An area of low pressure will pass the country from Sunday accompanied by a burst of cold air, which could see temperatures dropping below -20 on the mountain tops and bringing near/below freezing wind chill to sea level parts of Southland and Otago by Monday. Daytime highs at sea level on Monday may only be 5 degrees around Southland and Otago and inland highs may only be two or three degrees above freezing.
WeatherWatch.co.nz says isolated snow flurries may reach sea level or low level (around 100 to 200m) for a time but the main widespread heaviest snow will be in the mountains and ranges, mostly above 300m. (Please note, snow levels may change a bit due to subtle air flow shifts, keep up to date with your local forecast for more details/fine tuning).
From Saturday night snow will begin to accumulate over the Southern Alps for the most part. The area of snow will expand southwards as time goes by and include Fiordland on Sunday.
Across Sunday locally heavy rainfall is likely on the West Coast, possibly with lightning and strong wind gusts.
Heavy snow is in the mountains and ranges, lower down (below 200m) there may be a few snow flurries, especially on Sunday night and Monday AM.
Snow accumulation will further expand into wider areas in the southern part of South Island from Monday.
As a result of the air flow much of Canterbury should be fairly dry.
Snow showers are possible in the North Island’s Central Plateau from Sunday night to Tuesday night, including State Highway One the Desert Road and potentially SH5 Napier to Taupo.