MetService: 'That is a large amount of rain'

A quick turn to avoid a more flooded road. Te Kupe Rd, Paraparaumu Beach, Monday. Photo / David Haxton

An incredible amount of rainfall in Kāpiti created raging rivers and streams, flooded streets, road diversions, debris slips, and more on Monday.

MetService meteorologist Lewis Ferris said weather recordings from Kāpiti Coast Airport showed in the 24 hours up to 9am on Tuesday there was 80.2mm of rain.

"In the rain event, from midnight on Monday, to 9am on Tuesday, there was 132.4mm recorded at the airport.

"That is a large amount of rain.

"December, on average, would see about 80mm of rainfall at the airport, so this was over one and a half times that in less than a two-day period.

Flooding in Te Kupe Rd, Paraparaumu Beach, on Monday. Photo / David Haxton

"The weather station at Waikanae showed similar amounts."

Key moments included a "quite intense period around dawn on Monday".

"In the hour up to 7am there was almost up to 20mm of rain, and bearing in mind we classify heavy rain as about 6mm in an hour."

Ferris said the weather system was the same that had dumped a lot of rain on the West Coast during the weekend.

A raging Waikanae River on Monday. Photo / David Haxton

"As it moved northwards it brought a lot of the fuel for the rain that fell in Wellington and Kāpiti.

"The most important thing to note with the weather system was just how warm and humid it was compared to an average air mass.

"A warm and humid air mass has the potential to bring even more heavy and persistent rain.

Flooded Otaihanga Domain on Monday. Photo / David Haxton

"And another thing that really aided the development and accumulations was a southerly wind on the surface and above that northwesterly wind bringing all the moisture.

"It was the southerly at the surface. which helped to create those heavier falls, bringing in another set of moisture very close to the ground.

A flooded Wharemauku Stream. Photo / David Haxton

"When it rains, if the raindrops are falling into dry air, they do start to evaporate and get smaller, but what we had on Monday was basically almost saturation through the entire depth of the atmosphere, so as the raindrops were falling they were getting bigger and bigger."

Rain continued today but was expected to peter out later on.