Weather: Heavy rain brings flooding risk in north after unusually dry February

After what’s been an unusually dry February, forecasters are warning of potential flooding from a low-pressure system delivering periods of heavy rain onto hard, dry ground over the upper North Island. Photo / Alex Burton

Forecasters are warning of a localised flooding risk in parts of the north, as expected heavy rain falls on ground hardened by an abnormally dry February.

As at Sunday evening, MetService had heavy rain watches in place for Auckland and Northland south of Dargaville up to 10am Monday – and for all of Bay of Plenty from 9am to midnight Monday.

🌧 This morning's rain band weakened weakened just before reaching #Auckland, so it hasn't actually been that rainy yet this afternoon

👨‍🍳 However, the ingredients are all present to produce localised pockets of heavy rain, but it is very tricky to determine where exactly these… pic.twitter.com/PzYBqRN4jh

— MetService (@MetService) February 25, 2024

While the rain would be welcome after long spells of dryness, the forecaster warned peak intensities coupled with parched ground could cause water to run off and pool, carrying a risk of surface flooding in localised areas.

MetService forecaster Aidan Pyselman said a slow-moving front was expected to bring more rain to northern regions on Tuesday, with rain also expected about southern New Zealand on Wednesday.

Whether it’d be enough to alleviate widespread dryness, however, wasn’t yet clear.

“There are a lot of places that are still wanting a lot more [rain],” he said.

Niwa meteorologist Ben Noll said that, as at last week, much of the country had received less than half the rain that’d normally fall in February.

Rainfall has been below normal (🟤) across most of the country during February.

A front this weekend into next week may bring beneficial rainfall to some regions, particularly in the North Island 🌧️

There's uncertainty as to how fast it will track up the country - stay tuned. pic.twitter.com/5KeiZVnTnn

— NIWA Weather (@NiwaWeather) February 21, 2024

“Cumulatively, dryness this summer has been the worst for a number of years; Marlborough, for instance, has been experiencing its driest conditions since the summer of 2019-20,” he said.

In some inland pockets of the South Island, it’d been a decade since a summer with this little rainfall, he said.

As of Saturday, virtually all of the North Island and the north and east of the South Island had recorded more than 45 dry days over the season.

How many dry days has your region had this summer? ☀️

Most of the country experienced at least a month's worth of dry days this summer.

Areas shaded in red (🔴) experienced over 50.

Tasman/Nelson/Marlborough had the highest number of dry days (65 days, over 2 month's worth) 🏜️ pic.twitter.com/4yNZaRllEO

— NIWA Weather (@NiwaWeather) February 24, 2024

The trend had been influenced by an El Nino climate pattern driving warm northwesterly winds over New Zealand – and often making for hazardous fire conditions.

“Conditions this fire season around New Zealand are generally a return to normal compared to the last two years, which were very wet in many areas,” Fire and Emergency New Zealand’s (Fenz) wildfire manager Tim Mitchell said.

“However, fire danger levels in some areas of North Canterbury, Wairarapa and Otago are close to the highest they have been on record.”

By mid-February, Fenz had recorded nearly 2900 vegetation fires or incidents over the season, affecting an area covering just over 4300ha.

That included the 650ha scorched in this month’s Port Hills blaze near Christchurch, where fire crews were still dampening down buried “hot spots” late last week.

A fire that broke out on February 14 scorched more than 650ha in the Port Hills, near Christchurch.

Fire season restrictions remained in place across the country, including Auckland, Northland and Bay of Plenty.

Mitchell said rain received over the weekend would help lower the risk, “but is unlikely they will lead to an easing of fire restrictions”.

“We will have a better understanding of this after the weekend, once we see exactly how much rainfall there has been and in which areas.”

While the New Zealand Drought Index now showed regions including Northland, Wairarapa, Marlborough and Central Otago as running “dry” or “very dry”, nowhere had yet reached the threshold for meteorological drought.

Noll said it’d helped that temperatures over February had hovered close to average.

“While soil moistures around the country are low, they aren’t extreme as they’d have been if we’d had the warmth that was so persistent over January,” he said.

“The other factor has been the odd, spotty rain event that’s delivered five millimetres here and there, and prevented things from tipping into even worse conditions.”

February's rainfall anomalies as at Saturday. Source / Niwa

In Hawke’s Bay, farmers had happily been spared the severe summer drought that a strong El Nino was initially feared to bring.

“We’re just starting to get normal dryness now, but we’ve had feed all the way through,” said Jim Galloway, a Bridge Pa sheep and beef farmer serving as Federated Farmers’ local provincial president.

“Compared to what we were expecting as far as dry goes, most people here would be very pleased.”

Niwa’s most recent long-range outlook until April gave El Nino a 100 per cent chance of persisting, with more northwesterly winds in store for the country over coming weeks.

But also in the mix was potential for more decent downpours, Noll said, courtesy of a warmed-up West Pacific Ocean that’d disrupted El Nino’s traditional climate signals.

“We’re watching for a pulse of moisture to reach New Zealand around the second week of March.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.