Air temperatures are expected to be above average across all New Zealand regions for this winter season, according to NIWA's Seasonal Climate Outlook for June - August 2025. While cold snaps and frost will occur, they are expected to be less frequent than usual, says NIWA Principal Scientist Chris Brandolino.

Rainfall totals during the three months ahead are expected to be above normal for the north of the North Island and near normal for the east of the South Island. For all remaining regions of the country, rainfall is forecast to be near normal or above normal.

Higher than normal atmospheric pressure is expected to the east of the country, leading to north-easterly flow anomalies that may shift to a more north-westerly direction towards the end of the three-month period. Low pressure systems forming north of the country are still expected to affect New Zealand and may lead to heightened risks for heavy rainfall events.

Outlook Summary

• Neutral El Niño – Southern Oscillation (ENSO) conditions remain present in the tropical Pacific.
• Residual signals indicative of weak La Niña conditions persist, and occasional La Niña-like atmospheric patterns may still emerge over the next three months
• International guidance indicates about a 70% chance for the tropical Pacific to remain in an ENSO-neutral state over the outlook period.
• For New Zealand, higher than normal atmospheric pressure is expected to the east of the country, leading to north-easterly flow anomalies that may shift to a more north-westerly direction towards the end of the three-month period.
• Low pressure systems forming north of the country are still expected to affect New Zealand and may lead to heightened risks for heavy rainfall events.
• Seasonal air temperatures are expected to be above average across all New Zealand regions for this winter season. Thus, while cold snaps and frost will occur, they are expected to be less frequent than usual.
• Sea surface temperatures (SSTs) remain well above average around most of New Zealand, except in offshore areas east of the North Island. In May, coastal SST anomalies ranged from +0.39°C to +1.47°C. Large regions offshore are currently experiencing Marine Heatwave (MHW) conditions. Global climate models suggest that SSTs in the New Zealand region are likely to stay above average over the next three months, although the intensity of the anomalies may ease. Monitor the SST update for updates.
• June – August rainfall totals are expected to be above normal for the north of the North Island and near normal for the east of the South Island. For all remaining regions of the country, rainfall is forecast to be near normal or above normal. Sub-seasonal, or monthly, projections of rainfall and dryness are updated daily through the NIWA35 forecast.
• During June – August 2025, near normal soil moisture levels and river flows are expected for all regions of the North Island, while near normal or above normal soil moisture levels and river flows are forecast for the South Island.

Regional predictions for June – August 2025

The tables below show the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome for any of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance that is expected for the coming three-month period. All outlooks are for the three months averaged as a whole. During these three months, there will inevitably be relatively wet and dry periods, as well as hot and cold periods. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C relative to the long-term mean, whereas for rainfall the “near normal” category lies between 80 percent and 120 percent of the long-term (1991-2020) mean.

Northland, Auckland, Waikato, Bay of Plenty

• Temperatures are very likely to be above average (65% chance).
• Rainfall totals are most likely to be above normal (45% chance) for June – August 2025 as a whole, with occasional heavy rain events expected.
• Soil moisture levels and river flows are most likely (45-50% chance) to be near normal

Central North Island, Taranaki, Whanganui, Manawatu, Wellington

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are most likely (40-45% chance) to be near normal.

Gisborne, Hawke’s Bay, Wairarapa

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are most likely to be above average (45% chance).
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).
• Soil moisture levels and river flows are most likely (45% chance) to be near normal.

Tasman, Nelson, Marlborough, Buller

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are equally likely to be above normal (40% chance) or near normal (35% chance).
• Soil moisture levels and river flows are equally likely to be near normal (40% chance) or above normal (40% chance).

West Coast, Southern Alps and foothills, inland Otago, Southland

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).
• Soil moisture levels and river flows are about equally likely to be near normal (40% chance) or above normal (35% chance).

Coastal Canterbury and the nearby plains, east Otago

Probabilities are assigned in three categories: above average, near average, and below average.

• Temperatures are very likely to be above average (60% chance).
• Rainfall totals are most likely to be in the near normal range (40% chance).
• Soil moisture levels and river flows are about equally likely to be near normal (40-45% chance) or above normal (35-40% chance).

The full probability breakdown is:

Graphical representation of the regional probabilities

Background

Sea surface temperatures (SSTs) anomalies in the central equatorial Pacific (Niño 3.4 Index) in May (values estimated using data to 26 May) are in the ENSO-Neutral range (-0.08˚C). SSTs currently are slightly below average in parts of the central Pacific, and cooler than average surface waters have re-emerged along the Equator east of about 120oW. The warmer than average ocean waters previously observed off the South American coast have dissipated. The relative Niño 3.4 Index (RONI*) for the past 30 days (to 26 May) was -0.47˚C, placing it on the La Niña side of neutral, indicating that ocean temperatures in the central and eastern Pacific remain cooler than the tropical average.

As of 26 May 2025, the preliminary Southern Oscillation Index (SOI) value for May is +0.68, indicating conditions on the La Niña side of neutral. Atmospheric indicators – such as enhanced trade winds (positive zonal wind anomalies) and stronger-than-normal convective activity over the Maritime Continent – continue to reflect lingering influences from the previous La Niña event.

