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According to Blue Skies Weather and Climate Services, a southwesterly airstream will spread over Canterbury on Monday and become progressively colder as a trough of low pressure deepens east of the South Island.
On Monday, snow is expected to fall to 500m in Canterbury, with accumulations of 5-10cm likely above that level later in the day (70-90 per cent risk), a spokesperson said.
"Snow showers should clear during the afternoon, with gusty southwesterly winds easing."
Much colder air is expected to arrive late on Monday night or early Tuesday morning with winds becoming strong.
On Tuesday, snow showers to sea level are possible, with a 30-50 per cent chance, across Canterbury.
Snow may affect coastal Marlborough above 300m and roads on the eastern plains and Banks Peninsula may see some light snowfalls during the daytime.
"At this stage, it looks more likely to affect the eastern Canterbury Plains and Banks Peninsula, and have less impact on the western plains and foothills," they said.
Airflow will ease later on Tuesday and next Wednesday.
"Given the lead time, this snow is not locked in, and subtle changes in the forecast weather system could easily remove the risk of this snow.
"However, guidance data has been consistent on this risk for the last few days."