Severe Weather Outlook

Valid from Thursday 24-Sep-2020 to Sunday 27-Sep-2020

Issued: 2:32pm Tuesday 22 Sep 2020

A front, preceded by strong northwesterlies, is expected to move northeast over the North Island on Thursday, while a trough spreads showers north over the South Island. This trough then moves northeast over the North Island on Friday, followed by a weak ridge of high pressure.

On Saturday, a front approaches the country from the Tasman Sea, preceded by strong to gale northerlies. This front is expected to move east over New Zealand during late Saturday and Sunday, bringing heavy rain to some western areas and snow about the Alps. A trough brings a cold southwest change to the South Island later on Sunday, with snow showers to 400 metres in the far south.

There is a low confidence of rainfall accumulations approaching warning amounts about Fiordland, south Westland, Taranaki, the Central Plateau and King Country on Thursday, with a moderate confidence about the West Coast from Fiordland to Buller including the Otago and Canterbury Headwaters late Saturday to Sunday. Also, on Sunday there is a low confidence of heavy rain about Taranaki and the Central Plateau.

In addition, there is low confidence of severe northwesterly gales in exposed places of Hawkes Bay and Gisborne during Thursday. Also, there is a moderate confidence of severe north to northwesterly gales about Taranaki, Whanganui, Horowhenua, Kapiti Coast, Wairarapa, Wellington, Nelson, Marlborough and Canterbury from late Saturday to Sunday, then a low confidence about Northland, Auckland, Coromandel Peninsula and parts of Waikato on Sunday.

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 Wednesday, 23-Sep-2020

Issued: 8:05am Wednesday 23 Sep 2020

A southerly change moves across South Canterbury this afternoon and early evening, while wind convergence sets up over eastern Otago as upper temperatures cool. There is a low risk of thunderstorms for eastern Canterbury south of Banks Peninsula and eastern Otago this afternoon and early evening. A front is moving northwards from Westland to Nelson today, and there is a low risk of a few thunderstorms with this front as depicted on the chart. The northwest flow over Fiordland becomes increasingly unstable this evening as temperature in the upper atmosphere cool, with a moderate risk of thunderstorms. These thunderstorms may produce localised rainfall rates of 10 to 15mm/hr. 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.