Niwa National Climate Centre forecasting principal scientist Chris Brandolino said during early-to-mid December, a period of stronger-than-usual westerly winds was expected to bring rainfall, possibly heavy, to most western regions of the country.
But La Nina conditions were expected to become more established around New Zealand as the summer season unfolded, meaning air temperatures were very likely to be above average across the country between December 2024 and February 2025.
"Recent signals in both the ocean and the atmosphere point to the tropical Pacific transitioning to more distinct La Nina-like conditions over the course of December.
"There is, however, still uncertainty as to whether conventional La Nina thresholds will be exceeded.
"International guidance indicates about equal chances for La Nina [50%] and neutral [49%] categories during December 2024 and February 2025.
"Regardless of whether La Nina is formally declared over the forecast period, weather patterns broadly consistent with La Nina conditions are expected to become more established around New Zealand as the summer season unfolds."
He said coastal sea surface temperatures (SSTs) remained generally above average around the country, ranging from about 0.5°C to 1°C above average during the past 30 days.
However, marine heatwave conditions had receded in both extent and amplitude recently.
"Global climate models forecasts are for SSTs to remain above average around New Zealand during the coming summer, and marine heatwaves will remain possible."
On the West Coast, the Southern Alps and foothills, inland Otago and Southland, temperatures were "very likely" to be above average, he said.
"More northeasterly winds may contribute to more days above 25°C than normal."
Rainfall totals were most likely to be near normal, and soil moisture levels and river flows were equally likely to be near normal or below normal, he said.
Along coastal Otago, temperatures were also "very likely" to be above average.
"Northeasterly quarter wind anomalies could lead to night times being more unusually warm than the day times.
"Rainfall totals are likely to be near normal, but dry periods may be interspersed with short and sharp rain events for the area as the season progresses."
Soil moisture levels and river flows were most likely to be near normal, he said.
