The southwest flow would also affect parts of the North Island, bringing showers to western areas and a cold southerly change later in the day on the east coast.
Auckland was in for a high of 15C with periods and sun and scattered showers.
Generally the wintry weather was on an easing trend, Glassey said.
"It will all be easing through the day, and the rest of the week is not looking too bad.
A ridge of high pressure would move east over the country Thursday and Friday, bringing a period of settled weather.
However, overnight frosts were likely, especially about the South Island.
Northwesterlies would develop over the South Island on Friday ahead of an oncoming front.
This front was expected to reach the lower South Island late on Saturday and cross the Island on Sunday, bringing rain to the West Coast and mountains, and strong northwesterlies about exposed eastern areas.
NIWA said the northerly weather pattern arriving this weekend and into next week would raise temperatures across the country above average.
Today's weather
Whangārei
Sunny spells and a few showers. Southwesterlies. 16C high, 9C overnight.
Auckland
Sunny spells and a few showers. Strong southwesterlies, easing from afternoon. 15C high, 10C overnight.
Hamilton
Sunny spells and a few showers. Southwesterlies. 15C high, 6C overnight.
Tauranga
Mainly fine, but a shower or two is possible. Westerlies. 16C high, 6C overnight.
New Plymouth Sunny spells and a few showers. Fresh westerlies, easing at night. 14C high, 7C overnight.
Napier Mainly fine. A few showers developing afternoon and westerlies dying out. 17C high, 4C overnight.
Whanganui Sunny spells, chance shower from afternoon. Strong westerlies, easing evening. 14C high, 6C overnight.
Wellington A fine start. Southerly breezes developing around midday, bringing cloud and brief showers. 13C high, 6C overnight.
Nelson Fine. Gusty southwesterlies. 15C high, 4C overnight.
Christchurch Fine, apart from isolated showers about Banks Peninsula. Cold southwesterlies. 12C high, 1C overnight.
Dunedin Occasional showers. Snow flurries about the hills. Strong, cold southwesterlies easing evening. 8C high, 5C overnight.