Highlights:

- Lower than normal air pressure south of Aotearoa New Zealand will lead to unsettled and windy weather patterns throughout September, particularly in the west

- There is a 50% chance that a La Niña event officially develops by the end of spring

- Winter snowfall, catchment-level river flows, and groundwater recharge were below normal in several regions, which could present challenging conditions for water-reliant sectors heading into spring

NIWA Outlook: September – November 2024

Outlook Summary

• The effect of a minor Sudden Stratospheric Warming (SSW) event in July-August is expected to contribute to lower than normal air pressure south of Aotearoa New Zealand during September, leading to unsettled and windy weather patterns, particularly in the west.

• La Niña-like patterns may develop in mid-to-late spring, favouring the return of higher than normal air pressure to New Zealand. A La Niña Watch continues and there is a 50% chance that an event officially develops by the end of spring. See the background for more.

• Spring temperatures are most likely to be above average in the north and east of the North Island and east of the South Island. Spells of unusual spring warmth are likely as warm air masses from Australia occasionally flow eastward into New Zealand. Near average or above average temperatures are about equally likely for all remaining regions.

• Spring rainfall is about equally likely to be near normal or above normal in the west of the North Island and north and west of the South Island, consistent with more westerly quarter winds, fronts, and low pressure systems, especially early in spring. Near normal spring rainfall is most likely in the north of the North Island with near normal or below normal rainfall about equally likely in the east of both islands. Monitor NIWA35 for updates.

• Coastal sea surface temperatures (SSTs) ranged from 0.45C to 0.64C above average during August, an increase of 0.1C to 0.2C compared to July. Monitor the SST update for updates.

• Winter snowfall, catchment-level river flows, and groundwater recharge were below normal in several regions. These factors could present challenging conditions for water-reliant sectors heading into spring.

• Spring soil moisture levels are most likely to be near normal in all regions of the country. 

• Spring river flows are most likely to be near normal in the west of the North Island and the north and west of the South Island and about equally likely to be near normal or below normal in all remaining regions.

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