Severe Weather Outlook

Valid from Thursday 24-Aug-2017 to Sunday 27-Aug-2017

Issued: 02:17pm Tuesday, 22-Aug-2017

A ridge of high pressure is forecast to lie over the North Island on Thursday and Friday, before moving away to the east on Saturday allowing a north to northeast flow to spread over the Island.

Meanwhile, a northwest flow is expected over the South Island, and a slow moving trough is forecast to lie embedded within this flow over Fiordland on Thursday and Friday. The trough is then forecast to move slowly northwards into Westland on Saturday and remain slow moving there through Sunday.

There is low confidence of rainfall totals meeting warning criteria in Fiordland from Thursday through to Saturday, then low confidence for Westland on Saturday and Sunday as the trough moves north into the region.

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 Tuesday, 22-Aug-2017

Issued: 08:09am Tuesday, 22-Aug-2017

A trough moving northeast over the north of the North Island is expected to bring cool temperatures at upper levels and instability, increasing the chance of afternoon and early evening showers/convection. Over the Bay of Plenty, Rotorua and Coromandel Peninsula there is a moderate risk of thunderstorms this afternoon and evening. These thunderstorms may bring localised heavy rain and 10 to 20mm hail. There is a low risk in a broader area as indicated on the chart including Gisborne, Waikato, Auckland and Northland. These storms may be accompanied by bursts of heavy rain and hail. Elsewhere, no thunderstorms or significant convection expected.



Valid to Noon Wednesday, 23-Aug-2017

Issued: 07:27pm Tuesday, 22-Aug-2017

Chart reviewed at 7:25pm Tuesday and no changes made. No thunderstorms or significant convection expected during this period.



Valid to Midnight Wednesday, 23-Aug-2017

Issued: 07:33pm Tuesday, 22-Aug-2017

Chart reviewed at 7:30pm Tuesday and no changes made. Wednesday evening a front is expected to approach the south of Fiordland from the west. There is a low risk of thunderstorms in Fiordland towards midnight. Elsewhere, no thunderstorms or significant convection expected.



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.