Seasonal Climate Outlook August - October 2023

28 July 2023

Outlook Summary

El Niño Alert criteria continued to be met during July. Central equatorial Pacific sea surface temperatures (SSTs) exceeded El Niño thresholds but the Southern Oscillation Index (an atmospheric indicator) was neutral. Full ocean-atmosphere coupling for El Niño is expected in the next three months, likely persisting through summer.
Atmospheric pressure is forecast to be above normal to the northwest of Aotearoa/New Zealand and below normal to the south, leading to more southwesterly quarter winds than normal across the country for the season as a whole. Wind speeds are forecast to be stronger than normal, particularly across the South Island and lower North Island.
Rainfall is most likely to be below normal in the north of the North Island, near normal in the west of the North Island and east of the South Island, about equally likely to be near normal or below normal in the east of the North Island, and about equally likely to be near normal or above normal in the west and north of the South Island.
During the second half of August, frequent westerly winds and low pressure systems are expected to result in occasional heavy rainfall for the South Island, particularly in the west, as well as the western North Island.
Temperatures are about equally likely to be near average or above average in the northern North Island and most likely to be above average in all other regions.
Westerly winds will bring warm air into the region from Australia at times.
Coastal sea surface temperatures (SSTs) ranged from 0.3˚C to 1.8˚C above average during July. The warmer than average seas may reduce the intensity of episodic cold, southerly air masses that typically occur during late winter and spring.
Soil moisture levels are most likely to be below normal in the north and east of the North Island, near normal in the South Island, and about equally likely to be near normal or below normal in the west of the North Island.
River flows are most likely to be near normal in the west and east of the South Island and about equally likely to be near normal or below normal in all other regions.

NIWA Seasonal Climate Outlook: August-October 2023

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Regional predictions for August – October 2023

Northland, Auckland, Waikato, Bay of Plenty

The table below shows 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 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. The following outcomes are the most likely for this region:

Temperatures are about equally likely to be above average (50% chance) or near average (45% chance).
Rainfall totals are most likely to be below normal (50% chance). An increased frequency of high pressure systems near the region will likely lead to spells of drier than normal weather.
Seasonal wind speeds may be stronger than normal.
Soil moisture levels are most likely to be below normal (45% chance) while river flows are about equally likely to be below normal (45% chance) or near normal (40% chance).

	Temperature	Rainfall	Soil moisture	River flows
Above average	50	15	20	15
Near average	45	35	35	40
Below average	05	50	45	45