A forecaster is predicting a "tale of two extremes" for New Zealand over coming months - with hot, dry weather interspersed with heavy deluges like one expected to hit the West Coast hard this week.
As Niwa issued its three-month seasonal outlook today, MetService declared a "red warning" for heavy rain in Westland and Buller from 1am on Wednesday, with the potential for hundreds of millimetres to drop within just 48 hours.
The event could include downpours potentially bringing 40mm of rain per hour, along with thunderstorms.
Rain accumulations are forecast to approach 750mm about the West Coast ranges in a 42-hour period, with 150 to 250mm near the coast, in a deluge hitting this week. Image / MetService
"The West Coast is one of the wettest parts of New Zealand, but this system is far from a normal wet day on the coast," MetService meteorologist Lewis Ferris said.
Closer to the coast, the highest risk of flooding came when the tides were high – and delays on the road network were also expected.
"We have been working with West Coast Regional Council and Civil Defence to provide as much early warning of this impactful event as possible for locals and those who might be planning on travelling to the region for the long weekend."
As well, MetService has issued orange warnings for heavy rain in most neighbouring regions.
The current warnings were in place until Thursday night but were likely to be extended – and rain was forecast to make it across the North Island during the weekend.
Meanwhile, Niwa was forecasting warmer-than-average temperatures everywhere except eastern areas - where there was an equal chance of average temperatures - between now and April.
Source / MetService
These balmy conditions would help create days with high humidity and warm overnight temperatures – though fewer expected westerly winds in the east might bring fewer scorching hot days.
More stretches of hot, fine weather wouldn't be so welcome in several regions where soil conditions were extremely dry - particularly in areas in Northland and Waikato already experiencing meteorological drought.
But forecaster Ben Noll said brief bouts of heavy rain could also be expected over the next three months.
Along with this coming event, moderate-to-heavy rain may affect the North Island during the second week of the month, with another big downpour potentially coming in March.
Some of those deluges would be what are called atmospheric rivers – systems capable of funneling moisture from the tropics and sub-tropics hundreds of kilometres away from New Zealand.
It was just these events that caused what was New Zealand's costliest year for natural disaster insurance claims last year: the storm that hit the West Coast in July cost $122m in claims alone.
🐍 A snake tongue-like atmospheric river (🟢) will cause very heavy rainfall in the western South Island from Wednesday...
...this is the result of two separate tropical moisture plumes converging.
"We'll likely have these periods of potentially extreme weather interspersed with our status quo of high pressure and more tranquil conditions," Noll said.
"These two things are obviously very contrasting in what they offer: one's ideal for holiday-making and enjoying the outdoors – the other can have you worried about your property or even your safety.
"So, it's a tale of two extremes, these next three months."
Further in the background, a La Nina climate system that's been colouring our summer weather was expected to linger on into early autumn.
And warm coastal sea conditions that recently reached "marine heatwave" thresholds could keep driving more abnormal warmth on land across the country.
