Heavy rain is expected for the Bay of Plenty at times today, as well as Northland, Auckland, Coromandel, Waikato and northern Gisborne, with a severe weather watch being issued

A complex low pressure system and associated fronts are forecast to slowly spread onto the upper North Island from today through to Christmas Day, then move away to the east on Boxing Day.

This system is expected to bring periods of heavy rain and possible thunderstorms to northern parts of the North Island.

In a statement issued by MetService, it says this watch covers Northland, Auckland including Great Barrier Island, Coromandel Peninsula, Waikato, Bay of Plenty, Rotorua and northern Gisborne.

'This is for the possibility for short periods of intense rain, especially with any thunderstorms, that reach warning criteria.

'Also, total rainfalls during this extended period could exceed 100mm in parts of eastern Northland, northern Auckland, Coromandel Peninsula, Bay of Plenty and northern Gisborne, especially about the hills and ranges.

'There is still some uncertainty regarding the timing and passage of this low and associated fronts which will affect the distribution of the heaviest rainfall.

'Therefore, people in these areas are advised to stay up to date with the latest forecasts in case parts for this watch are upgraded to a full warning or new areas are added.

'Note, the combination of the approaching low, strong onshore winds and King Tides late Monday and during Christmas Day could cause some coastal inundation about eastern coasts, including parts of Auckland such as Tamaki Drive and about the Firth of Thames.”

Heavy Rain Watch:

Area: Bay Of Plenty, including Rotorua, also Gisborne from Tolaga Bay northwards
Valid: 44 hours from 1:00 am Monday to 9:00 pm Tuesday
Forecast: Periods of rain, heavy at times with isolated thunderstorms possible. There could be short periods of intense rain, especially with any thunderstorms, that reach warning criteria (for example 50mm or more falling in a 3 to 6 hour period). Also, total rainfalls during this extended period could exceed 100mm in places, mainly about the ranges.

Area: Waikato
Valid: 41 hours from 1:00 am Monday to 6:00 pm Tuesday
Forecast: Periods of rain, heavy at times with isolated thunderstorms possible. There could be short periods of intense rain, especially with any thunderstorms, that reach warning criteria (for example 50mm or more falling in a 3 to 6 hour period). Also, total rainfalls during this extended period could exceed 100mm near the Kaimai Range.

Area: Northland
Valid: 52 hours from 1:00 pm Sunday to 5:00 pm Tuesday
Forecast: Periods of rain, heavy at times with isolated thunderstorms possible. There could be short periods of intense rain, especially with any thunderstorms, that reach warning criteria (for example 50mm or more falling in a 3 to 6 hour period). Also, total rainfalls during this extended period could exceed 100mm about the eastern hills.

Area: Auckland, Great Barrier Island, Coromandel Peninsula
Valid: 48 hours from 6:00 pm Sunday to 6:00 pm Tuesday
Forecast: Periods of rain, heavy at times with isolated thunderstorms possible. There could be short periods of intense rain, especially with any thunderstorms, that reach warning criteria (for example 50mm or more falling in a 3 to 6 hour period). Also, total rainfalls during this extended period could exceed 100mm in parts of northern Auckland and Coromandel Peninsula.