- latest GFS is giving 517 over Chch Friday night/Saturday with 850 temps down to -8. Still looking for 520 over southeast of the NI, and under 524 (the usual indicator of snow to low levels) for much of the Island., probably not heavy snow, but cold enough for falls to be widespread in most populated areas of the country, and some really extreme frosts to follow.
Interesting observation.tich wrote:There is an area in the east of the South Island that tends to get a lot less snow to low levels from very cold southerly outbreaks than places further to the south and and the north. This area is covers most of the land between Dunedin and inland mid-Canterbury. Relatives of mine who live near Palmerston, between Oamaru and Dunedin, say that when Dunedin is having snow, they're usually getting fine weather instead. Why? This whole area (including North Otago and South Canterbruy) is exposed to the south, but maybe it's because there are no significant high hills right by the coast, like there are near Dunedin and Christchurch.
Ben
Christchurch
Warm advection snowstorms tend to affect inland areas like Central Otago and the Mackenzie Basin, rather than coastal places. June 2002's big snow in Canterbury was an exception - but even that one missed most of Banks Peninsula, while blanketing much of the plains.Almost all heavy snow events in NZ originate from warm advection storms where warmer air looses its moisture above a very cold surface layer. This is not expected to be one of these storms, its a cold advection jobbie, and consequently the moisture content will be low. GFS is predicting about 8-9mm of rain over about 36 hours for Chch, which could easily translate to 8-9cm of snow given the temperatures. Certainly enough to bring the city to a standstill, but hardly in the heavy category. You might expect twice that depth on the hill country with some orographic lifting.
I've wondered about that possibilty. The models seem to push the depression off to the east before the coldest air arrives up north, but if the 2 systems happen to collide - the scenario could be almost unimagineable. Civil Defence should be aware of this possibility. Low-lying towns and cities in the North Island I don't think could easily cope with a major snowfall event like the ones that periodically hit places like Dunedin from time to time.Note well though that if this cold air gets far enough north to interact with the depression(s) which the models want to bring across the north of the NI then warm advection snow is a possibility for the NI high country (and maybe low country as well).
Both of the heavy snow events in Canterbury in 1992 were warm advection storms (yes Aaron, there were 2 that winter, you probably only remember the one which put snow in Chch, but there was one a few weeks earlier which put heavy snow down to about 300m from memory).tich wrote:
Warm advection snowstorms tend to affect inland areas like Central Otago and the Mackenzie Basin, rather than coastal places. June 2002's big snow in Canterbury was an exception - but even that one missed most of Banks Peninsula, while blanketing much of the plains.
,especially with no high forming to the south of the country until at least Saturday.
