The Dance of Two Systems

May 14, 2017

A low sat in the western wings this week sending periods of rain to the South Island’s west coast throughout the week. However, it wasn’t until Thursday that the system sent its final performer over the entire New Zealand ‘stage’ in the form of a trough which eventually merged with the former Tropical Cyclone (TC) Donna. This duet between the trough and remnants of TC Donna meant large rainfall accumulations and gale force gusts over parts of the North Island.

Much of the North Island and northwest corner of the South Island were put under severe rainfall watch or warning from Thursday through to Friday as the trough, fuelled with moist air from the remnants of Donna, made conditions ripe for persistent rain and brief downpours. “Nelson was soaked early Thursday, with a torrential downpour of 49mm in one hour recorded in Collingwood” reported April Clark, meteorologist at MetService.

“Prolonged rain affected the North Island later Thursday and into Friday, with the heaviest falls observed about the higher ground north of the central plateau. A rain accumulation of 269mm was recorded over a 36 hour period from Thursday midday in the Gisborne ranges” she continued.  As the remnant low of Donna passed to the east of the North Island on Friday winds turned strong southerly with severe gale gusts recorded in exposed places on the eastern coasts of the North Island.

Though it wasn’t a dry end to the working week, the South Island generally fared better during the performance of the trough and Donna, though southerly winds and clear skies meant frosty temperatures for those east of the Alps this Mother’s Day morning.

Looking ahead, another low is expected to move eastwards over the country on Thursday bringing periods of rain ahead of it, and cold showery southerlies behind. The southerlies bring the potential of snow falling to the lowest level since last season.

The image above shows gale force winds over the western flank of the remnant low of former TC Donna Friday morning taken by NOAA’s advanced scatterometer.

Official Severe Weather Watches and Warnings are reviewed and re-issued by MetService at least every twelve hours, and more often if necessary. To get the most up to date information on severe weather around the country, or any other forecasts, see metservice.com or on mobile devices at m.metservice.com. You can also follow our updates on MetService TV, at MetService New Zealand on Facebook@metservice  and @MetServiceWARN on Twitter and at blog.metservice.com

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