Very cold air starts to make progress over the South Island today then onto the North Island overnight. The main issue today will be the potential for heavy snow about the lower South Island from evening onwards down to sea level.

Northland, Auckland
Any early showers clear then expect sunny spells and some cloud with westerly breezes. Late afternoon or evening showers move in again.
Highs: 17-18

CNI, BOP & Waikato
Westerly winds with a mix of sunny spells and some cloud, showers develop late afternoon or evening.
Highs: 13-18

Eastern North Island
Gusty northwesterlies strong to gale force about Wairarapa. A period of shower passes over Wairarapa late morning then clears in the afternoon. Rain moves back in there during the evening however. Sunny further north.
Highs: 18-20

Western North Island
Early sunny spells then expect showers from midday. Winds gusty from the northwest. Rain setting in during the evening, winds tend more southerly later.
Highs: 15-16

Wellington
Cloudy spells, a few showers move through late morning then clearing early afternoon. Winds strong to gale force from the northwest. Later in the evening a southerly change brings further showers with a risk of hail. Snow flurries to 500m overnight.
Highs: 14-15

Nelson
Morning showers with gusty northwesterlies, clearing around midday and sunny spells develop with winds tending westerly. There could be a further shower later in the evening.
Highs: 13-14

Marlborough
Strong northwesterlies with areas of cloud, a few showers move over from the west around late morning then clear. Later in the evening winds change southerly with the risk of a shower.
High: 14

Canterbury
Rain, mainly about South and perhaps Mid Canterbury frist thing. Clear areas and some cloud further north. Rain moves further north to about Banks Peninsula by midday with southwest winds then clears during the afternoon. The odd sunny break then develops. Later in the evening showers move onto Banks Peninsula and perhaps Christchurch with snow to 300m.
Highs: 10-11

West Coast
Heavy thundery rain this morning eases to showers as gusty northwesterlies change southwest. Showers clear later in the evening for most although only slowly clearing away overnight for areas north of Greymouth.
Highs: 12-13

Coastal Otago
Sunny spells and some cloud in the morning with breezy westerlies, in the afternoon winds tend a little more southwest and wintry showers move into areas about and south of Dunedin. Snow to 400m lowers to sea level overnight. Snow showers become more widespread overnight into much of coastal Otago.
Highs: 6-7

Central Otago
Wintry showers with small hail at times, snow to 500m lowering to low levels (200m or lower) by midnight. Westerlies tend more southwest in the afternoon. Very cold overnight with snow, heavy in areas.
Highs: 4-5

Southland
Showers, heavy at times with thunderstorms and hail. Snow to 500m lowering to sea level later in the evening and becoming heavy especially inland. Gusty westerlies tend a little more to the southwest by midday.
Highs: 6-7

WeatherWatch.co.nz