Tomorrow's spring equinox will signal that days and nights are now of equal length.

The spring equinox is this Wednesday at 8:21pm NZT (or 08:21 UTC Sept 23).

The spring equinox is this Wednesday at precisely 8:21pm NZT. Photo: Weatherwatch.co.nz

'This will continue on until the summer solstice on December 22,” says the weather organisation.

The equinox is also when the windy westerlies usually kick off - with El Nino fuelling them even more.

'But not this year,” they add. 'Because NZ can have chaotic weather patterns we are currently experiencing an easterly flow - the complete opposite.

'This general easterly flow will remain with us for the rest of September.”

However, in October Weatherwatch.co.nz is predicting westerlies to arrive in New Zealand.

'For those in England for the Rugby World Cup, the Autumn Equinox for them signals the slide towards wetter, wintrier weather in October.”