the mid 1970s. If I have understood media reports, this would hold true for both Wellington and Auckland.

I have not seen much in the way of snowfall reports, although on-line images show what must be at least 6-inch (15-cm) snow depth. Forecasters reportedly spoke of half-meter snowfalls on the South Island, which is believable to me.

CAUSE OF THE SNOWS

500 millibar height field on 0000 UTC Tuesday as initialized by the GFS numerical forecast model. An exceptional deflection of the jetstream over New Zealand is seen at the lower right.

An unusually strong northward outbreak of Antarctic cold, centered upon the meridian of New Zealand, has coincided with a sharp northward buckling of the jetstream.

Rendered deeply unstable and moist in crossing long stretches of increasingly “warm” sea surface, the blast triggered waves of numbing-cold rain, sleet/graupel and snow. Inland and a few hundred meters up, mostly snow seems to have fallen.

Over the next few days, the cold outbreak will weaken, and the time for low-elevation snow will come to an end.