Severe Weather Outlook

Valid from Sunday 25-Feb-2018 to Wednesday 28-Feb-2018

Issued: 02:39pm Friday, 23-Feb-2018

A front is expected to move over most of the South Island during Sunday before weakening as it moves onto the North Island during Monday. A strong northwest flow precedes the front and a period of heavy rain is likely in the west of the South Island.

There is high confidence that a rainfall warning will be required during Sunday for Fiordland, Westland and southern Buller, with moderate confidence that the warning will extend over the Alps to the headwaters of the Otago and Canterbury lakes and rivers. Confidence is moderate that a wind warning will be issued for severe gale northwesterlies about Southland and inland areas of Otago and Canterbury during Sunday.

Later Sunday and during Monday there is moderate confidence that a rainfall warning will be needed for northern Buller and northwest Nelson. There is also moderate confidence that warning strength winds will be experienced about northern Marlborough, Wellington and Wairarapa on Sunday and Monday.

On Monday, there is low confidence that a rainfall warning will be needed for the Tararua Range.

A large high should move onto the South Island late Monday and over the rest of the country on Tuesday. The high is expected to remain over the country on Wednesday.

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, 23-Feb-2018

Issued: 08:08am Friday, 23-Feb-2018

No thunderstorms or significant convection expected during this period.



Valid to Noon Saturday, 24-Feb-2018

Issued: 09:37am Friday, 23-Feb-2018

No thunderstorms or significant convection expected during this period.



Valid to Midnight Saturday, 24-Feb-2018

Issued: 09:37am Friday, 23-Feb-2018

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.