SnowCast

SnowCast is SIMA's regional snow forecast for the South Island. It is issued when snow is forecast below 1000 metres, and covers expected snow levels, accumulations, timing and the likely impact on alpine passes and lowland roads.

How SnowCast is produced

Forecasters analyse the depth of cold air, freezing levels, precipitation intensity and the timing of cold southerly or southwesterly changes to estimate where and how low snow will fall. Short-term (0 to 48 hour) and extended (days 3 to 7) outlooks are written for the alpine areas, with accumulation maps and regional bulletins added as events approach.

SnowCast severity matrix

Regional snow bulletins are graded on a four-step severity scale.

• Slight: non-disruptive; snow possible on higher-level roads.
• Enhanced: accumulations expected; road restrictions possible.
• High: heavy snow risks; potential for low-level disruptive snow, with road closures likely.
• Significant: heavy snow risks at lower levels and widespread road closures.

Snow and the alpine passes

The South Island's main alpine routes, including Arthur's Pass, Lewis Pass, Porters Pass, Burkes Pass, Lindis Pass, the Crown Range and the Milford Road, are regularly affected by snow between May and October, and occasionally outside those months. Snow to low levels can also close roads across the Canterbury Plains, Central Otago and Southland during strong polar outbreaks.

For official road closures, chain requirements and highway conditions, consult NZTA Waka Kotahi. SnowCast provides independent meteorological guidance and is not a substitute for official road reports.

South Island Met Agency is an independent private forecaster and does not issue official government severe weather warnings. For official New Zealand watches and warnings, visit MetService. Read our About Us & Terms and Privacy Policy.