Weekend weather dubbed 'Tasman Tempest'

Dalberth Road, Ngongotaha, 60mm of rain fell in 45 minutes. Supplied photos.

A deluge of heavy rain in the weekend caused flooding and evacuation in the Bay of Plenty and Hauraki areas.

At the peak of the deluge across Auckland, Waikato and into Rotorua the Fire Service was receiving a 111 call every 24 seconds.

Roads in Rotorua turned into streams and the Utuhina River burst its banks.

The army was called in to help hundreds of people to safety when floods hit the Sundaise Festival near Waihi on Friday night.

NIWA says Auckland rainfall typically ranges from 75mm to 110mm in March but some places, such as Albany and Mangere, recorded more than 100mm within 24 hours.

The downpour included 27.6mm of rainfall between 5pm and 6pm on Friday to equal Auckland's wettest March hour since records began in 1965.

So why all the rain in such a short period of time?

NIWA meteorologist Ben Noll, who dubbed the weather tha Tasman Tempest, says an autumn trifecta for meteorologically induced misery: strong and slowing moving low pressure, deep tropical moisture, and a blocking ridge of high pressure to the south.

Strong and slow moving low pressure: The driving force behind all of the adverse weather.

Deep tropical moisture: An atmospheric moisture source from the Indian Ocean, Coral Sea, and Pacific Islands—this is known as an atmospheric river.

Tasman tempest meteorology [Image:NIWA. Background satellite source: Japanese Meteorological Agency]

Blocking ridge of high pressure: High pressure building to the south and east of New Zealand encourages slow movement of the Tasman Tempest, effectively preventing it from moving over the next several days.

“These three factors are causing a weather traffic jam,” he says.

“Think Auckland at 7.30am on a Monday in March.”

After the Tempest

The weather is expected to slowly settle across most of New Zealand by the middle stages of next week. By late week and into next weekend, high pressure may be the dominant weather player across the country, bringing plenty of dry weather and gradually warmer temperatures.

Warm and dry “beach-type” weather may actually be the flavour into late March as well: which will be much deserved after the extreme weather that Tasman Tempest brings.


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