Severe Weather Outlook

Valid from Tuesday 24-Oct-2017 to Friday 27-Oct-2017

Issued: 02:48pm Sunday, 22-Oct-2017

A trough of low pressure is forecast to move onto the lower South Island from the Tasman Sea late Tuesday then slowly move northeast across the South Island on Wednesday and the North Island on Thursday. West to northwest winds preceding this front is expected to affect the country with strong to gale force winds likely in exposed places at times. However, these winds are not expected to become severe. The front is also expected to bring rain to the western districts, but rainfall totals are not expected to reach warning criteria at this stage.

On Thursday, a ridge of high pressure is forecast to build across South Island and then spread onto the North Island 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 Monday, 23-Oct-2017

Issued: 11:13am Monday, 23-Oct-2017

Updated to add a moderate risk of localised downpours over the northern North Island as covered in the latest Severe Thunderstorm Watch. A low is forecast to approach Northland late this morning and cross the northern North Island this afternoon and evening, bringing brief heavy rain. There is also a broad low risk of thunderstorms from northern parts of Waitomo, Taupo and Gisborne northwards for a time today as the low crosses. However, this risk of thunderstorms and very heavy rain is considered moderate about Northland, Auckland, Coromandel Peninsula and northern Waikato late this morning and especially this afternoon, and about coastal and eastern Bay of Plenty this afternoon and early evening. Any thunderstorms that eventuate are expected to produce heavy rain of 10 to 20mm per hour. There is also a moderate of localised downpours with or without thunderstorms producing rainfall rates of 20 to 30mm per hour and a Severe Thunderstorm Watch has been issued to cover this possibility. If any downpours eventuate they are likely to cause localised surface flooding and hazardous driving conditions. Cooling upper level temperatures are expected over the South Island this afternoon and evening, bringing a low risk of thunderstorms to Buller and northern Westland, as well as parts of Canterbury and North Otago, as indicated on the chart. No thunderstorms or significant convection expected elsewhere.



Valid to Noon Tuesday, 24-Oct-2017

Issued: 10:27am Monday, 23-Oct-2017

A weakening front is forecast to cross northern Westland and Buller before dawn bringing a low risk of thunderstorms to these areas. This front should then spread onto the west of the North Island up to Waikato during the morning, with a similar low risk of one or two thunderstorms, as indicated on the chart. No thunderstorms or significant convection expected elsewhere.



Valid to Midnight Tuesday, 24-Oct-2017

Issued: 10:35am Monday, 23-Oct-2017

A weakening southerly change is forecast to spread up the east of the North Island during Tuesday afternoon and evening. There is a low risk of one or two thunderstorms triggering on this change from northern Wairarapa up to Gisborne,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.