Severe Weather Outlook

Valid from Sunday 18-Apr-2021 to Wednesday 21-Apr-2021

Issued: 3:05pm Friday 16 Apr 2021

A westerly flow should weaken later on Monday as a ridge of high pressure develops over the country. On Sunday and early Monday, there is low confidence of severe gale west to northwesterlies about the far south coast including Stewart Island.

On Tuesday, a low approaches northern New Zealand from the North Tasman Sea and may move southeast across the North Island on Wednesday. This system spreads a moist northeast flow across the North Island, followed by a southwest flow later on Wednesday. Winds are likely to be strong to gale, especially in those places near the low centre. There is some uncertainty with the speed and track of the low, but at this stage there is low confidence of significant rainfall affecting Northland later on Tuesday and early Wednesday, and then Coromandel Peninsula and Bay of Plenty 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 Saturday, 17-Apr-2021

Issued: 7:30pm Friday 16 Apr 2021

Chart reviewed at 7:30pm, no changes. Conditions stabilise from the west during the second half of Saturday. A low risk of thunderstorms persists during the afternoon for eastern coastal areas of the North Island, as indicated on the chart, and into the evening for Mahia Peninsula. In the far south, a cold front is expected to bring a low risk of thunderstorms to far southern Fiordland and Stewart Island Saturday night. No thunderstorms or significant convection expected elsewhere.



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.