Severe thunderstorm warning for North, South Islands after downpour hits Alexandra

Tuesday, 28 November 2017

With more thunderstorms predicted for both islands, MetService says around 4000 lightning strikes were recorded over New Zealand on Monday.

Further storms were picked for Tuesday afternoon and evening, as a static weather pattern settled in over the country.

Scattered heavy showers and thunderstorms were expected about inland parts of the central and lower North Island on Tuesday afternoon and evening. 

Thunderstorms could become severe in southern Waikato, Waitomo, Taumarunui, Taupō, Taihape, inland parts of Taranaki, Whanganui, Manawatū and the ranges of Hawke's Bay and Wairarapa, MetService warned.

READ MORE: Roxburgh flash flood 'like a train' going past

Localised downpours could result of 25 to 40mm per hour. Rainfall of that intensity could cause flash flooding, slips and hazardous driving conditions, the forecaster warned.

In the South Island the risk of severe thunderstorms covered many inland areas, but not Southland or western areas apart from the Nelson ranges.

Storms could hit there as well as Marlborough, the Canterbury High Country, Central Otago and inland parts of North Otago and Dunedin, MetService warned in a severe weather watch.

The 4000 strikes was a typical number of lightning hits from thunderstorms triggered by afternoon convection, although probably more toward the upper end, meteorologist Tui McInnes said.

'It's quite similar to what we've been seeing in the past couple of days.'

While the heaviest downpours in the past couple of days - in Roxburgh and Alexandra - had been in Central Otago, that part of the country was shown having fewer lightning strikes than the upper South Island, and central and southeastern areas of the North Island.

The downpours came from thunderstorm clusters, but lightning and rainfall intensity weren't technically correlated, McInnes said.

In Alexandra the rainfall was likely to have come from a single thunderstorm cell that lasted a long time, and produced around 20 lightning strikes, McInnes said.

Further north sea breezes could push a considerable distance inland to the ranges, leading to a lot more thunderstorms. In Central Otago the sea breezes weren't as dominant.

The sort of afternoon convection and lightning strikes happening now tended to be more common in the middle of summer, McInnes said.

That's when it was more usual for a high pressure system to hang around, bringing calm conditions and warm conditions, and trapping low level moisture under it -  good conditions for thunderstorms.

DOWNPOURS

After around 42mm of rain in a couple of hours caused flooding in the Central Otago town of Roxburgh on Sunday afternoon, it was Alexandra's turn on Monday.

There 38.4mm fell over four hours, nearly 23mm of that between 7pm and 8pm, MetService said.

Other places to get some of the heaviest thunderstorm rain on Monday were Moturoa, near Taupō, with nearly 30mm of rain, while Dannevirke had more than 23mm.

'It's going to be the same today,' MetService meteorologist Michael Marten said.

'Basically it will repeat everyday until probably Friday. Every day we just get summertime thunderstorms, although it's technically still spring,' he said.

'These thunderstorms are quite isolated and don't move much.' For instance, the high rainfall total for Alexandra on Monday was recorded at the town's airport, while a nearby station had more than 15mm less rain.

MetService said a persistent ridge of high pressure meant many places were seeing little change in the weather from day to day.

'However, warm temperatures, sea breezes and humidity combine to form thunderstorms during the afternoons and evenings for inland areas of both the North and South Islands,' MetService meteorologist Hannah Moes said.

'Coastal areas remain mainly dry, while inland areas are targeted by showers and thunderstorms.'Along with lightning, risks associated with thunderstorms could include heavy or torrential downpours and hail.

With light winds across the country, storms that did form this week were unlikely to travel far, so rainfall in localised spots could be very high, bringing risks of flash flooding.

There was some respite from the showers in sight for the lower south in the second half of the week, with conditions stabilising there from Thursday, Moes said.

As well as thunderstorms, some inland areas are also in for high temperatures, with Alexandra expected to get to 26 degrees Celsius on Tuesday, and up to 29C by the end of the week.

Auckland has a chance of showers from late morning, with sunny spells, light winds and a high of 23C. After morning low cloud or fog, Wellington is expecting a fine day with light winds and a 22C high.

Christchurch is expected to get to 21C and should be fine apart from low cloud morning and night.