Severe Weather Outlook

Valid from Wednesday 11-Dec-2019 to Saturday 14-Dec-2019

Issued: 2:30pm Monday 9 Dec 2019

A ridge of high pressure over New Zealand weakens away on Wednesday as a weak front moves over the far south of the South Island from the southwest and a trough moves onto East Cape from the north.

There is low confidence of rainfall accumulations reaching warning criteria in Gisborne on Thursday.

A complex low is expected to approach the west of the South Island on Friday, and the system should then move over the North Island and weaken on Saturday. There is low confidence of warning amounts of rain in Fiordland and the ranges of southern Westland on Friday.

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, 10-Dec-2019

Issued: 7:28am Tuesday 10 Dec 2019

No thunderstorms or significant convection expected during this period.



Valid to Noon Wednesday, 11-Dec-2019

Issued: 10:22am Tuesday 10 Dec 2019

No thunderstorms or significant convection expected during this period. draft



Valid to Midnight Wednesday, 11-Dec-2019

Issued: 10:30am Tuesday 10 Dec 2019

Wind convergences are expected to develop over eastern parts of the lower South Island during the second half of Wednesday, along with cooling temperatures aloft. There is a low risk of thunderstorms for southern parts of the Canterbury High Country, central and eastern Otago and inland eastern Southland as indicated on the chart. 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.