The Tasman Tempest takes its toll - time to tally up

As the Tasman Tempest headed east off New Zealand today, it’s time to look at the statistics and see how many records it washed away.

NIWA meteorologist Ben Noll says among other milestones this weather pattern produced: 

• More rain than typically falls for the whole of March
• Instances of 1 in 100 year rain
• A tie for Auckland’s wettest March hour
• Widespread flooding.

The Tasman Tempest strengthened off the east coast of Australia a week ago and churned very slowly to the northwest of the North Island over the Tasman Sea for days on end. Warmer than average sea surface temperatures east of Australia contributed to the storm’s strength and duration. In fact, the structure of the Tasman Tempest was similar to a tropical cyclone, Mr Noll said.

Tempest totals

Here are some of the Tasman Tempest's most impressive records:

• Whangamata received 260mm of rain in 24 hours from Tuesday into Wednesday. That was 156% of the normal March rainfall in one day.
• From Tuesday 7 Mar to Sunday 12 Mar, Whangamata recorded 475 mm of rainfall, the highest total at any one location from the Tasman Tempest. This was 112% of the normal rainfall for the entire autumn season.
• According to NIWA’s High Intensity Rainfall System, the 225mm that fell in 12 hours on Tuesday night-Wednesday in Upper Hunua exceeded a 1 in 100 year event.
• Between 3am and 4am on Saturday 11 Mar, Kaitaia had its wettest March hour on record (44.6 mm) since hourly records began in 1962.
• Auckland (Mangere) tied its wettest March hour on record with 27.6 mm between 5pm and 6pm on Friday since hourly records began in 1965.
• Between 9am on Friday 10 Mar and 9am Saturday 11 Mar, Auckland (Mangere) recorded its wettest March day on record (100 mm) since 1959. This was also the 3rd wettest autumn day on record in Auckland (Mangere).
• Hamilton observed its 4th wettest March day on record (since 1907) on the 10th with 77 mm.
• Kaitaia had its 2nd wettest March day on record (since 1948) on the 10th with 104 mm.
• With all the rain, Kaitaia is already on track for a top-4 wettest March on record with 216 mm since the beginning of the month (267% of the March normal).
• Paraparaumu recorded its 2nd wettest March day on record (since 1951) on the 11th with 74 mm.
• In the Auckland suburb of New Lynn, it is estimated that 60mm fell in two hours on Sunday afternoon, causing flash flooding. The return frequency on such a rainfall is approximately 1 in 30 years.
• Kumeu recorded 41.4 mm of rain in one hour on Sunday 12 Mar, which is a 1 in 20 year event according to NIWA’s High Intensity Rainfall System.
• Whangaparaoa is on track for its wettest or 2nd wettest March on record since 1948. 226 mm have fallen since the 1st of the month, which is 317% of the March normal and 84% of the entire autumn normal.

But is it climate change?

While no one weather event is caused by climate change, all events are influenced by climate change since the atmosphere is now warmer and moister than it was in the past. Climate change increases the likelihood of extreme rainfall, given the appropriate weather setup. Research suggests that there will be up to 8% more intense rain for every 1°C of warming.

Goodbye Tempest, hello sunshine

High pressure will build over New Zealand this week, causing much calmer conditions across the upper North Island and country as a whole. The humidity that the Tempest brought will be replaced with typical crisp autumn mornings, though afternoons will be filled with sunshine and comfortable warmth for many.

For those looking to have some late-season beach days, next weekend is looking pretty fabulous across the upper North Island, Mr Noll says.

And here’s the silver lining: the Tasman Tempest caused some warming of the sea temperatures around the northern portion of the country as warm winds blew from the north for many consecutive days.

Through to the end of the month, high pressure will be favoured to the west and over the country, leading to long stretches of dry weather and sunshine with temperatures near or above average.