Severe Weather Outlook

Valid from Monday 25-Sep-2017 to Wednesday 27-Sep-2017

Issued: 03:17pm Saturday, 23-Sep-2017

An active front, preceded by strengthening northwesterlies, moves north across the South Island during Monday and the North Island on Tuesday. Another front should move over the South Island from the Tasman Sea on Tuesday. The front should then move over the North Island during Wednesday and weaken.

There is high confidence of rain reaching warning amounts for Fiordland and Westland south of Hokitika on Monday and a warning is in force for these areas. The confidence for heavy rain then decreases to moderate for these areas on Tuesday. There is high confidence that a rain warning will be needed for Westland north of Hokitika, Buller and the ranges of northwest Nelson during Monday and Tuesday. Also, there is moderate confidence for warnable amounts of spillover rain into the headwaters of the Canterbury and Otago Lakes and Rivers on Monday, and a watch is in force for the headwaters of Otago at this stage.

For the North Island, there is high confidence that rainfall warnings will be needed for the Tararua Range during Monday and Tuesday, and for Mount Taranaki, Tongariro National Park and Taumarunui on Tuesday and Wednesday. There is also low confidence of warning amounts of rain about the ranges of eastern Bay of Plenty on Tuesday and Wednesday.

In addition, gale northwesterlies are expected in central and eastern areas from Gisborne down to Southland until late Tuesday. For exposed parts of Fiordland, Southland and Otago, there is moderate confidence that the winds will reach severe gale on Monday and a watch is in force. In Canterbury, the confidence is high for severe gale northwesterlies in exposed places on Monday and a warning is in force. For Marlborough, Wellington and Wairarapa there is high confidence that northwesterlies will reach severe gale in exposed places on Monday and Tuesday and a watch is in force for these areas. Meanwhile, there is moderate confidence of severe gale northwesterlies in southern Taranaki, Taihape, Whanganui, Manawatu, Horowhenua Kapiti Coast, Hawkes Bay and Gisborne on Monday and Tuesday.

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 Saturday, 23-Sep-2017

Issued: 08:26am Saturday, 23-Sep-2017

Daytime heating is expected to produce a few afternoon and evening showers in the North Island from Bay of Plenty and the ranges of Gisborne to Taihape and the ranges of Hawkes Bay, and in the South Island over the ranges of Marlborough and about North Otago, Dunedin and Clutha. In these areas there is also a low risk of isolated thunderstorms as well. No thunderstorms or significant convection expected in other areas of New Zealand during this period.



Valid to Noon Sunday, 24-Sep-2017

Issued: 09:47am Saturday, 23-Sep-2017

No thunderstorms or significant convection expected during this period.



Valid to Midnight Sunday, 24-Sep-2017

Issued: 12:55pm Saturday, 23-Sep-2017

A front from over the Tasman Sea is expected to move onto the southwest of the South Island Sunday evening, bringing a period of significant heavy rain - see the latest Severe Weather Warnings and Watches for more information. Embedded thunderstorms are also expected with the frontal rain, with a moderate risk of them occuring in Fiordland and Westland south of Haast, and with a lower risk extending north to the Glaciers and east towards the Divide. These thunderstorms will boost the already heavy rainfall rates. No thunderstorms or significant convection expected in other areas of New Zealand during this period.



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.