Earth Sciences New Zealand (ESNZ) forecasting principal scientist Chris Brandolino said a westerly-dominated September was expected to transition into a pattern of higher-than-normal pressure over southern parts of the country in October and November.
"This scenario will lead to alternating periods of settled weather and northeasterly flow anomalies."
He said El Niño-Southern Oscillation-neutral (Enso) conditions remained in the tropical Pacific, but La Nina-like patterns became more established last month.
"While remaining in the neutral range, sea surface temperature anomalies in most of the key Enso regions became more negative during August.
"Subsurface ocean cooling continued in August, with cooler-than-average waters moving eastward and rising closer to the surface in the eastern Pacific Ocean, reinforcing the shift towards La Nina-like conditions."
Mr Brandolino said international guidance suggested La Nina conditions were favoured during spring and early summer (2025-26), which would bring more northeasterly winds.
"This was further supported by experimental forecasts developed by Earth Sciences New Zealand for the Relative Oceanic Nino Index, which accounted for the broader warming trend across the tropical Pacific and indicated that La Nina conditions were likely to emerge over the September-November 2025 period."
The West Coast, Southern Alps and foothills, inland Otago, and Southland, were most likely to have above-average temperatures, he said.
However, rainfall totals, soil moisture levels and river flows were about equally likely to be below normal or near normal for the September-November season as a whole.
"Below-average hydro storage would persist under the scenario of a drier-than-normal spring for much of the South Island.
"Currently, many sites report total storage, river flows, and snow cover are below typical levels for this time of year."
Along coastal Otago, temperatures were about equally likely to be above average or near average, while rainfall totals were about equally likely to be near normal or below normal and soil moisture levels and river flows more likely to be near normal.
At this stage, Mr Brandolino said the La Nina conditions were expected to return to Enso-neutral early next year.
