Seasonal Climate Outlook June - August 2026

Outlook Summary

• Air flow patterns are expected to tend towards a west or southwesterly flow anomaly, characteristic of El Niño conditions, as the season progresses. June may behave as a transitional month, and the northerly air flow anomaly during the first week should not be representative of the upcoming season. 
• Seasonal air temperatures for June – August 2026 are about equally likely to be near average or below average in the west of the South Island. They are about equally likely to be near average or above average in the east of the South Island. Near average temperatures are most likely elsewhere.
• Rainfall totals for June – August 2026 are about equally likely to be near normal or below normal for the north of the South Island and the west of the North Island. Rainfall is most likely to be below normal for the remainder of the North Island, and for the east of the South Island. Meanwhile, rainfall is about equally likely to be near normal or above normal for the west of the South Island.
• New Zealand has experienced several heavy rain events from the north over the past year. These events are expected to become less frequent over the coming months, and beyond. Areas hit by heavy rainfall are expected to shift to generally drier-than-usual conditions. Note that the wet conditions occurring in early June should not be characteristic of the upcoming season.
• The prospect of below normal winter rainfall in several regions is likely to translate to below-normal groundwater recharge, creating challenges for water-reliant sectors.
• During June – August 2026, soil moisture levels and river flows are expected to be near normal or below normal for the east and north of both islands, and the west of the North Island. In the west of the South Island, they are expected to be near normal.
• Neutral El Niño–Southern Oscillation (ENSO) conditions are currently present in the tropical Pacific; however, the coupled ocean–atmosphere system continues to transition rapidly toward El Niño.
• Warm subsurface ocean temperature anomalies have continued to intensify and expand in the tropical Pacific, consistent with the ongoing transition toward El Niño and raising the potential for a significant event.
• There is a 95% chance for El Niño conditions to emerge over the forecast period (June – August 2026). 
• El Niño–related influences on regional weather patterns are expected to become more apparent later in the season as ENSO signals strengthen.
• In a New Zealand context, later this winter, this is expected to translate to periods favouring active weather including, unusually windy conditions, along with notable temperature variability.
• Peak El Niño conditions are expected to occur during the austral summer of 2026–27, with the potential for this event to have significant impacts (see From our forecasters: Winter 2026 may see the start of a formidable El Niño event).

Regional predictions for June – August 2026

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.

The intensification and expansion of warmer‑than‑average subsurface ocean waters are increasingly consistent with the early stages of a strong El Niño event later this year.

Rainfall and Convection: During May 2026, enhanced convection and rainfall - indicated by negative outgoing longwave radiation (OLR) anomalies – were restricted to the Indian subcontinent and central Indian Ocean, associated with an early monsoon onset, and in the North Pacific Ocean, associated with an active and persistent Intertropical Convergence Zone. Positive OLR anomalies, indicating suppressed convection, were recorded over the eastern Maritime Continent, the SW Pacific and Central America. These patterns comprise a mixture of both the MJO and the developing El Niño.

Trade winds strengthened in the eastern Pacific relative to April, partially driven by extratropical developments near South America, and by tropical intraseasonal factors. These trades have weakened recently, and should continue to do so through June. Meanwhile, in the central Pacific, weakened trade winds, and occasional wind reversals are expected to persist over the next few weeks, which will further aid El Niño development and ocean-atmosphere coupling.

In summary, oceanic conditions are still setting the stage for the development of El Niño in the very near future, with particularly significant warmth (positive heat content anomalies) stored in the ocean along the Equator in the Pacific. These signals are only growing ever-stronger with time. Meanwhile, the atmosphere is also increasingly resembling El Niño. A fully coupled El Niño is favoured to emerge by the middle of winter.  

The Relative Oceanic Niño Index forecasts indicate a 95% chance for El Niño conditions over the next three-month period (June – August 2026), and a 5% chance for ENSO-neutral. The IRI/CPC forecasts also support this outlook. Further strengthening of the event is likely far beyond the outlook period, with a strong possibility of this developing into a historic event during spring or summer. 

The Southern Annular Mode (SAM) was more commonly positive than negative during May 2026, reflecting the settled weather conditions both early and late in the month, with only a short unsettled spell at the end of the first week.

The Indian Ocean Dipole (IOD) value for May 2026 is negative (-0.31°C). Forecasts from the Australian Bureau of Meteorology indicate that the IOD is likely to return to positive values over the next three month period, and may reach thresholds (+0.4°C) required to declare a positive IOD event during the winter.

During May 2026, the Madden Julian Oscillation (MJO) behaved very differently to the previous month. It stalled in the Indian Ocean, and likely contributed to the early development of the summer monsoon over South Asia. It lost amplitude, as expected, when crossing the Maritime Continent, but then emerged into the Western Pacific and strengthened near the end of the month. It is forecast to continue tracking eastwards, with amplitude increasing further as it encounters favourable conditions towards the central Pacific.
The MJO moving through the Pacific phases is consistent with the current northerly-sourced rain event; further migration of the MJO into the Western Hemisphere and beyond is often associated with a “return to westerlies” for the NZ region.

Local Sea Surface Temperature (SSTs) are near average or above average around the majority of the country, and marine heatwave (MHW) conditions are emerging near the south of the South Island. Forecasts from coupled ocean-atmosphere models for the next three-month period (see Sea Surface Temperature Update | Earth Sciences New Zealand | NIWA) indicate SSTs should continue to warm around the coastal waters of the country, especially around the South Island.

[1]The Relative Oceanic Niño 3.4 Index (RONI) is a modern way of measuring oceanic El Niño and La Niña that is complementary to traditional oceanic 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