Weather: North Island east coast in line for 'weather bomb'

A weather bomb off New Zealand is expected to bring "very strong" southeasterly winds and heavy swells - potentially higher than four metres - to parts of the North Island later this weekend.

"A low-pressure system develops northeast of New Zealand in the coming days and rapidly deepens, even meeting the 'weather bomb' criteria", MetService said in a Facebook post this afternoon.

The low centre doesn't make it on land, sparing the country most of the brunt of the weather bomb.

But some parts of the North Island will be affected.

"Very strong southeasterly winds are expected in exposed locations, along with heavy swells, possibly larger than four metres, for the eastern coasts, especially Gisborne."

No watches or warnings have been issued, but MetService has low confidence of severe southeast gales for eastern Bay of Plenty on Sunday, Monday and Tuesday, and low confidence rainfall amounts will reach warning criteria north of Napier on Monday and in Gisborne on Sunday and Monday.

Thames-Coromandel District Council warned residents and visitors to prepare for a potential "hammering" from tomorrow night in its Facebook post yesterday.

"This is potentially a severe weather event lasting almost a week, therefore experience tells us we are quite possibly in for a hammering", Civil Defence Controller Garry Towler said.

"The eastern seaboard of the Coromandel Peninsula could expect extremely heavy rain from Saturday night, due to a large sub-tropical low tracking our way. The low is expected to hang around from Saturday evening for four or five days, bringing with it high winds, very rough seas, and potentially heavy rain."

From Stony Bay, north of Colville, through to Whangamatā and the wider Bay of Plenty in the south could be affected.

Coastal erosion, inundation of low-lying property, roads and reserves, trees down and likely power outages were the main concerns, Towler said.

Those in the area should keep up to date with the council's Facebook, MetService and Waka Kotahi/NZTA websites and take precautions including securing outdoor furniture.

Those travelling in campervans and high-sided vehicles are also urged to check conditions from Sunday morning onwards, he said.

A weather bomb is a low-pressure system which deepens by 24hPa in 24 hours at a latitude of 60 degrees - also known as explosive cyclogenesis, the MetService wrote.

"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.

"At the latitude of this low (S31°), the low only needs to deepen by 15hPa ... this is to do with a natural pressure gradient across Earth, with higher pressures at the equator and lower pressures at the poles."

The low was expected to drop 24hPa between 9pm tonight to 9pm tomorrow, from 1003hPa to 979hPa.

Weather bombs are usually associated with significant weather impacts including strong winds, heavy rain and large swell and storm surges.