Severe Weather Outlook

Issued: 10:20am Wednesday 18 Sep 2024

Valid Friday 20-Sep-2024

On Friday, several fronts move across New Zealand, bringing bursts of rain to western regions, and strong northwest winds at times.

There is low confidence of rainfall reaching warning accumulations about the Tararua range and the central North Island hill country.

Valid Saturday 21-Sep-2024

On Saturday, a ridge of high pressure moves onto the North Island, while a moist northwest flow affects the South Island, bringing rain to western regions.

There is low confidence of rainfall reaching warning accumulations about Fiordland and the ranges of southern Westland from late in the day.

Valid Sunday 22-Sep-2024

During Sunday, a moist northerly flow affects the South Island, bringing rain to Fiordland and Westland. There is low confidence of rainfall reaching warning accumulations about Fiordland and the ranges of the Westland District.

Valid Monday 23-Sep-2024

A front is expected to move onto the lower South Island. A low in the north Tasman Sea is forecast to spread a front onto the upper North Island later in the day, bringing rain to Northland, Auckland and Great Barrier Island, with low confidence of warning accumulations of rain there.

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 Wednesday, 18-Sep-2024

Issued: 7:29am Wednesday 18 Sep 2024

An unstable westerly flow covers southern and central New Zealand, with heavy showers likely in some places. For the Westland District, Fiordland, Stewart Island and coastal Southland, there is mostly a low risk of thunderstorms, but the risk is moderate for Stewart Island and Fiordland south of Milford Sound this morning. Any thunderstorms that occur will likely be accompanied by heavy rain with intensities of 10 to 20 mm/h. For the Grey and Buller Districts, also northwest Tasman, there is a low risk of thunderstorms this afternoon and evening. Lastly, for coastal areas from Manawatu to Waitomo, there is a low risk of thunderstorms from evening. No thunderstorms or significant convection expected elsewhere in New Zealand today.



Valid to Noon Thursday, 19-Sep-2024

Issued: 7:51pm Wednesday 18 Sep 2024

A front embedded in an unstable west to northwest flow moves onto southern New Zealand Thursday morning. There is a moderate risk of thunderstorms for Fiordland and the Westland District south of Haast, spreading northwards to include the remainder of Westland plus the Grey and Buller Districts after dawn. A low risk of thunderstorms extends slightly farther north and east, as depicted on the chart. Any thunderstorms that occur will likely be accompanied by brief heavy rain with intensities of 10 to 20 mm/h, and small hail in the south from late morning. No thunderstorms or significant convection expected elsewhere in New Zealand during Thursday morning.



Valid to Midnight Thursday, 19-Sep-2024

Issued: 8:02pm Wednesday 18 Sep 2024

A front moves north over the South Island in the afternoon and onto the North Island in the evening. Cold air at upper levels is likely to bring unstable conditions and showers to the lower and central South Island. In the afternoon there is a moderate risk of thunderstorms for Westland and also Otago and Canterbury near the main divide as indicated on the chart. These storms may be accompanied by heavy rain, 10 to 20 mm/h and hail. There is a moderate risk of thunderstorms for Canterbury south of the Banks Peninsula and North Otago. Those storms may be accompanied by heavy rain and 5 to 15 mm hail. In the afternoon and evening there is a moderate risk of thunderstorms for Buller and the far west of Nelson. Those storms may be accompanied by heavy rain, 10 to 20 mm/h and hail. In the evening and at night there is a moderate risk of thunderstorms for Kapiti, Taranaki and coastal Waitomo. Those storms may be accompanied heavy rain, 10 to 20 mm/h and hail. There is a low risk of thunderstorms extending slightly east and south from the moderate risk area over the North Island. There is also a low risk of thunderstorms over the far south of the South Island. No thunderstorms or significant convection expected elsewhere in 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.