Severe Weather Outlook

Valid from Tuesday 20-Aug-2019 to Friday 23-Aug-2019

Issued: 2:01pm Sunday 18 Aug 2019

An active front is forecast to move east across New Zealand from the Tasman Sea on Tuesday, followed by a complex and slow moving low pressure system on Wednesday through to Friday. These weather systems are expected to bring rain to many parts of the country with the heaviest falls about the western regions of the South Island and the central and southern districts of the North Island.

There is moderate confidence of warning amounts of rain in Westland and Fiordland on Tuesday, then again in Westland on Wednesday and Thursday.

The risk for warning amounts of rain in northwest Nelson is low on Tuesday, then again on Wednesday and the risk area extending south across all of Buller, with the risk increasing to moderate on Thursday as indicated on the chart. On Thursday and Friday, there is moderate confidence that a rain warning will be required for Waitomo, Taumarunui, Taupo, Taihape, northern Taranaki including the Mountain, and also about the Tararua Range, meanwhile the risk is low for ranges of Bay of Plenty east of Whakatane.

In addition, on Tuesday there is moderate confidence of severe north to northeast gales affecting inland parts of Marlborough, the Canterbury High Country, the Otago Lakes, and also Westland and Fiordland. On Thursday and Friday, there is low confidence of northwest gales becoming severe in exposed parts of the North Island from Taranaki across the central high country to Gisborne and northwards.

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 Monday, 19-Aug-2019

Issued: 8:19pm Sunday 18 Aug 2019

No thunderstorms or significant convection expected during this period. Reviewed at 8:15pm Sunday - No changes.

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.