WeatherWatch.co.nz head weather analyst Philip Duncan said conditions for the start of summer will be like a "slowed down" version of November.
"Our long range maps indicate December will also be a mixture of cloudy and dry days, calm and sunny days, and still a few more wind and rain warnings to come. It will be like a slowed down version of November with fronts and highs gradually losing some speed."
The first weekend of the summer will see low cloud and drizzly patches of rain across both islands, as a front passes over. This area of rain may turn into a small low just east of the North Island next week but next week another high is expected to cross New Zealand.
"It's been so dry in the top half of the North Island lately and very windy and changeable from Auckland southwards and that is directly due to constant highs north of New Zealand. These highs are stopping rainmakers coming down from the north and are leaving the country vulnerable to strong winds.
"The change we're seeing in December is a shift in those highs further south. Instead of tracking north of New Zealand they may now cross central New Zealand - which will bring longer periods of settled spells and more easterlies to the north."
But Mr Duncan says between each high is still a windier, wetter, period of weather.
"We may see conditions still fairly unsettled up until Christmas but plenty of dry, sunny, days too. Summer will gradually start this year."
Niwa seasonal outlook breakdown
According to Niwa's seasonal outlook, temperatures in Northland, Auckland, Waikato, and the Bay of Plenty are expected to be near average or above average, as are rainfall totals. Soil moisture is expected to be in the normal range, while rivers flows are likely to be normal or below normal.
In the central North Island, Taranaki, Wanganui, Manawatu and Wellington, temperatures are also expected to be near average or below average. Rainfall totals, river flows and soil moisture levels are likely to be near normal or below normal.
Temperatures in Gisborne, Hawke's Bay, Wairarapa, Nelson, Marlborough and Buller are forecast to be near average or above average, while rainfall totals, soil moisture levels and river flows are all likely to be in the near normal range.
Average summer temperatures are expected on the West Coast, the Southern Alps and foothills, inland Otago, and Southland, with normal rainfall totals and normal or below normal soil moisture levels and river flows.
Summer temperatures are likely to be near average or above average of coastal Canterbury and eastern Otago. Rainfall totals are likely to be near normal or below normal, while soil moisture levels and river flows are likely to be normal.
