Severe Weather Outlook

Valid from Tuesday 22-Oct-2019 to Friday 25-Oct-2019

Issued: 2:54pm Sunday 20 Oct 2019

A series of fronts, preceded by strong westerlies and followed by cold strong southwesterlies, are forecast to move north across New Zealand through to Thursday. Late Friday, a narrow ridge is expected to move onto the country from the Tasman Sea.

There is Low Confidence that snow might at times approach warning criteria over the lower South Island above about 500 metres from Tuesday through to Thursday, and parts of Canterbury and Marlborough from Tuesday evening and on Wednesday, affecting some higher roads and passes.

There is also Moderate Confidence that westerly gales could become severe over Central Hawkes Bay and the Tararua District on Tuesday and again on Thursday. Also, on Tuesday and Wednesday, there is Low Confidence of severe southerly gales in exposed parts of Wellington and the Marlborough Sounds. On Wednesday there is Low Confidence of severe southwest gales in exposed coastal parts of Hawkes Bay north of Wairoa and also in coastal Gisborne. Finally, on Wednesday and Thursday there is Low Confidence of severe west to southwest gales from Northland to Taranaki and eastwards to East Cape.

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 Sunday, 20-Oct-2019

Issued: 8:12am Sunday 20 Oct 2019

A front moves slowly northwards over Fiordland this morning and Westland during the second half of the day. There is a moderate risk of thunderstorms this morning and afternoon about Fiordland and Westland from Haast southwards, especially with the front. Any thunderstorms that occur will likely bring localised heavy rain with intensities of 15 to 25 mm per hour. There is also a risk of a localised tornado about coastal areas. For Westland north of Haast there is a moderate risk of thunderstorms this evening and tonight, mainly with the front. Any thunderstorms that occur are likely to bring heavy rain with intensities of 15 to 25 mm per hour. A lower risk of thunderstorms extends north to Buller, east to the main divide and also to remaining areas of Fiordland, coastal Southland, Clutha and Dunedin. This evening, a cold front over the south Tasman Sea is expected to approach southern New Zealand, bringing a moderate risk of thunderstorms to Fiordland and Stewart Island this evening and tonight. These thunderstorms will likely bring localised heavy rain of 10 to 20 mm per hour, hail and strong wind gusts of 80 to 100 km/h. No thunderstorms or significant convection expected elsewhere in New Zealand today.



Valid to Noon Monday, 21-Oct-2019

Issued: 10:37am Sunday 20 Oct 2019

The cold front over northern Westland continues to move northwards and onto the west of the North Island during Monday morning, while weakening. An unstable southwesterly flow follows this front, with an embedded cold front moving over southern New Zealand. There is a moderate risk of thunderstorms for Buller and northwest Nelson with the front before dawn, and for Westland throughout the morning but especially with the two fronts. Any thunderstorms that occur will likely be accompanied by heavy rain of 10 to 25 mm per hour, with hail in Westland. There is also a moderate risk of thunderstorms for Fiordland and Stewart Island before dawn, especially with the cold front. Any thunderstorms that occur in this area will likely be accompanied by heavy rain of 10 to 20 mm per hour, hail and strong wind gusts of 70 to 90 km/h. A broader low risk area extends north and east to include southern and western parts of the South Island plus western parts of the North Island, as indicated on the chart. No thunderstorms or significant convection expected elsewhere in New Zealand during Monday morning.



Valid to Midnight Monday, 21-Oct-2019

Issued: 10:52am Sunday 20 Oct 2019

The weak cold front over the west of the North Island continues to move eastwards during the second half of Monday. An unstable and showery west to southwesterly flow covers central and southern New Zealand, with an embedded cold front moving northwards. This second cold front brings a southerly change to the east of the South Island, which is expected to trigger a few showers, some with hail, as far north as Canterbury. During the afternoon and evening there is a low risk of thunderstorms for southern and western parts of central and southern New Zealand, plus coastal parts of Otago and the Canterbury Plains and Christchurch, as indicated on the chart. No thunderstorms or significant convection expected elsewhere in New Zealand during this period.



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.