Wet, wild week of tropical pulse
Several rain bouts will be needed for rain-starved areas
It’s kind of like an energy drink for mother nature .
Ben Noll, Niwa
Aweek of wet, muggy weather is being delivered to much of New Zealand courtesy of a pulse of tropical moisture that one meteorologist has likened to “mother nature’s energy drink”.
But many more downpours will be needed to alleviate widespread dryness — with some parched areas still facing severe moisture deficits of 50mm weeks after summer.
After last week’s dry and settled weather, MetService is forecasting a wet and wild week for most areas, as a humid air mass hovers over the country.
A slow-moving front embedded in a warm and moist northerly flow brought periods of rain to the North Island yesterday, with severe weather warnings and watches in place for Northland, Taranaki, Waitomo, Taumarunui, Taupo¯ and Taihape.
The humid conditions were also favourable for thunderstorm development, with much of the northwest of the North Island under a “moderate” thunderstorm risk.
While weather remained mainly fine over the South Island, some low cloud was forecast to linger along the east coast, and a few showers were expected in inland areas.
A complex low pressure system was expected to deepen in the Tasman Sea today, before moving over the country tomorrow and Thursday bringing another dose of rain and possible thunderstorms.
Further watches and warnings for heavy rain were expected to be issued.
On Friday, a cold front was forecast to move swiftly up the country, accompanied by another period of rain with strong southwest winds, and bringing relief from humid nights.
Over Easter weekend a ridge was due to build over the country and bring more settled weather for most places.
Niwa forecaster Ben Noll said some of the humidity being recorded around the country was abnormal for the time of year. At noon yesterday humidity was close to 90 per cent in Hamilton, 83 per cent in Tauranga, and 75 per cent in Auckland.
Niwa forecaster Ben Noll said the wet weather was the result of a longawaited surge of moisture, ferried here by the Madden-Julian oscillation (MJO) — a pulse of rain and thunderstorms that circled the globe every 30 to 40 days.
In certain phases, it could power weather patterns and drive big downpours over New Zealand.
“It’s kind of like an energy drink for mother nature — and when it’s aligned with your line of longitude, looking northward to the tropics, it can spark some pretty active weather,” Noll said.
“It just so happened that the MJO had been tracking from the eastern Indian Ocean around the time when Australia had its recent major flooding, and helped transport moisture across the continent.
“Now, it’s continuing to track eastward, and from now, and into the first 10 days of April, it’s going to be navigating the Pacific Ocean.”
Noll said the system’s presence closer to our neighbourhood didn’t mean New Zealand’s weather would be “constantly busy” over the coming weeks.
“But there are going to be several opportunities, as we go through the first half of April, for some rain and windy weather at times, which is a certainly different flavour to what we’ve seen over the last couple of months.”
Noll said the high pressure that had dominated New Zealand through much of March, bringing fine, settled, summer-like days, could be linked to the fact the MJO had been centred so far away.
For many rain-starved parts of the country, the big wet would be welcome. Virtually all of the North Island, along with eastern and much of inland South Island, was running drier than usual.
The driest pockets could be found around Pukekohe, inland Manawatu¯Whanganui, southern Hawke’s Bay, Marlborough, northern and central Canterbury, and northern and central
Otago.
“I think we can expect this current activity to do quite a number on some of those soil moisture deficits that have been recorded,” Noll said.
“But, given that we’re talking about deficits that are as high as 50mm, it’s likely going to take two or three weather systems like this before we can start to move back closer to normal.”
Noll suggested there’d be a low chance of New Zealand receiving an ex-tropical cyclone system before the cyclone season officially ended on April 30.
“However, because the weather patterns that may be affecting us here over the coming half-month will be coming from a northerly direction, we still have to be on heightened alert for things like flash flooding and slips.”