Severe Weather Outlook

Valid from Wednesday 13-Dec-2017 to Saturday 16-Dec-2017

Issued: 02:14pm Monday, 11-Dec-2017

Minimal risk of broadscale severe weather during the outlook period.

A front, followed by a ridge of high pressure, moves over the South Island and lower North Island on Wednesday, then the remainder of the North Island on Thursday. A trough preceded by northwesterlies approaches the southwest of the South Island on Friday, then moves away to the southeast on Saturday.

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, 12-Dec-2017

Issued: 08:55pm Monday, 11-Dec-2017

Updated at 8:55pm Monday - minor changes. The front and trough mentioned above should lie over the north of the South Island during Tuesday afternoon and evening. Meanwhile another slow-moving front lies over the north of the North Island. Daytime heating and humid conditions over the North Island are also likely to lead to some showers developing during the afternoon. There is a low risk of thunderstorms about northern Westland, Buller and northwest Nelson during the afternoon and from North Otago up to the Kaikoura Coast during the afternoon and early evening, as indicated on the chart. In the North Island, there is a generally low risk of thunderstorms developing in Northland, Auckland, Coromandel Peninsula, Bay of Plenty and the ranges of Gisborne and northern Hawkes Bay during the afternoon or evening. However, this risk of thunderstorms is considered moderate about central and eastern parts of Northland during the afternoon. If any thunderstorms occur here they are likely to be accompanied by localised heavy rain with intensities of 10 to 25mm per hour. No other areas of severe convection or thunderstorms are 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.