The worst of the incoming front, fuelled by a subtropical low-pressure system parked just off the country, was expected to hit Northland this afternoon.
Much of the region - which MetService has placed under a Red Warning for heavy rain until 4am tomorrow - has already been soaked by overnight downpours that brought as much as 50mm to some places.
This system, however, could bring Northland 100mm to 140mm of rain in the north and east - with some areas possibly receiving as much 140mm to 220mm - in a 24-hour period.
At their hardest, the downpours could reach peak rates of 10mm/h to 20mm/h in the northeast of the region - including localised rain hitting volumes as high as 40mm or more per hour.
There was also currently a moderate risk of severe thunderstorms for Northland, possibly driving localised downpours of 25mm per hour to 40mm/h.
“Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips,” MetService reported.
From 5pm tonight, meanwhile, as much as 80mm to 120mm could be dumped on the Auckland region north of Ōrewa over 15 hours - with the potential for thunderstorms and localised downpours of 25mm/h to 40mm/h.
The deluge was expected to swell rivers and cause “significant” flooding, MetService warned.
“Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.”
After the front moved further southward later this evening, totals of 50mm to 80mm could drop over the Auckland region from Ōrewa southwards from 6pm, with an Orange heavy rain warning in place overnight Tuesday and Wednesday.
“While there will be some rain before nightfall in Auckland, the heaviest is going to happen overnight,” Noll said.
Peak rates were forecast to reach up 20mm/h, while some isolated spots - especially in the north - could see up to 40mm/h.
Northland and Auckland have also been placed under a Strong Wind Watch until early tomorrow morning, with strong northeast winds potentially rising to severe gale in exposed places.
“What can happen during intense periods of rain, some of that really strong wind can be mixed or brought down to the surface - and that is a concern for overnight tonight and into early Wednesday morning,” Noll said.
Potential thunderstorms also carried the risk of wind shear - or a sudden change in wind direction or velocity - which may create severe wind speeds in some areas.
“If we do see thunderstorm activity, this could bring potentially damaging winds to parts of the region – so it’s certainly no time to let your guard down.”
Noll said that, like Friday’s system, the coming system would also include a feature called a low-level jet – or a ribbon of strong wind in the atmosphere, positioned not far above the surface.
On Friday, it ran north-to-south across the city and helped dump rain over much of the urban area.
Tomorrow morning, it appeared the jet would be moving faster, and running more northeast to southwest.
“That means there’ll be less of the region exposed to extreme weather at any one time,” Noll said.
Elsewhere, Coromandel - also under a Red warning - could receive 100 to 150 mm about the ranges, with lesser amounts about the coast, between 10pm tonight and 3pm tomorrow.
Those downpours could come with peak rates of 15mm to 25mm - especially about the ranges - along with thunderstorms, flooding, slips and rising rivers.
Adding to the weather woes, MetService has also warned the system could bring waves as high as 4 metres at times in northern regions.
In Waikato, where rainfall could reach warning criteria in some places, a Heavy Rain Watch is in place between 2am and 3pm tomorrow.
Further east, the Bay of Plenty west of Whakatane was under an Orange Heavy Rain warning – with totals of 100mm to 150mm, and peak rates of 15mm/h to 25mm/h, possible between 3am and 9pm tomorrow.
Heavy Rain Watches have also been issued for Mt Taranaki tomorrow, for much of Westland over Thursday and Friday, and for Canterbury High Country, Central Otago, Clutha, Southland this afternoon and into the evening.
