Niwa climate scientist Gregor Macara said an outbreak of exceptionally cold temperatures occurred early last month as a lobe of the tropospheric polar vortex passed over New Zealand, bringing a series of strong cold fronts and snow to very low elevations in Otago and Southland.
"The heaviest snowfalls were reported for inland and northern parts of Southland, where approximately 25-30cm of snow was recorded down to elevations of approximately 300m above sea level near Five Rivers and Garston.
"Snowfall was recorded at sea level along the southern coastline and in Rakiura Stewart Island.
"Dunedin motorists were asked to stay at home due to treacherous roads, particularly in the hill suburbs, with snowfall blanketing the city down to the beaches at sea level.
"This unseasonable outbreak and the resulting string of cold, frosty nights was so pronounced that by the middle of the month, dozens of locations were tracking towards record or near-record low October mean temperatures."
Manapouri and Invercargill recorded their lowest mean minimum air temperatures for October with 2degC and 4degC, respectively; while Middlemarch, Dunedin and Clyde posted their second lowest on record, he said.
Around this time, Puysegur Point equalled its strongest wind gust on record when wind speeds hit 169km on October 10.
"In contrast, relatively high temperatures were common during the second-half of October, with record and near-record high daily maximum and minimum temperatures occurring on October 23 and 29."
Overall, the nationwide average temperature for October this year was 12.2degC — 0.2degC above the 1981-2010 October average.
Mr Macara said rainfall patterns in the south were mixed.
For much of the South Island, rainfall was below normal, although above normal rainfall was observed about some inland parts of Otago and Southland.
At the end of the month, and for the time of year, soil moisture levels were lower than normal in northwestern Otago, and near-normal elsewhere across Otago and Southland.
