While the unsettled conditions that characterised the end of November are expected to continue across the country this week, high pressure systems will become an increasingly common occurrence as December progresses.
And as La Niña kicks in, dryness will become "a concern" from mid-December and into January for many parts of the country, including the South.
Earth Sciences New Zealand forecasting principal scientist Chris Brandolino said moderate La Niña conditions were present in the tropical Pacific and both the Southern Oscillation Index (SOI) and the Relative Oceanic Nino Index remained above La Niña thresholds during November, giving a "consistent" and "persistent" signal that La Niña conditions were on the way.
Northeasterly winds tend to become more common during La Niña events, bringing reduced rainfall to the lower and western South Island.
"This tendency towards a steadily increasing presence of high pressure makes regional dryness a concern from mid-December and into January, especially as it combines with high sun angles at this time of year," Mr Brandolino said.
Global forecast models indicated ocean temperatures around New Zealand were likely to remain warmer than average over the summer period, which was expected to have an upward influence on seasonal air temperatures.
"However, a rapid weakening of La Niña conditions is expected over summer, with about a 50% chance for El Nino Southern Oscillation conditions to return by late summer.
"The final month of summer may transition to high pressure systems frequently sitting further south than normal — over the lower South Island, or the Southern Ocean — bringing an easterly wind anomaly and increased humidity over large parts of the country.
"This pressure pattern may allow occasional rainmakers in, from the north."
He said temperatures on the West Coast, Southern Alps and foothills, inland Otago, and Southland, were equally likely to be above average or near average.
Rainfall totals for the summer season as a whole were about equally likely to be near normal or below normal, and soil moisture levels and river flows were most likely to be near normal.
Around coastal Otago, temperatures were also equally likely to be above average or near average, and rainfall totals for the summer season as a whole were about equally likely to be near normal or below normal, but soil moisture levels and river flows were about equally likely to be below normal or near normal, he said.
