Severe Weather Outlook

Valid from Thursday 24-May-2018 to Sunday 27-May-2018

Issued: 02:11pm Tuesday, 22-May-2018

Strong westerlies with bursts of heavy rain and gales.

A strong disturbed westerly flow affects to country, bringing bursts of heavy rain. Snow may lower to 300 metres at times over the south of the South Island but at this stage warning amounts of snow are not considered likely. The disturbed westerly flow is expected to turn cold southwesterly on Friday.

On Thursday a front is expected to sweep east across the country and there is then a moderate confidence in heavy rain over the west of the South Island, from the west of Nelson to Fiordland. There is a low confidence in severe gales over the entire country but the risk is considered high in Wellington and Wairarapa.

On Friday a southerly or southwesterly flow is expected to spread north onto New Zealand. There is a low confidence in warning amounts of rain over the central North Island, extending south to the Tararua Ranges. There is a low confidence that westerly gales will become severe over the central and upper North Island.

On Saturday and Sunday a strong southwesterly or southerly flow is expected to affect the country. There is then a low risk of severe gales over the central and upper North Island.

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 Tuesday, 22-May-2018

Issued: 01:24pm Tuesday, 22-May-2018

A strengthening and unstable northwest flow will cover New Zealand today ahead of an embedded front with thunderstorms expected in most northern and western areas. This front wil move northwards over the South Island during the afternoon and evening to lie over the lower North Island at midnight. A cold unstable southwest flow follows the front. Over the northern North Island, there is a moderate risk of a few thunderstorms over Northland and northern Auckland this morning and afternoon, and over many other areas from North Taranaki and eastern Bay of Plenty northwards during the afternoon and evening. These thunderstorms will bring brief heavy rain and small hail. Around central New Zealand, the front brings a high risk of rain and embedded thunderstorms to Buller and Nelson this afternoon or evening, and about Taumarunui, Taranaki, Horowhenua, Kapiti, Wellington and northern Marlborough tonight. These thunderstorms will bring a period of heavy rain, strong northwest wind gusts, and possibly one or two small tornadoes near the coast. In TARANAKI, HOROWHENUA, KAPITI, WELLINGTON and the MARLBOROUGH SOUNDS however,it is possible a few of the thunderstorms could be severe with DAMAGING WIND GUSTS in excess of 110km/hr. These damaging wind gusts could occur with or without small tornadoes. Over the south and west of the South Island, there is a high risk of embedded thunderstorms about Westland, Fiordland and the Southern Alps through until late afternoon or evening, bringing heavy rain, strong northwest wind gusts, and possibly a small coastal tornado about Westland. This thunderstorm activity eases for a time, but then returns this evening or tonight in the colder southwest flow bringing snow above 500 metres and small hail to Fiordland and coastal Southland. Elsewhere, there is a general low risk of a few thunderstorms spreading further east (as indicated on the chart), and also a low risk of a few thunderstorms about eastern Otago and Canterbury this evening or tornight with a southerly wind change.



Valid to Noon Wednesday, 23-May-2018

Issued: 01:36pm Tuesday, 22-May-2018

A front with a band of heavy rain and embedded thunderstorms will move quickly eastwards over the North Island on Wednesday morning. A cold unstable west to southwest spreads over the country behind the front. There is a moderate risk of thunderstorms associated with the front spreading over western North Island areas from Kaipara Harbour to Waitomo before dawn, and across other northern and western areas from Northland to Bay of Plenty later in the morning. These thunderstorms will bring brief heavy rain, small hail,strong northwest wind gusts, and possibly a small tornado. Over AUCKLAND, WAIKATO, WAITOMO and TAUMARUNUI however, it is possible a few of the thunderstorms could be severe before dawn, with DAMAGING WIND GUSTS in excess of 110km/hr. If these damaging wind gusts occur, they may or may not be associated with small tornadoes. In the unstable west to southwest flow behind the front, the thunderstorms ease for a time over the North Island, but then return later in the morning to western areas from Waitomo to Manawatu with brief heavy rain and hail. Over the South Island, there is a moderate risk of thunderstorms about Kahurangi National Park, Buller and northern Westland through the whole morning, and about Fiordland and Southland before dawn. These thunderstorms will bring brief heavy rain, hail, and snow to 7-800 metres in the north of the South Island and to 4-500 metres about Fiordland and Southland. A low risk of thunderstorms covers many other parts of New Zealand as indicated on the chart.



Valid to Midnight Wednesday, 23-May-2018

Issued: 01:37pm Tuesday, 22-May-2018

A very unstable west to southwest flow will cover northern and central New Zealand during the second half of Wednesday, bringing heavy showers,thunderstorms and hail to many northern and western areas. As indicated on the chart, there is a moderate to high risk of thunderstorms over northern and western North Island areas, and also about Buller and western Nelson through the afternoon and evening. These thunderstorms are expected to be more frequent about the North Island west coast and the Auckland region. Brief heavy rain and hail will accompany the thunderstorms, with snow to 7-800 metres about the northern South Island. A low risk of thunderstorms covers the remainder of the North Island and the northern and eastern South Island as indicated on the chart.



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.