In its latest seasonal outlook NIWA tips a strong likelihood El Nino conditions will dominate weather patterns in late spring and early summer.
It puts an 88% chance on non-traditional El Nino conditions developing from November to January.
An 85% chance is put on temperatures at or above average for most of the North Island while coastal Canterbury and Otago have the greatest odds, at 35%, of below average rainfall.
But Farmers Weekly forecaster Philip Duncan of Weather Watch hesitates to agree on the likelihood of a major El Nino.
“I think the El Nino factor is being overplayed at this point.
“Typically, with El Nino you would find we would be experiencing much cooler conditions in the Tasman Sea and we are not.”
Australia’s Meteorology Bureau reports the sea temperatures off the coast of Sydney and Melbourne are 1-2C above normal, in a band that extends across the Tasman Sea to NZ’s coastline.
NIWA suggests the development of a modoki or non-conventional El Nino pattern, where the maximum sea surface temperature anomaly is located more towards the central than eastern Pacific.
“I do support NIWA’s November forecasts but not out to January,” Duncan said.
“Seasonal forecasting is a really fraught business.
“We are a group of small islands in a very large ocean. We lack a large, stable, land mass and we can have a 50% chance of being wrong.”
NIWA’s shorter term outlook for November predicts frequent high-pressure systems to circulate, delivering several very warm spells over the next four weeks.
“I tend to focus more on the south-southeast of Australia and what is happening there at present. They are experiencing patterns similar to ours and we are tending to follow them.”
The region has experienced a spring dry spell followed by some snow and rain, particularly in coastal areas, similar to what NZ has experienced.
“Really, what we are experiencing is not being driven by any particular index or system at present. We are having the similar patterns to southeast Australia and it is generally a chaotic activity that we have had for the past 18 months.”
Despite recent concern over hydro lakes levels and irrigation issues in the South Island Duncan’s greater concern is dry spells in the North Island in the next month.
“We are going to see some significant rain events hit the South Island in coming weeks as sub-tropical northerly conditions develop off the West Coast, giving some horsepower to the rain they will receive into the southern lakes district.”
Meantime, the North Island is expected to be influenced by high pressure systems centred on South Australia extending right across the Tasman and even beyond in an unbroken belt.
“I can’t see a lot of rain coming for the North Island for at least the next 10 days.”
This will add moisture pressure on regions that have already come through a drier than normal October.
“I would be concerned about Waikato, Northland, Hawke’s Bay and even Manawatu as the winds we are experiencing this week dry up the rainfall they did receive at the end of October.”
That dry period might be broken by rain spinning out of what he describes as the very active tropical zone, bringing some welcome north-northeasterly moisture patterns.
“The main thing is we don’t want farmers to start worrying too much about what we cannot really see yet.”
However, he estimates December should shape up as reasonable in terms of moisture levels, with a better-than-average chance reasonable rain-making weather systems will pass through.
“The high pressure systems we get are there but they are moving quickly and that gives me confidence we won’t get stuck in a pattern too much.”
