Winter storm delivers wettest day on record

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The latest atmospheric river over New Zealand delivered one town’s wettest day on record, and broke several other long-held rainfall statistics, according to NIWA data.

Observed storm rainfall in the South Island, Friday 21 - Sunday 23 July 2017. [Image: NIWA]

Lower pressure than normal has fuelled several high impact weather events during July.

Standout statistics include:

  • Oamaru’s wettest day on record since daily rainfall records started in 1950. That happened on Friday when 161.2mm was recorded in 24 hours. The previous record was 128mm in April, 2006. In a normal July Oamaru’s total rainfall is 40.6mm.
  • Following last week’s rain, winter 2017 became Oamaru's wettest winter on record and July 2017 the wettest July for the town on record.
  • Dunedin had its wettest July day on record since 1918 on Friday when 89mm fell – beating the 2007 record of 75.8mm.
  • At Winchmore, South Canterbury, between 9am on Friday and 9am on Saturday, 111.8mm of rain was recorded – making it the 2nd wettest day on record.
  • Christchurch recorded 88.6 mm at the airport. Christchurch’s normal July total rainfall is 64.7 mm.

Atmospheric river

Atmospheric rivers are long, narrow regions in the atmosphere—like rivers in the sky—that transport water vapour from the tropics.

Emerging research indicates that for New Zealand, atmospheric rivers occur about 40 days per year and are associated with up to 48% of rainfall extremes for parts of the country. Atmospheric rivers were also prevalent during events like the Tasman Tempest and the remnants of ex-Tropical Cyclone Debbie earlier this year.

Atmospheric moisture 21 July 2017 [Image: NIWA]

High tides

The extreme rainfall over the weekend coincided with some very high storm tides for Christchurch. The  peak storm tide about about 3:15 pm on Saturday was equal highest recorded in last 13 years at NIWA’s Sumner gauge, which exacerbated flooding in low lying areas.

NIWA scientists were able to forecast a drop in the next tide at 3am on Sunday but further high tides combined with rainfall caused more flooding. Fortunately for Timaru and Christchurch, the storm arrived just ahead of peak king tides yesterday and today.

What’s next

The Tasman Sea is expected to remain an active place this week, sending several fronts up the west coast of the South Island and western and upper North Island. From Thursday into Friday, a slow moving area of low pressure may bring more drenching rain to areas that have been hit hard this month, including middle and upper Canterbury and Wellington-Wairarapa. However, rainfall isn’t expected to be as heavy as the last storm.

Heading into the weekend and early next week, high pressure may arch over the country, delivering a period of dry weather for many places aside from the eastern North Island where an onshore breeze may deliver rounds of rain.

Contact

Ben Noll, NIWA meteorologist
Tel 09 375 6334
 
 
 
 

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