Canterbury and Otago are being warned about possible severe thunderstorms that could generate intense downpours and large hail from Wednesday afternoon.
MetService issued a severe thunderstorm watch for Dunedin, eastern Central Otago, North Otago, and the Canterbury Plains and foothills from around Ashburton southwards between 2pm and 9pm.
Severe thunderstorms would generate localised downpours with rainfall rates of 25 to 35 millimetres per hour, and large hailstones, MetService added.
Rainfall of this intensity can cause surface and flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.
Large hailstones can also cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.
MetService meteorologist Lewis Ferris said a very humid air mass was sitting across the country.
"So, very unstable atmosphere which means those bubbly clouds can get very tall and potentially bring downpours, maybe even some large hail. People might hear or see some lightning," he said.
Warm, humid air was currently down near the surface, sitting below cooler air in the upper atmosphere. This created buoyancy and as the warm air rose, it built clouds, and the higher and more intense it got, the more intense the storms would be, he said.
"With severe thunderstorms watches in the middle of summer time when it is very hot, you might not expect that you would get hail but hail is something that is quite common around these types of severe thunderstorms at this time of the year."
The severe thunderstorm watch comes after large hailstones caused damage in the Clutha District after an intense storm on Tuesday night.
Clutha District mayor Bryan Cadogan said hailstones were "huge" in some places, and came down "heavily" for a short period causing localised damage.
The district council said roads were closed amid the rain and thunder, and are assessing roads in the area.
State Highway 8 between Millers Flat and Lawrence was closed by flooding and toppled trees on Tuesday night.
Rongahere Road was closed overnight between Beaumont and John O'Groats Road, the council said.
Contractors were assessing roads throughout the district including in West Otago, Waipori and the Lawrence areas.
Ferris said there was a reprieve from the stormy conditions in sight.
"As we move forward to the rest of the week, things do look to settle down a little but around those eastern parts of the South Island," he said.
A high pressure system was expected to move in from the Tasman Sea into the weekend, which would bring more settled weather as the week wore on, he said.
'Unlike anything we have seen before'
David Robertson, who lives near Beaumont, said a small creek topped its banks in the storm and nearly took out his mailbox in last night's storm.
He believed more than 70mm of rain fell in 30 minutes.
It was "very loud, continuous thunder and lightning, huge hailstones and overflowing spoutings. [It was] unlike anything we have seen before", he said yesterday.
The creek had nearly dried up before last night’s downpour, he said.
"We were actually going to check our shop in Lawrence, but would have needed a boat. No way we were going to safely make it down tonight."
In a video he took of the storm, he describes the hail as "absolutely nuts" before exclaiming: "Look at the size of that hail!".
Roads affected, more thunderstorms forecast
Police said there had been extensive surface flooding on State Highway 8 between Millers Flat and Raes Junction and they closed the road between Raes Junction and Lawrence.
Motorists were asked to take alternative routes or delay their travel.
A spokeswoman for the Clutha District Council said this morning that Rongahere Rd was closed overnight between Beaumont and John O'Groats Rd due to flooding and fallen trees.
The New Zealand Transport Agency advised that SH8 between Millers Flat and Lawrence has been reopened, as has Tima Burn Rd near Millers Flat.
Craig Flat Road at Millers Flat is closed due to a slip.
Surface flooding may be present on roads and motorists were urged drive to the conditions and take care.
The Central Otago District Council placed general flood warnings on Ranfurly and Patearoa community areas due to torrential rain.
It advised caution as the extensive amount of water lifted manhole covers in the streets.
-ODT and RNZ
