Localised parts of Northland - especially north of Whangārei - could receive in excess of 200mm of rain fall in around 24 hours which, given the already sodden ground, could bring about flooding quickly, MetService reported.
MetService has issued multiple severe weather watches and warnings – including Red heavy rain warnings for Northland from 24 hours from 4am Tuesday, and for Coromandel for 18 hours from midnight Tuesday.
The rain-maker could bring Northland 100mm to 140mm of rain in the north and east, with some areas possibly receiving as much 140mm to 220mm.
At their hardest, the downpours could reach peak rates of 10mm to 20mm an hour in the northeast of the region – including localised rain hitting volumes as high as 40mm or more per hour – with thunderstorms possible.
After the system moved southward later on Tuesday, totals of 50mm to 80mm could drop over the Auckland region from Orewa southwards from 6pm, with an Orange heavy rain warning in place overnight Tuesday and Wednesday.
Peak rates were forecast to reach up 20mm per hour, while some isolated spots – especially in the north – could see up to 40mm hour.
While less rain was likely about and south of the city, amounts could still exceed “warning” limits in some places, MetService reported.
Niwa Weather forecaster Ben Noll said the developing system was yet another example of an “atmospheric river” funneling warmth and moisture from the subtropics, thousands of kilometres above New Zealand.
Over Tuesday, a subtropical low parked off the country – and still being blocked by a high pressure system lying off to our southeast – was expected to strengthen and push further southwest into the Tasman Sea.
“As it does so, all of that associated moisture will come along with it – meaning much of Northland is going to be exposed to some pretty heavy rainfall amounts, perhaps eclipsing 100mm.”
Aucklanders would see steady rain through much of tomorrow also – but overnight Tuesday was the “period of concern”, Noll said.
“That’s when we’ll see those really intense convective lines – which could bring some extremely high moisture values - potentially move down through Auckland.
“Within these cells, we’re looking at 25mm-plus an hour on Tuesday night, and that could carry on for a couple of hours.”
That would mean more flooding, he said, at a time forecasters have warned that it wouldn’t take much more rainfall to create more slips and damage across the region.
On Friday, those convective features happened to line up straight across the city, helping deliver Auckland’s wettest day in history.
“It’s one thing to get those intensities, but it’s another to have them right over the most populated part of the country, where you know the impact is going to be dramatic,” Noll said.
“On Tuesday, while it does look like we’ll get these narrow corridors of intense rain, the main question is where they stretch across, and whether it’s north, central or south Auckland.”
This likely wouldn’t become clearer until closer to the event, he said.
However, it appeared the system would bear one of the key features of last week’s.
That was a “low-level jet” - a ribbon of strong wind in the atmosphere, positioned not far above the surface, that extended from the system’s flank and siphoned warmth and tropical moisture straight across Auckland on Friday.
“The orientation of the jet on Friday ran almost due north to south – or straight through Auckland – which is probably the worst-case scenario as it can cover more of the geographic area at once,” Noll said.
“The one on Tuesday, however, appears to run more northeast to southwest, with a faster forward speed, and will probably be overhead from 7pm Tuesday to the small hours of the morning.
“In that timeframe, moisture values for Auckland will be maximised.”
Noll said it was possible that its values of “total column water” - or the amount of water packed into the column between the surface and the top of the atmosphere – could reach Friday’s.
Another question was whether the system would also pack thunderstorm activity – something blamed for the heaviest rain in the last storm.
In that case, a long band of thunderstorms were squeezed within the moisture-charged system to the point they couldn’t take on more water, ultimately unleashing torrents of rain on the city below.
“[On Tuesday], I’d say there’s certainly a chance that we’ll get those convergence lines that could ultimately lead to isolated thunderstorm activity.”
MetService, too, warned that thunderstorms were possible in Auckland on Tuesday night.
WeatherWatch described the system – also bringing gales to Northland, and possibly damaging gusts – as one packed inside a “squash zone” between two pressure systems.
Rainfall values for Northland, Auckland and Coromandel Peninsula over coming days could top a normal month’s rain for January – and areas around the edges could also see falls of 50mm to 100mm.
“I know there are a lot of people with anxiety at the moment,” WeatherWatch’s Phillip Duncan said.
“The best I can say is no two severe weather events are perfectly the same, they are as unique as fingerprints, so it’s not always a carbon copy for everyone of what we just experienced.
“For those most worried about the rain, track it live on the MetService rain radar which you can find at WeatherWatch and RuralWeather or from MetService’s website, that way you can see where the heaviest and driest areas are and sometimes tracking, seeing and understanding it can help a little”.
By Thursday, Duncan said this week-long-event would finally be over.
“Waitangi Weekend looks much more positive for the North Island, it may be the South Island’s turn to get some wetter and windier weather.”
