As the upper North Island only now started to dry out after a run of weather that had some areas 50% above their annual rainfall average, weather forecaster Tony Trewinnard predicted more rain might be on the way heading into Christmas and New Year.
Trewinnard, Blue Skies Weather’s forecaster and director, said weather patterns indicated a slow, late developing La Nina that would continue to build in influence over the coming months, with potential to continue well into next year and beyond.
“The El Nino is very much reflected in what we have seen so far.
“That includes the stormier conditions over winter that were generally not as cold as usual with early spring westerlies then dropping into more of a summer pattern, which we are seeing now.”
The high pressure systems were likely to dominate in Canterbury and Otago into summer as the North Island started to fall under the effect of tropical systems moving off the Queensland Coast-Lord Howe Island area, bringing damp, easterly weather with them.
So Canterbury was likely to see less norwesterly wind and the accompanying heat, instead having calm, hot conditions.
Generally, the further south, the drier and hotter conditions would get. A drought on the West Coast was possible.
The emerging humid easterly conditions in the North Island later in January would make it almost a summer of two halves, with ideal drier haymaking conditions turning to good grass-growing weather later in summer.
“It is likely though that these conditions will move further south, through Marlborough and Canterbury, and we do not expect the dry conditions Canterbury is experiencing now to still be dry come autumn.”
He predicted the wetter weather for Canterbury might start later in January as the sub-tropical weather crept further south.
Rather than preparing for drought-like conditions in any particular part of the country Trewinnard said the bigger risk for farmers along east coast areas of the North Island and Northland would be floods starting mid-late January.
Other indicators supporting the easterly conditions included the Quasi-biennial Oscillation (QBO), which was pushing further east.
Met Service forecaster Georgina Griffith expected the effect of the La Nina to be light and almost secondary to other influences in the weather systems.
“This La Nina is a late developer and is fairly weak and almost borderline.
“But there are other players in the mix bringing easterly-La Nina like conditions. This includes the Southern Annual Mode (SAM), a big driver of our southern weather.”
The SAM identified the north-south movement of fierce storm systems that ranged between Antarctica and New Zealand.
The high pressure or positive value it was recording brought more blocking highs to the South Island and fewer southwesterly storm systems.
In contrast to Trewinnard, Griffith expected La Nina to deteriorate heading into autumn.
But, meantime, the lack of northwesterly winds and the creep of wetter, humid weather from the north could pose a challenge for Canterbury grain growers trying to harvest crops. Trewinnard recommended they consider booking drying space early.
“Horticultural crops including grapes on the east coast could also face a challenge with increased humidity causing fungal issues like botrytis and lower sugar levels due to lower sunshine hours.”
While the weather would be good for grass growing through late summer it would also bring a significantly elevated risk of facial eczema under the warm humid conditions.
