Gisborne and Westland are bracing for heavy rain over the next couple of days, while other parts of the North Island could see severe thunderstorms.
MetService is forecasting up to 170mm of rainfall in parts of Gisborne, which is under an orange heavy rain warning, and severe thunderstorms from Taranaki to Taupō.
Moving to the South Island, the already-soaked West Coast could receive thunderstorms and localised downpours. It comes after the Westland District was plunged into a local state of emergency on Friday after heavy rain caused flooding and road closures.
Meanwhile, MetService meteorologists are monitoring a tropical low in the Coral Sea, which is expected to develop into a cyclone over the next few days.
North Island
On Sunday night and Monday, a humid east-to-northeast flow is expected to affect Gisborne, bringing a period of heavy rain to the region, MetService said.
An orange heavy rain warning is in place for Gisborne about and north of Tolaga Bay until 1pm Monday. MetService expects 120 to 170mm of rainfall with peak intensities of 20 to 30 mm/h on Monday morning.
A heavy rain watch has also been issued for Gisborne south of Tolaga Bay on Monday, when heavy rainfall may approach warning criteria.
There is also the potential of localised downpours for Taranaki south of New Plymouth, southern Taumarunui, southern Taupō, Taihape and northern Whanganui. A severe thunderstorm alert has been issued for these areas.
"Between 1pm and 9pm tonight, localised downpours of 25-40 mm/h may affect parts of these areas. These downpours could occur with or without thunderstorms," MetService said.
"Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
"Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."
South Island
An active trough is forecast to move across the Westland and Grey Districts, bringing a period of heavy rain with localised thunderstorms and downpours possible.
A watch has been issued for the Westland and Grey Districts from 11pm Sunday until noon Monday,
"Periods of heavy rain and amounts may approach warning criteria. Thunderstorms and localised downpours of 25 to 35 mm/h possible. Note, further heavy rain is likely Monday night and early Tuesday morning," MetService said.
It comes after heavy rain on Friday and Saturday triggered a state of emergency in the Westland District.
A red heavy rain warning was issued for the district, as downpours caused flooding and road closures.
Tropical cyclone
Metservice said tropical low 05U lies in the Coral Sea and is expected to deepen into a tropical cyclone over the next few days.
"Although the environment is currently marginal for development, conditions should improve this weekend and there is a low risk the system will become a tropical cyclone today, increasing to moderate on Sunday," MetService said.
"There is a high risk that tropical low 05U will be a tropical cyclone from Monday onwards."
Most outlooks suggest the system will deepen further and drift west or southwest across the Coral Sea during Tuesday and Wednesday.
MetService meteorologist Clare O'Connor told Newshub most models are predicting the potential tropical cyclone could reach Queensland's coast later this week. She said there is some indication it could come back towards New Zealand but since that is quite far ahead in terms of outlook, it is difficult to say for now.
While O'Connor said there is still a lot of variation in the forecast tracks, if the low pressure moves towards New Zealand, she expects it will bring the usual characteristics associated with intense low pressures.
"With any of these more intense low-pressure areas, strong winds, rainfall and thunderstorms are usually what we'd expect with tropical cyclones and even after they've done their tropical cyclone dash," O'Connor said.
Warnings and watches
Severe Thunderstorm Watch
Area: Taumarunui, Taupo, Taranaki, Taihape, Wanganui
Period: 8hrs from 1pm - 9pm Sun, 21 Jan
Forecast: This is an alert for the potential of localised downpours for Taranaki south of New Plymouth, southern Taumarunui, southern Taupo, Taihape and northern Whanganui.
Between 1pm and 9pm tonight, localised downpours of 25-40 mm/h may affect parts of these areas. These downpours could occur with or without thunderstorms.
Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.
Heavy Rain Warning - Orange
Impact: Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.
Area: Gisborne/Tairawhiti about and north of Tolaga Bay
Period: 16hrs from 9pm Sun, 21 Jan - 1pm Mon, 22 Jan
Forecast: Expect 120 to 170 mm of rain. Peak intensities of 20 to 30 mm/h Monday morning.
Heavy Rain Watch
Area: Gisborne/Tairawhiti south of Tolaga Bay
Period: 13hrs from 3am - 4pm Mon, 22 Jan
Forecast: A period of heavy rain, and amounts may approach warning criteria.
Area: The Westland and Grey Districts
Period: 13hrs from 11pm Sun, 21 Jan - noon Mon, 22 Jan
Forecast: Periods of heavy rain and amounts may approach warning criteria. Thunderstorms and localised downpours of 25 to 35 mm/h possible. Note, further heavy rain is likely Monday night and early Tuesday morning.
Short Forecast
Northland, Auckland, Waikato, Coromandel, Bay of Plenty and Gisborne
Isolated showers, turning to occasional rain in Coromandel, Bay of Plenty and Gisborne from evening, More persistent in Gisborne, with heavy falls.
Waitomo, Taupo and Hawke's Bay
Cloudy periods, isolated showers, some possibly heavy this afternoon and evening.
Taranaki to Wellington including Taumarunui and Taihape, also Wairarapa
Fine spells. Scattered showers developing, with possible thunderstorms and localised downpours inland this afternoon and evening.
Marlborough, Nelson, Buller and Westland
Mainly fine. A few showers this afternoon and evening, mostly inland with possible thunderstorms from inland Buller to Nelson and Marlborough.
Canterbury, and Otago except Clutha
Fine, apart from areas of evening cloud.
Clutha and Southland
Cloudy periods. Isolated showers, clearing evening.
Fiordland
Showers, becoming isolated from afternoon.
Chatham Islands
Low cloud, possible drizzle.
