Auckland and Northland are expected to be lashed with thunderstorms, heavy rain and hail today, bringing a threat of flash flooding.

MetService earlier issued a severe thunderstorm watch for Northland and Auckland for seven hours from midday.

The forecaster says slow-moving electrical storms are expected to roll over the top of the country until 7pm, with intense localised downpours leading to surface and flash flooding in low-lying areas.

Between 10 to 25mm/h of heavy rain is expected to fall, with hail anticipated north of Auckland city.

At 3pm, MetService also issued a severe thunderstorm warning for Kaipara, the Far North and Whangarei.

It comes after their weather radar detected a line of several thunderstorms lying from Motutangi and Awarua.

“This line of severe thunderstorms is moving towards the southwest, and is expected to lie from Motutangi to Tutamore to Waihue at 3.22pm and from offshore, west of Ninety Mile Beach to Waipoua Forest to Dargaville at 3.52pm,” MetService said.

Torrential rain is expected to accompany these thunderstorms, possibly causing surface and/or flash flooding about streams, gullies and urban areas, and making driving conditions extremely hazardous.

MetService meteorologist John Law said areas north of the Harbour Bridge are expected to see some “pretty intense bursts of rain”.

Law said the storm’s lack of pace combined with the large amounts of rain is the perfect storm for flash flooding, surface flooding and slips in low-lying areas.

“It will make extra hazardous conditions on the roads as well.

“We need to be taking an extra bit of care, travelling and watching out for those as we head through this.”

Further north, between 25 to 40mm/h of rain is tipped to drench Northland, with or without the thunderstorms.

“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 said.

There is also a low risk of thunderstorms over the remainder of Auckland, Coromandel Peninsula, Waikato, Waitomo and Taranaki.

MetService said an unstable atmosphere over the north of the North Island is causing the wild weather.

Forecaster Niwa said since the thunderstorms were slow-moving this increased the risk of surface flooding.

“Although they’ll be quite isolated, these heavy cells will likely be slow-moving and may produce rainfall that’s heavy enough to cause surface flooding,” Niwa said.

“The action will be driven by converging winds in the lower atmosphere and weak winds aloft that will enable the cells to remain nearly stationary.”

Tropical Cyclone forming in South Pacific

Beyond New Zealand shores, a tropical low near the Solomon Islands is expected to intensify into a Tropical Cyclone later this week.

Forecasters are keeping a close eye on the path of the cyclone, which is currently expected to make landfall in northern Queensland, Australia.

“There’s no indication that we’ll get a tropical cyclone coming toward New Zealand at this stage, but we are flagging a moderate risk of a tropical cyclone, somewhere around that area, from around the middle of the week,” said meteorologist Stephen Glassey.