Combination of factors cause 'persistent' rain for Whanganui

High winds, a low-pressure system and abnormally high ocean temperatures caused the week of persistent rain recorded in Whanganui, according to Niwa's Chris Brandolino. Photo / Bevan Conley

A "traffic jam" between weather systems over the country, plus unstable winds and abnormally high water temperatures in the Tasman Sea, caused a week of persistent rain in Whanganui.

MetService meteorologist Lewis Ferris said the Whanganui region had recorded 81.8mm of rain during June, as of Friday, June 17.

It was likely that Whanganui would record rainfall above the June average of 90mm by the end of the month, Ferris said.

The highest amount of rain over a day so far this June was around 20.4mm, which was less than half of the highest daily June rainfall of around 48mm recorded for Whanganui, he said.

However, the persistence of the rain over the month was more unusual, with 12 days in June being declared as "wet days" thus far, meaning there was more than 1mm of rain, Ferris said.

National Institute of Water and Atmospheric Research (Niwa) principal scientist Chris Brandolino said periods of heavy rain and thunderstorms were expected in winter, as well as damaging wind and even occasional tornadoes.

This weather persisting in the region over multiple days was what made the last week notable, Brandolino said.

"The fact that happened more than once over successive days, I think that's what separates it from other events. It's not so much the intensity but the longevity that makes it more unusual than anything."

He said the extended periods of rain had many causes, the main factor being a quasi-persistent low-pressure weather system over the lower Tasman Sea.

"Think of low-pressure like a cog spinning, and that cog or low-pressure it spins clockwise. So because that low was positioned to the southwest of New Zealand, that clockwise flow encouraged a wind from the northwest," he said.

Northwest winds were historically unstable and looking to rise through the atmosphere, which created an environment prone to heavy rain and thunderstorms, and also pulled the unstable winds back down, Brandolino said.

A high-pressure system to the east of the country also kept the low-pressure system in place over the west coast.

The rain was fueled by ocean temperatures being abnormally high for this time of year, particularly off the west coast of the country.

"It's not warm enough to go for a dip without a wetsuit for most people, but it's unusually warm for mid-June and early winter," Brandolino said.

The unusually warm ocean temperatures created more water vapour which was fuel for rain.

A combination of these factors led to the week of persistent and "sharp" conditions felt across the region, Brandolino said.

Ferris said this June was unlikely to break any records for rainfall or the number of wet days, as weather for the region should be much more settled for the next week.