Warmer temperatures and lower than average rainfall are expected to impact snow accumulation and storage in Central Otago and Southland, increasing the likelihood of dry conditions as spring approaches.
Earth Sciences New Zealand principal scientist Chris Brandolino said more frequent northeasterly-to-easterly air flows were expected over the next three months (August-October), elevating the likelihood of tropical and subtropical systems affecting New Zealand.
"This is associated with an increased risk of heavy rainfall events, including those linked to atmospheric rivers, and flooding."
However, in the South Island, a shift towards more persistent easterly flow anomalies was expected as the season progressed, increasing the likelihood of dry conditions in the southwest.
On the West Coast, the Southern Alps and foothills, inland Otago, and Southland, temperatures were very likely to be above average, he said.
Rainfall totals were about equally likely to be below normal or near normal, and soil moisture levels and river flows were about equally likely to be near normal or above normal.
"However, warmer temperatures and an increasing likelihood of dry conditions may continue to impact snow accumulation and storage in the region as spring approaches," he said.
"These factors could have implications for water availability and seasonal run-off beyond the August-October period," he said.
Along coastal Otago, temperatures were about equally likely to be above average or near average, rainfall totals were equally likely to be near normal or above normal, and soil moisture levels and river flows were about equally likely to be near normal or above normal.
Cold snaps and frosts could still occur across the southern regions, but they would be less frequent than usual.
While conditions in the tropical Pacific remained officially Enso-neutral, oceanic trends throughout July had moved closer to La Nina territory, he said.
"Subsurface temperatures and ocean heat content are showing clear signs of change, boosting the chances of a La Nina developing later in 2025.
"The chances for La Nina increase to 41% by the end of the calendar year.
"Sea surface temperatures remain above average around New Zealand, particularly off the west coasts of both islands, where marine heatwave conditions persist."
