Known by locals as the Nor'wester, the föhn effect is a warm, dry, down-slope wind that occurs in the lee side of a mountain range.
In the case of the South Island, it's a nor'wester wind blowing down off the Southern Alps from the west towards those in the east.
The size of the Southern Alps means the effect can be very pronounced, with temperatures skyrocketing in a short period of time.
Mosgiel, the "pearl of the Taieri Plain"' south of Dunedin, noticed the sudden increase yesterday - from 4C to 16C in just one hour.
Above the Taieri Plain, temperatures were around 12C with strong northerly winds. They were able to mix the warmer air to the surface, which saw the temperature rise.
The speed of the wind also shot up, increasing from a light 5km/h to a gusty 30km/h.
At 7am, Cooptown in Banks Peninsula recorded a temperature of 18.6C.
Warmer than normal temperatures continued overnight in Otago and Canterbury, with many places only dropping to around the 10C mark.
The overnight temperatures last night were stark in comparison to the previous night when the mercury reached the freezing point.
MetService meteorologist Tahlia Crabtree says those in the south should expect "a very sharp change" tomorrow.
Another front will move in and temperatures should be warm again but they would not be as prominent as yesterday or today, she said.
The cold change would hit Southland about midday before creeping up the South Island and reaching Kaikōura in the east and Hokitika in the west around midnight.
"This cold change is looking like it'll bring snow down to 500 metres," Crabtree said. MetService would issue any road snow warnings on their website.
Cooptown in Banks Peninsula recorded 18.6°C at 7 am this morning! 🥵 An unseasonably warm start to a winter morning,...
Posted by MetService New Zealand on Monday, June 15, 2020
Rain is spreading through lower parts of the North Island today and there was a chance of scattered showers again tomorrow.
On Thursday, the strong southerly change will arrive and there will be a "big shift" in the weather conditions, Crabtree said.
However, the news is not all bad with temperatures tomorrow around the North Island expected to be reasonably warm.
The high for Napier tomorrow is meant to be 20C, with overnight temperatures ahead of the front warmer than you would expect for this time of year.
On this day in 1983, waterspouts were seen off the coast of Whanganui at 1.50pm as a cold front approached the area.
