Severe Weather Outlook

Valid from Saturday 21-Sep-2019 to Tuesday 24-Sep-2019

Issued: 2:47pm Thursday 19 Sep 2019

A large high remains slow-moving over New Zealand on Saturday. On Sunday, a front preceded by gale north to northwest flow, is forecast to approach the lower South Island from the southwest. There is low confidence of north to northwest gales becoming severe in exposed parts of southern Fiordland and coastal Southland including the Stewart Island on Sunday.

This front is forecast to move north across the South Island on Monday and North Island on Tuesday bringing a period of heavy rain. There is low confidence of rainfall accumulations reaching warning amounts in Westland and Fiordland on Monday, and about the Tararua Range and Mount Taranaki on Tuesday.

A ridge spreads onto the South Island from the Tasman Sea late 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 Friday, 20-Sep-2019

Issued: 7:24am Friday 20 Sep 2019

No thunderstorms or significant convection expected during this period.



Valid to Noon Saturday, 21-Sep-2019

Issued: 9:02am Friday 20 Sep 2019

No thunderstorms or significant convection expected during this period.



Valid to Midnight Saturday, 21-Sep-2019

Issued: 9:02am Friday 20 Sep 2019

No thunderstorms or significant convection expected 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.