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A bit like Christmas, El Nino is on its way – but not here yet, according to Niwa’s September/November outlook.
Northland, Auckland, Waikato, Bay of Plenty temperatures are most likely to be near average, rainfall totals are most likely to be below normal, and seasonal wind speeds may be stronger than normal.
Consequentially, soil moisture levels are most likely to be below normal, while river flows are about equally likely to be below normal or near normal.
Although the first half of September will be unsettled at times, an increased frequency of high-pressure systems near the region during spring will likely lead to longer spells of drier than normal conditions.
Prevailing areas of high pressure near the region may also cause clear, cold nights and mornings and the potential for some unseasonable frosts.
The El Nino alert level criteria continued to be met during August and central equatorial Pacific sea surface temperatures (SSTs) exceeded El Niño thresholds while atmospheric indicator, the Southern Oscillation Index (SOI) was near the El Niño threshold.
The conditions needed to officially classify El Niño in a New Zealand context may be reached during September.
Air pressure is forecast to be above normal to the northwest of the country and below normal to the south, leading to more southwesterly quarter winds than normal across the country for the season.
However, the pattern during the first half of September is expected to have more easterly quarter winds. Spring rainfall is likely to be near normal or above normal in the west of the South Island and most likely to be below normal in all other regions.
Spring may be marked by unusually long dry spells for several regions, owing to a combination of high pressure near the country and lower chances for atmospheric rivers.
Near normal or above normal rainfall in the northern and eastern North Island during early-to-mid September may not be indicative of the season as a whole.
Coastal sea surface temperatures (SSTs) ranged from 0.1 degree Celsius below average to 1.1˚C above average during August.
As El Niño becomes firmly entrenched in the climate system, the risk for the development of widespread marine heatwave conditions during spring is low.
