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NORMAL and average are the bywords for Niwa’s April/June weather outlook with El Niño and La Niña making a changeover.
For the King Country it means temperatures are likely to be near average or above average. Rainfall is also likely to be near normal, though a period of northerly winds in mid-April may increase the chance for heavy rainfall.
But rainfall may tend lower than normal during early winter as southerly winds are expected to become more common. Soil moisture levels and river flows are likely to be near or below normal.
La Niña ended during March, after a three-year run. A dramatic change in tropical trade winds resulted in warming seas across the equatorial Pacific which by late March saw ENSO-neutral conditions occurring.
El Niño conditions could arrive as early as winter, and Niwa has issued an El Niño Watch to cover this potential.
Niwa expects ENSO neutral conditions to have an 80% chance of continuing through June.
Through winter there is a 65-70% chance of El Niño developing through winter and continuing through spring.
The transition means changeability will be a theme in New Zealand’s weather patterns. Mixed air flows during autumn could tend southerly around the start of winter, consistent with a developing El Niño. As such, the weather during the previous month may not be a particularly useful indicator of the weather in the month ahead.
During mid-April, a northerly quarter air flow may cause tropical moisture to move toward the country, resulting in potential heavy rainfall.
For the three-months, rainfall is likely to be near normal or above normal in the north and west of the South Island and most likely to be near normal in all other regions.
Should the climate system migrate toward El Niño in the months ahead, June could turn out less wet than normal, particularly if southerly quarter winds take hold.
Seasonal temperatures are most likely to be above average in the east of the South Island and likely to be near average or above average in all other regions.
While a colder than average season is unlikely due to the persistence of warmer than average sea temperatures, more variable circulation and wind patterns will lead to more variable temperatures.
Should El Niño arrive, it could result in cooler winter temperatures, particularly relative to recent record warm winters, including an increased risk for frosts.
