"That's when you basically have two opposing wind directions meet, and it pushes the clouds right up," he said.
"So we see that happening in areas like Auckland, where you've got two coast lines right next to each other, and you see it inland as well, where you have got daytime heating, which causes the air to rush up and that causes these big, big clouds."
Meanwhile, forecasters offered the first prediction of what the weather could look like for Christmas.
It's shaping up to be a fine and dry holiday period for much of the North Island, but there are warnings that the weather could be changeable on Christmas Day.
MetService forecaster Heath Gullery said Christmas Day in the upper North Island should be mostly fine with some southwest winds.
"It looks like the wind direction for the weekend and the first few days before Christmas is from the west or the southwest.
"Sometimes in Auckland that can keep the temperatures down to the low 20s, it just depends how strong that wind is."
Next weekend should also be mostly fine, he said.
Mr Duncan said people in Auckland and the far north could look forward to dry weather come Christmas Day.
"Northern New Zealand - from Auckland, Waikato and Bay of Plenty - at this stage they're looking dry, but it might be cloudy in the west, and it looks like it could be quite windy through central New Zealand and maybe eastern areas."
However, the bottom half of the North Island could be in for some surprises.
"The weather pattern is still unsettled," Mr Duncan said.
"It doesn't mean it's going to be raining on Christmas Day, but it means that the weather is changeable, it means that you've got clouds, you've got rain, you've got sun all in the mix and some windy weather too.
"It's down to timing as to whether the sun will arrive on December 25."
This week's forecast
Today: Mostly fine, although a few heavy showers this afternoon and evening with light winds, 25.
Tomorrow: Southwesterlies, a few showers, 24.
Wednesday: Fine spells and a few showers, 21.
