The sudden stratospheric warming event is expected to cause a chilly start to October for New Zealand, with MetService warning this week will run "extremely cold".
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"The polar jet stream is expected to routinely influence New Zealand's weather patterns to start October," NIWA says.
"What does that mean? In one word: COLD. This is associated with the sudden stratospheric warming event that peaked in mid-September."
MetService's October outlook shows the Southern Ocean setting down - with a shift to warmer temperatures coming up.
"The Southern Ocean is forecast to quieten down during October, taking a break from driving stormy weather over New Zealand," MetService says.
"Instead, a Tasman Sea rainmaker is expected to develop next week and should bring a spell of wetter weather to northern and western North Island areas."
The second half of the month looks quieter, with a ridge of high pressure showing up over Northland and a gradual return to near-average October temperatures through the rest of the month.
"Rainfall wise, monthly rainfall totals are predicted to run on the drier side of the ledger (normal to below normal) north of about Taupo, also Gisborne, Hawkes Bay, Nelson, Marlborough," MetService says.
"For all other regions, near-normal October totals are forecast."