Despite summer being less than a month away, Kiwis are being warned about a "roller coaster" of hot and cold temperatures in November.
NIWA said this is because of a November "air mass", which will provide trends of "cold, then hot" temperatures.
"All aboard the temperature roller coaster," NIWA said in a tweet.
NIWA said Southern Ocean air masses will be in full effect through the first 10 days of November.
"An Australian air mass says g'day from around the 12th or 13th, seeing temperatures turn more summer-like for a time, sticking around until the 18th or 19th," NIWA said.
"A brief cooler change is possible around the 20th before more heat at the end of the month. Hope you have both sweaters and singlets ready!"
It comes as MetService warns there is a low risk of thunderstorms for parts of the South Island on Sunday.
These areas include northern and western South Island areas from Tasman/Nelson and inland Marlborough to northern Fiordland and Central Otago.
"However, about the Tasman/Nelson region, inland Buller, the far west of Marlborough, Southern Westland, and near the main divide of the Southern Alps in Otago and the south of Canterbury the risk of thunderstorms is considered moderate in the afternoon and early evening," MetService said.
"Any thunderstorms that occur will bring localised heavy rain of 10-25mm/h and hail of 5-15mm diameter."
The forecasting agency said Sunday afternoon cloud buildup over the central North Island is also expected to produce scattered showers and there is a low risk of some thunderstorms for Gisborne/Tairāwhiti, Hawke's Bay, the Tararua District and the Central North Island high country.
