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A developing El Niño is expected to result in a lower likelihood of tropical moisture plumes, such as those that frequently affected the country during the first half of this year, according to Niwa’s July/September weather outlook.
El Niño Alert criteria was reached during June when sea surface temperatures in the central equatorial Pacific exceeded El Niño thresholds – but atmospheric indicator the Southern Oscillation Index was neutral.
The weather system is expected to progress toward El Niño conditions from late winter into spring.
As El Niño develops, air pressure is forecast to be above normal over the Tasman Sea, leading to more southwesterly quarter winds than normal across the country for the season.
Episodic easterlies are also possible, particularly early in the three-month period, owing to a La Niña-like lag effect in the atmosphere.
Equinoctial gales may return around the start of spring.
Rainfall is most likely to be near normal in the east of the North Island, about equally likely to be near normal or above normal in the west of the South Island, and about equally likely to be near normal or below normal in all other regions.
Temperatures are about equally likely to be near average or above average in all regions.
Regional predictions for July – September 2023 for Northland, Auckland, Waikato, Bay of Plenty are for temperatures likely to be above average or near average, with rainfall totals about equally likely to be below normal or near normal.
Soil moisture levels are most likely to be near normal while river flows are about equally likely to be below normal or near normal.
There is also a likelihood of short sharp cold snaps, as happened in early July.
Coastal sea surface temperatures remained from 0.4˚C to 1.8˚C above average during June, which was expected may reduce the intensity of cold, southerly air masses.