In May 2025, upper-ocean heat content (temperature anomalies in the upper 300 metres of the ocean) remained above average across the western tropical Pacific, particularly west of approximately 170°W. Anomalies exceeding +2°C persisted south of the Equator, from the Maritime Continent to the International Dateline, near the latitude of the Solomon Islands. East of the Dateline, while isolated areas north and south of the Equator exhibited negative anomalies, heat content along the Equator itself was generally near average. This distribution is consistent with the tropical Pacific Ocean having transitioned away from La Niña conditions.

In the subsurface equatorial Pacific, ocean temperatures remained significantly above average (anomalies > +3°C) between approximately 100- and 200-metres depth west of 170°W. Over the past week, these warm anomalies have expanded eastward and risen in the water column, with above-average temperatures now observed below roughly 50 metres depth west of about 130°W. In contrast, the central and eastern Pacific show limited cooler anomalies, with only a single isolated pocket of below-average temperatures near 50 metres depth off the South American coast.

Taken together, these indicators suggest that the tropical Pacific ocean has largely transitioned away from La Niña conditions observed earlier this year. Notably, the previously prominent east–west gradient in heat content and subsurface temperature anomalies has diminished.

Based on the dynamical and statistical models monitored by NIWA, ENSO-Neutral conditions remain the most likely outcome (73% chance) over the next three-month period (June – August 2025). However, intermittent weak La Niña-like atmospheric patterns may still occur over the winter season. For the remainder of the calendar year, ENSO-Neutral conditions are expected to persist. The guidance available however suggests the possibility for La Niña conditions to return by summer 2025 – 2026.

The Southern Annular Mode (SAM) was mostly positive during the month of May (85% of days positive) and the monthly average anomaly was +0.5 std. A positive SAM index is typically associated with higher pressure than normal over New Zealand. Over the next two weeks, the SAM is forecast to become temporarily negative and then return to slightly positive values.

The Indian Ocean Dipole (IOD) is currently in the neutral range, with the average anomaly for the month of May* 2025 being +0.36oC. The guidance from the Australian Bureau of Meteorology is for the IOD to remain in the neutral range until at least July after which it is forecast to become negative.

During May 2025, intra-seasonal convective activity associated with the Madden-Julian-Oscillation (MJO) was generally weak, despite a temporary increase in MJO amplitude over the Maritime Continent mid-May. The MJO is likely to remain weak or indiscernible in the coming two-week period.

Sea surface temperatures (SSTs) remain significantly higher than average around much of New Zealand. The exception is off the east coast of the North Island, where SSTs are currently near average. Marine Heatwave (MHW) conditions – defined as five or more consecutive days with SSTs above the 90th percentile – are ongoing along the west and southeast coasts of the South Island, as well as the west and northeast coasts of the North Island. SST forecasts for the June – August 2025 period suggest a slight weakening of these anomalies. However, above-average ocean temperatures are expected to persist around New Zealand through the winter season.

*The Relative Oceanic Niño 3.4 Index (RONI) is a modern way of measuring oceanic El Niño and La Niña that is complementary to oceanic traditional indices. While traditional oceanic indices like the Niño 3.4 Index monitor SSTs in one region, the RONI compares the average SST in the central equatorial Pacific with the average SST across the global tropics. Since tropical rainfall patterns respond to changes in ocean temperatures, this new relative index can help forecasters better determine if the equatorial Pacific is warmer or cooler than the rest of the global tropics, which has become more challenging to discern as seas warm because of climate change.

Forecast Confidence

Notes:

1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole, relative to the 1991-2020 average. They are not ‘weather forecasts’ as it is not possible to forecast precise weather conditions three months in advance.
2. The outlooks are the result of the expert judgment of NIWA’s scientists and forecasters. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be useful indicators of likely overall climate conditions for a season.
3. The outlooks state the probability for above average conditions, near average conditions, and below average conditions for rainfall, temperature, soil moisture, and river flows. When a particular probability reaches or exceeds 60%, we conclude it is “very likely”.
4. This three-way probability means that a random choice would be correct only 33 per cent (or one-third) of the time. It would be like randomly throwing a dart at a board divided into three equal parts, or casting a dice with three numbers on it.
5. Where probabilities are within 5% of one another, the term “about equally” is used.
6. All outlooks are for the three months as a whole. There will inevitably be relatively wet and dry days, and hot and cold days, within a season. The exact range in temperature and rainfall within each of the three categories varies with location and season. However, as a guide, the “near average” or middle category for the temperature predictions includes deviations up to ±0.5°C for the long-term mean, whereas for rainfall the “near normal” category lies between 80 per cent and 120 per cent of the long-term mean.
7. The seasonal climate outlooks are an output of Predicting climate variability and change, a scientific research programme supported through NIWA’s Strategic Science Investment Fund.
8. The forecast confidence meter for temperature and rainfall represents the expert judgement of NIWA’s climate scientists. It aims to synthesize various forecast elements, such as global and local climate drivers, in order to clearly communicate forecaster confidence in how all the evidence has aligned for the seasonal outlook.

See all of NIWA’s media releases at: https://niwa.co.nz/news

Seasonal Climate Outlook June - August 2025 | NIWA: https://niwa.co.nz/climate-and-weather/seasonal-climate-outlook/seasonal-climate-outlook-june-august-2025

Home Page | Auckland | Previous Story | Next Story

Copyright (c) Scoop Media