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Otago and Southland temperatures are forecast to be warmer than average over the next three months, but the localised heavy rainfalls we have had of late may continue in what Niwa is calling a "non-traditional La Nina" summer.
During La Nina events, northeasterly winds become more common, bringing moist, rainy conditions to northeastern areas of the North Island and reduced rainfall to the lower and western South Island.
It also brings warmer-than-average air and sea temperatures around New Zealand.
Niwa National Climate Centre forecasting principal scientist Chris Brandolino said despite moderate La Nina conditions continuing during December, the impact was "non-traditional" for parts of New Zealand.
"This owes to persistently warm ocean waters in the tropical Indian Ocean and the flavour of the ongoing La Nina event.
"An example of the non-traditional impact of La Nina is the ongoing unusually dry conditions across the upper North Island," Mr Brandolino said.
Sub-tropical air flows might fuel localised heavy rainfall, which could cause flooding, similar to what was experienced in parts of Otago and Southland during late December and early this month.
"It is not possible to pinpoint exactly which regions may experience extreme weather months in advance, hence the need to keep an eye on day-to-day weather forecasts through the season.
"Extended dry spells will likely continue to be interspersed with the unsettled conditions - a by-product of the ongoing non-traditional La Nina and a predominantly positive Southern Annular Mode."
Air temperatures were most likely to be above average in all regions of the country and there would be elevated humidity levels from time to time, but an outlook of warmer-than-average three-month temperatures did not preclude cold snaps, he said.
Over the next three months, Southland and inland Otago temperatures are likely to be above average, rainfall totals are about equally likely to be below normal or near normal, soil moisture levels are most likely to be below normal, and river flows are about equally likely to be below normal or near normal.
Warmer-than-average temperatures and drier-than-normal conditions are forecast to persist along the West Coast and about the hydro-lake areas.
In coastal Otago, temperatures are most likely to be above average, rainfall totals are about equally likely to be near normal or above normal, and soil moisture levels and river flows are most likely to be near normal.