Severe Weather Outlook
Issued: 10:57am Friday 18 Apr 2025
Valid Sunday 20-Apr-2025
Cyclone Tam remains slow-moving over the Tasman Sea, but should weaken. Warm moist northerly winds continue to affect the North Island, while south or southeast winds develop over the southern South Island.
There is moderate confidence there will be warning amounts of heavy rain about Taranaki Maunga.
There is low confidence that northerly rain about eastern Bay of Plenty, northern Gisborne/Tairawhiti, Tasman/Nelson, and northern/western Marlborough will require a heavy rain warning.
Valid Monday 21-Apr-2025
Cyclone Tam is likely to lie just west of New Zealand on Monday, but should continue to slowly weaken.
There is low confidence that heavy rain warnings will be required for Nelson, northern/western Marlborough, and possibly the Buller district, however for the Tasman District confidence is moderate that a heavy rain warning will be required.
Valid Tuesday 22-Apr-2025
A weak Cyclone Tam shifts northwards west of the North Island, while a southerly flow spreads onto the far south of country.
Although rain or showers are expected to affect areas of New Zealand, there is minimal risk of severe weather.
Valid Wednesday 23-Apr-2025
The remains of Cyclone Tam move over, and to the north of the North Island, and southerly winds spread north over New Zealand.
Although these weather features are expected to bring rain or showers to areas of New Zealand, there is minimal risk of severe weather.
Low confidence:
A 20% likelihood (or 1 chance in 5) of a warning being issued.
Moderate confidence:
A 40% likelihood (or 2 chances in 5) of a warning being issued.
High confidence:
a 60% likelihood (or 3 chances in 5) of a warning being issued.
New issues of this forecast are made available on this site at or before 16:30 NZST
Thunderstorm Outlook
Valid to Midnight Friday, 18-Apr-2025
Issued: 8:01am Friday 18 Apr 2025
Cyclone Tam is expected to remain slow-moving over the Tasman Sea today. It directs a moist northeast flow over the country with several unstable fronts and troughs moving onto the country. There is a general low risk of thunderstorms for the northern part of the North Island, from Taranaki to northern Gisborne/Tairawhiti and about the Tasman District and northern Buller during the day. At night this low risk area spreads southeast to cover Taumarunui, Taihape, the Hawke's Bay ranges and southern Gisborne/Tairawhiti. This afternoon and evening, the risk of thunderstorms increases to moderate for northern Northland and Bay of Plenty from Rotorua eastwards. These thunderstorms may be accompanied by localised heavy rain with intensities of 10 to 25 mm per hour, and for Bay of Plenty there is a low risk of downpours with intensities of 25 to 40 mm per hour. For southern Northland, the risk of thunderstorms is moderate this evening and tonight, possibly bringing localised heavy rain with intensities of 25 to 40 mm per hour. Tonight there is also a low risk of downpours of 25 to 40 mm per hour. Tonight, the risk of thunderstorms about western Tasman District also increases to moderate and there is a chance of localised heavy rain with intensities of 10 to 25 mm per hour. No other areas of severe convection or thunderstorms are expected.
Valid to Noon Saturday, 19-Apr-2025
Issued: 10:27am Friday 18 Apr 2025
Cyclone Tam, over the Tasman Sea, moves very slowly towards the northern South Island during the day. Fronts and troughs associated with the cyclone continue to cross the country. There is a generally low risk of thunderstorms for the north and west of the North Island and for the northwest of the South Island. For southern Northland, Auckland, Waikato and Coromandel Peninsula, the risk of thunderstorms is moderate during the morning. These thunderstorms may bring localised heavy rain with intensities of 10 to 25 mm per hour and a low risk of downpours of 25 to 40 mm per hour. There is also a moderate risk of thunderstorms during the morning for Taranaki, Tasman District and Nelson, and for the pre-dawn period for eastern Bay of Plenty. These thunderstorms may bring localised heavy rain with intensities of 10 to 25 mm per hour. No other areas of severe convection or thunderstorms are expected.
Valid to Midnight Saturday, 19-Apr-2025
Issued: 10:32am Friday 18 Apr 2025
Cyclone Tam continues to approach the northern South Island. Some, generally weaker, troughs cross the country. There is a low risk of thunderstorms about the west and north of the North Island and the northwest of the South Island. During the afternoon, there is also a low risk of thunderstorms about the inland areas of Whanganui, Manawatu, Hawke's Bay, Wairarapa and Kapiti. During the afternoon, there is a moderate risk of thunderstorms about southern Coromandel Peninsula, far western Bay of Plenty, coastal Waitomo, Taranaki, Tasman District, Nelson and parts of western Marlborough. These thunderstorms may bring localised heavy rain with intensities of 10 to 25 mm per hour. No other areas of severe convection or thunderstorms are expected.
Severe Thunderstorm Criteria
In New Zealand, MetService classifies a thunderstorm as severe if one or more of the following criteria are met:
Heavy rain (from thunderstorms):
Rainfall of 25 millimetres per hour, or more.
Large hail:
Hailstones 20 millimetres in diameter, or larger.
Strong wind gusts (from thunderstorms):
Gusts of 110 kilometres per hour / 60 knots or stronger.
Damaging tornadoes:
Fujita F1 (wind speeds greater than 116 kilometres per hour / 63 knots) or stronger.