As the season progresses, the threat of frosts will increase says NIWA National Climate Centre's Chris Brandolino, Principal Scientist - Forecasting and Media.

"Overall, a mix of anticyclonic flows and easterly quarter flows are expected over New Zealand over the autumn season."

Oceanic and atmospheric conditions remained broadly consistent with a weak La Niña in February 2025, however, La Niña-like signals in both the ocean and the atmosphere have weakened significantly very recently.

Autumn rainfall totals are expected to be near normal or below normal for the west of the North Island and the west of the South Island, near normal for the north of the North Island and north of the South Island, and near normal or above normal rainfall in the east of both islands.

"Unusual dry spells can be expected at times, especially early in the forecast period."

Late summer through early autumn is the peak of the tropical cyclone season. The risk for an ex-tropical cyclone to pass within 550 km of New Zealand is normal-to-elevated through April, says Brandolino.

"Ocean waters remain much warmer than average around the South Island, with extended areas offshore as well as Cook Strait experiencing marine heatwave (MHW) conditions. Soil moisture levels and river flows are about equally likely to be near normal or below normal in all regions of New Zealand, except for the east of the South Island, where near normal soil moisture levels and river flows are likely."

Sub-seasonal projections of rainfall and dryness are updated daily through the NIWA35 forecast.