Reasonable rain in the past two weeks has eased farmers’ stress around the immediate impact of a seasonal drought in Northland, Auckland, Hawke’s Bay, Waikato and Bay of Plenty.
However, soil moisture levels remain precarious in those areas, also now well into their second year of less-than-average rain.
Weather Watch meteorologist Philip Duncan said it will not be clear until early spring whether winter has delivered enough rain to turn around what has become almost systemically dry conditions.
A lack of substantial winter rain will leave farms vulnerable to any lengthy dry spell in spring or early summer, lacking the soil moisture reserves they would normally have.
“With the exception of some very localised areas I have not had any complaints about how dry things have been for a month. However, we have to remember we are not just talking about this year. Many areas were well behind last year on rainfall and this year has continued to be the same.”
He sees little sign the pattern is likely to change significantly before spring.
MetService rainfall data shows Tauranga, Napier, Hamilton and Whangarei have had only 45% to 60% of their average year-to-date rain.
“So we are going to have to watch what this will do to dams, reservoirs, bores and rivers come summertime.”
To correct the deficit and catch up on averages will require about double the average monthly rain for the next three months.
The blocking high-pressure systems that dominated in summer and autumn have shifted south, helping some low-pressure systems penetrate the parched upper North Island.
“So they have helped chip away at the dry but we really need a lot more.
“If you got 200mm in a month in Waikato right now there is that much capacity in the soil it could be absorbed and many areas could do with that. Even Fiordland is relatively dry with 10 days of no rain.”
Duncan refutes claims the rain in dry areas came too late for farmers heading into midwinter.
“We are still experiencing temperatures well above average, even 11C in Fielding the other evening.”
NZ’s is getting high-pressure systems near the most intense possible, sitting on 1030-1040 hectopascals.
Typically, highs of such intensity are experienced only for a couple of weeks in August, bringing cold, frosty, spring mornings.
“But we are also not seeing the low low-pressure systems, even down at 980hPa that bring rain and we need some deep ones to really deliver some significant rain events.”
Most concerning to Duncan is the belief held by some farmers that seasons balance out.
“For several regions this is the second dry year in a row and I just do not see it balancing out.”
He said the circuit breaker for wetter conditions will come only if the high-pressure systems move further east, past the Chatham Islands.
“That would allow low-pressure systems in the north Tasman to be produced and to move across New Zealand.”
He compares the dry period to the last major widespread dry event in 2008.
“That had a very clear beginning and a definite end. This is tending to just drag on. It’s taken a year to form and is nowhere near over.”
Niwa’s soil moisture deficit indicator for late June highlights how tenuous soil moisture levels are.
Much of Hawke’s Bay and parts of Northland are still in deficit by 10-20mm while Waikato barely scrapes in as even average for moisture for this time of year.
“Come mid August we should have a good understanding if we are in major trouble.”
He is also concerned Australia is starting to exhibit a recurrence of high-pressure systems early in winter, the time the continent normally gets a significant portion of its rain.
“It raises the question that if it doesn’t rain for them now then when will it?”
