"So it comes back down the other side when it reaches sea level again, and ends up quite a lot hotter than when it was on the west, because of the energy it gained."
While this effect was mostly associated with Christchurch, because of the normal west-to-east flow over the South Island, on Sunday, the configuration would be different.
"It's going to go north to south, over the Southern Alps, while getting heated up and then warming up Invercargill."
The 30C forecast - there have been just five other times since 1993 when the mercury has hit that high - was also partly due to otherwise sunny conditions.
That warmth would largely be contained to the south, Adams said, although temperatures in some parts of the Wellington region could also hit the high 20s on Sunday. Meanwhile, thunderstorms were moving up the country this afternoon.
MetService this afternoon issued thunderstorm warnings for Palmerston North, as well as the wider Rangitikei and Manawatu regions.
Severe thunderstorms detected near Marton, Halcolm and Porewa were moving toward the south-east and were expected to lie near Feilding, Halcombe and Cheltenham, and also near Palmerston North, the Manawatu Gorge, Ashhurst and Pohangina.
The thunderstorms were expected to be accompanied by torrential rain and large hail, which could cause surface flooding, make driving conditions hazardous and damage crops, orchards, vines, glasshouses and vehicles.
