"As the sun rises low in the horizon to the east it briefly lights up the skies over us. The saying 'red sky in the morning' is a warning because it indicates something is brewing to the west and is moving in," Duncan said.
And the red or orange sky is because the early sunlight has to go through a thick layer of atmosphere, which gives it a "warm, spectacular look" especially when gloomy grey skies are above.
A big, bright, red sunset lights up the clouds above when the weather to the west is clear.
Duncan said a very large low is in the Tasman Sea and a southerly is moving up the country as a northerly spreads south, both bringing rain.
Clark said there is a very warm front with a lot of rain heading to parts of the North Island.
Rain is due to hit Auckland around midday and will be heavy in the evening, she said.
The heavy rain is expected to last for up to a week. At the same time the South Island faces an autumnal cold snap with temperatures in some regions barely registering above single digits.
As well as the intense rain, a severe thunderstorm watch has been issued for Auckland,
Coromandel Peninsula, Waikato and Bay of Plenty overnight to tomorrow morning.
Gisborne has occasional rain this evening, and afternoon rain is forecast in Palmerston North and Wellington. Rain was also expected over the Bay of Plenty, Gisborne, the Hawke's Bay ranges and the central North Island high country, but not to warning levels.
The MetService said rainfall accumulations could reach warning levels in Northland, Auckland, Great Barrier Island, Waikato, Coromandel Peninsula and Western Bay of Plenty from today through to Wednesday and Thursday.
Today's forecast
•Whangarei: Showers developing, chance heavy evening rain. 25C
•Auckland: Rain developing in afternoon, chance heavy. 24C
•Tauranga: Evening rain, chance heavy. 24C
•Wellington: Afternoon southerly change brings rain. 21C
•Christchurch: Showers, then fine. 17C
