Severe Weather Outlook
Valid from Monday 18-Jan-2021 to Thursday 21-Jan-2021
Issued: 3:28pm Saturday 16 Jan 2021
A northwest flow covers the country on Monday. This flow is expected to become increasingly unsettled on Tuesday as a deepening trough moves onto the South Island. The active trough moves slowly to the east of New Zealand on Wednesday, with the flow over the country turning southwest.
On Monday and Tuesday for the west of the South Island from Buller to Fiordland, and also for the Otago and Canterbury headwaters, there is mostly moderate confidence that rainfall amounts will reach warning criteria, however about Westland and the Canterbury headwaters confidence increases to high on Tuesday. On Wednesday, confidence for warnabale amounts of rain in all these areas decreases to low. For the Tararua Range on Tuesday, there is low confidence that rainfall amounts will reach warning criteia.
Northwest winds strengthen over southern and central New Zealand ahead of the deepening trough. A low confidence of severe west to northwest gales in exposed parts of Fiordland, Southland, Otago, inland Canterbury, Marlborough, Wellington and Wairarapa on Monday, increases to moderate on Tuesday. Additionally, there is low confidence of severe west to northwest gales in exposed parts of the Tararua District and Hawkes Bay south of Hastings on Monday, Tuesday and Wednesday.
As the active trough moves slowly to the east of the South Island on Wednesday, there is low confidence that rainfall amounts will reach warning criteria in Southland, Clutha and Dunedin on Wednesday, and there is also a low confidence that west to southwest gales will reach severe gale in these areas on Wednesday and Thursday.
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 Saturday, 16-Jan-2021
Issued: 8:23am Saturday 16 Jan 2021
The combination of another hot humid day and generally light winds will allow daytime cloud buildups to produce isolated afternoon and evening showers and possible thunderstorms about the central North Island and inland South Island today. The most likely area for thunderstorms is about the central North Island high country in the late afternoon and evening, especially about Taihape, Tongariro National Park, and the ranges of inland Hawkes Bay and eastern Taupo. A broader low risk covers other surrounding central North Island areas, and also inland parts of the South Island from Marlborough and Nelson down to North Otago as indicated on the chart. Any thunderstorms that develop today will produced localised heavy rain of 10-25mm/h, however as the storms will be slow-moving there is the possibility a few could be severe producing localised downpours of 25-40mm/h. If localised downpours do occur, they could cause surface and/or flash flooding and make driving conditions hazardous. No other thunderstorms or significant convection expected over New Zealand today.
Valid to Noon Sunday, 17-Jan-2021
Issued: 10:57am Saturday 16 Jan 2021
A cold front in the Tasman Sea will move onto the South Island on Sunday morning bringing a period of heavy rain and thunderstorms to northern and western parts of the South Island. A unstable northwest flow follows the front. Severe Weather Warnings and Watches have been issued for parts of the South Island. As this front moves over the South Island, there is a moderate risk of thunderstorms embedded within broader areas of rain about Westland and Buller though the morning, then spreading into Nelson towards midday. These thunderstorms will boost local rainfall rates and there is a risk of localised downpours in excess of 25mm/h. Strong wind gusts, small hail and even one or two small tornadoes are also likely with the thunderstorms (mainly on the west coast). In Fiordland, the initial risk of thunderstorms with the front is considered low over the land before dawn, however an unstable northwest flow will spread onto Fiordland from mid morning and the risk of thunderstorms increases to moderate. These thunderstorms will be accompanied by brief heavy rain and hail. A broader low risk covers areas further east (including Southland from late morning) as indicated on the chart. No other thunderstorms or significant convection expected over New Zealand during the morning.
Valid to Midnight Sunday, 17-Jan-2021
Issued: 11:51am Saturday 16 Jan 2021
The cold front mentioned above will sweep over central and northern New Zealand during the second half of Sunday with a period of rain and thunderstorms in western areas. There is moderate risk of thunderstorms with this front about Nelson and Marlborough in the early afternoon, about western areas from Taranaki to northern Wellington mid afternoon, then about areas from Auckland to Taupo in the late afternoon or early evening. These thunderstorms will bring localised heavy rain, with the chance of small hail and even a small tornado from Taranaki southwards. There is a lower risk of thunderstorms about eastern North Island areas as indicated on the chart. In the South Island, an unstable northwest flow with another embedded front brings a high risk of thunderstorms about western areas from Kahurangi National Park to Fiordland during the afternoon and early evening, with some of these thunderstorms spreading across the Southern Alps into the Canterbury High Country and western parts of Otago and Southland. Additionally, a southerly wind change is likely to bring thunderstorms to other parts of Southland and Otago in the afternoon and evening, which then spreads to South Canterbury later in the day. Localised heavy rain and hail will accompany these thunderstorms, along with squally wind gusts of 80-110km/h about the West Coast. There is also a low risk of a few thunderstorms spreading off the Alps into western and southern parts of Canterbury during the afternoon and early evening.
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.