Cold southwesterly winds hit Whanganui in September

There were several bursts of snow in the Waimarino last month. Photo / Bevan Conley

September was a month of two halves, with wild and stormy weather at both ends and a long spell of calm and fine, but cold, days in the middle.

On September 1 the air 30km above Antarctica heated up more than usual for the time of year, Niwa meteorologist Ben Noll said. That broke down the strong westerly winds that usually circulate around Antarctica and allowed colder surface air to escape.

The phenomenon is know as Sudden Stratospheric Warming (SSW) and this year's is the strongest on record for the southern hemisphere. The only other incidence of it was in 2002, also in September.

It's more common in the northern hemisphere, and caused the "Beast from the East" cold snap in Europe in early 2018. Our SSW made for cooler weather in Argentina, Uruguay, South Australia and New Zealand where it peaked in mid-September.

But overall, in a warming world, temperatures were around average for the month. For Whanganui they were slightly lower than average at 11.6degC, just below the usual 11.7degC.

Whanganui's hottest day was 22 degrees C on September 4, and the average high for the month was 15degC. Our lowest temperature was 3.6degC on September 21.

Between September 9 and 22 there were three nights when the temperature dipped to 4degC, low enough for frost to form under clear skies in windless conditions. The fine spell followed a day - September 8 - when Whanganui's temperature didn't top a chilly 9degC.

That patch of weather corresponds with the SSW, but MetService local weather consultant Georgina Griffiths doesn't believe it moved low enough in the atmosphere to make a difference to us. Instead, she said, the main reason for this year's cold and stormy spring is the stormy southern ocean.

It has cooled considerably since the marine heatwave a year ago and has a major effect on our weather. New Zealand's last cold and stormy spring was in 2016-17, also affected by the southern ocean, she said.

Our September had a lot of strong, cold southwest winds, with bursts of snow inland. The Desert Road was closed briefly on September 8 and overnight on September 25.

In Whanganui the strongest wind gust was 93km per hour on September 25, and that was a westerly wind.

Southwest winds typically lose most of their moisture over the South Island. Whanganui got a few brief heavy falls and rainfall for the month was just 45mm - two-thirds of normal.

Recent heavy rain has redressed the balance, with soil moisture now around normal.