Severe Weather Outlook

Valid from Monday 18-Nov-2019 to Thursday 21-Nov-2019

Issued: 2:51pm Saturday 16 Nov 2019

A series of fronts are forecast to move across New Zealand from the west through overnight Wednesday, bringing periods of heavy rain and northwest gales to parts of southern and central New Zealand.

There is high confidence of rainfall accumulations reaching warning criteria in Fiordland and Westland on Tuesday, and moderate confidence for the headwaters of the Otago and Canterbury Lakes and Rivers (within 15km east of the main divide). There is low confidence of warning amounts of rain for the ranges of Buller late Tuesday and early Wednesday, and also low confidence for the Tararua Range on Wednesday.

In addition, there is low confidence that northwest gales could become severe in exposed parts of Canterbury High Country on Tuesday, and in exposed parts of Marlborough, Wellington and Wairarapa late Tuesday and early Wednesday.

A ridge of high pressure should then move over the country from the Tasman Sea on Thursday.

Low confidence:

a 20% likelihood (or 1 chance in 5) that the event will actually happen.

Moderate confidence:

a 40% likelihood (or 2 chances in 5) that the event will actually happen.

High confidence:

a 60% likelihood (or 3 chances in 5) that the event will actually happen.

New issues of this forecast are made available on this site at or before 16:30 NZST

Thunderstorm Outlook

Valid to Midnight Sunday, 17-Nov-2019

Issued: 9:00am Sunday 17 Nov 2019

A front remains slow-moving over the lower North Island during Sunday afternoon, then starts to move northwards in the evening. Meanwhile, an unstable northwesterly flow persists over southern New Zealand, with an embedded trough moving over Fiordland and southern Westland. This trough also brings a southerly wind change to the southeast of the South Island during the afternoon and evening. There is mostly a low risk of thunderstorms from central Westland to Fiordland, and over Southland and eastern Otago. However about northern Fiordland and southern Westland this morning and early afternoon there is moderate risk of isolated thunderstorms, while over Clutha and Dunedin there is a moderate risk of thunderstorms this afternoon and evening. These thunderstorms if they occur could bring localised heavy rain with intensities of 10 to 20mm per hour and small hail. Further north, apart from a moderate risk of thunderstorms over the waters southwest of Taranaki there is initially a low risk of thunderstorms about the Marlborough Sounds, Wellington, Kapiti Coast, Horowhenua, Tararua Range, Taranaki, Taumarunui and Waitomo. However from late afternoon the risk of thunderstorms increases to moderate about Waitomo, Taumarunui, Taranaki and over the waters south of Taranaki and west of Horowhenua and Kapiti Coast. These thunderstorms will be embedded in bands of rain and boost heavy rainfalls. No thunderstorms or significant convection expected elsewhere in New Zealand during this period.



Valid to Noon Monday, 18-Nov-2019

Issued: 10:54am Sunday 17 Nov 2019

A front moves slowly north onto northern New Zealand tomorrow morning, bringing a low risk of thunderstorms to Northland, Auckland, Coromandel Peninsula, Waikato, Waitomo, Taupo and Bay of Plenty. Over the South Island an unstable west to southwest flow brings a moderate risk of thunderstorms to Westland, Fiordland, Stewart Island and coastal Southland. These thunderstorms could be accompanied by heavy rain of 10 to 20mm per hour and small hail. A lower risk of thunderstorms covers Nelson, Buller, inland Southland, eastern Otago and coastal Canterbury as far north as Banks Peninsula. No thunderstorms or significant convection expected in other areas.



Valid to Midnight Monday, 18-Nov-2019

Issued: 11:00am Sunday 17 Nov 2019

The front continues to move slowly north over northern New Zealand tomorrow afternoon and evening, bringing an associated low risk of thunderstorms to Northland, Auckland, Coromandel Peninsula, Waikato, Waitomo, Taupo and Bay of Plenty. For the South Island, an unstable west to southwest flow brings a low risk of thunderstorms to western Nelson, Buller, Westland, Fiordland, Southland and eastern parts of Otago and Canterbury as far north as Christchurch. No thunderstorms or significant convection expected in other areas.



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.