A low-pressure system northeast of New Zealand is forecast to bring very strong southeasterly winds and heavy swells for the east coast of the North Island.
MetService says it's rapidly deepening - and even meeting the 'weather bomb' criteria.
And now new graphics show the size of the system as it rages towards New Zealand.
What is a 'weather bomb'?
A weather bomb - technically known as bombogenesis - is when a depression drops 1 hectopascal (hPa) an hour for 24 hours at a latitude of 60 degrees. This is also known as 'explosive cyclogenesis'.
"Cyclogenesis is the technical term for the process of a low-pressure system developing and deepening. A low classified in this way is indicative of a very active and volatile weather system," MetService says.
What will it do to New Zealand?
MetService says a weather bomb usually leads to significant weather impacts including strong winds, heavy rain, large swell and storm surges. While the low will stay to the northeast of the country, it will still impact on the North Island.
NIWA says while the strongest winds will remain offshore, eastern coastal areas from Gisborne through to Northland will have gusts of near or more than 70 km/h, mainly on Sunday.
And these winds could generate impressive waves, with a forecast showing over 8 metres above New Zealand and around 7 metres just off the east coast.
What warnings are in place?
MetService has issued severe weather warnings for the eastern parts of the North Island.
"A period of gale force southeast winds are expected to affect exposed parts of the North Island, especially as the ridge of high pressure builds over the South Island. Heavy rain is also forecast to affect eastern parts of the North Island for a time," it says.
"There is high confidence rainfall amounts will reach warning criteria about Gisborne on Monday, and a Severe Weather Warning has been issued for heavy rain that isn't expected to ease until Monday.
"In addition, there is moderate confidence of severe southeast gales about eastern Bay of Plenty and exposed parts of Taranaki on Monday and Tuesday. A Strong Wind Watch has been issued for eastern Bay of Plenty, and other areas may be added to the Severe Weather Warning or Watch in the coming days."
