Severe Weather Outlook

Valid from Tuesday 28-Sep-2021 to Friday 01-Oct-2021

Issued: 2:41pm Sunday 26 Sep 2021

A low developing just to the northeast of the North Island on Monday is forecast to track southeast and deepen on Tuesday. There is still uncertainty about the depth and position of the low.

There is moderate confidence of warning amounts of rain about the Tararua District and Hawkes Bay south of Napier early Tuesday, and low confidence for the remainder of Hawkes Bay and Gisborne during Tuesday.

Meanwhile a narrow ridge covers much of the South Island on Tuesday. Southwesterly winds should then dominate much of the country on Wednesday and Thursday. A ridge is expected to move east over central and northern New Zealand on Friday, and northwesterlies develop in the south of the South Island. There is no broad-scale severe weather expected from Wednesday to 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, 27-Sep-2021

Issued: 8:07pm Sunday 26 Sep 2021

The unstable southeast flow continues to cover central New Zealand during the second half of Monday. There is a moderate risk of a few thunderstorms in the east of Wairarapa during the afternoon as indicated on the chart. These storms could be accompanied by heavy rain, 10 to 25 mm per hour and small hail. There is a moderate risk of isolated thunderstorms about Kahurangi National Park and inland Buller during the afternoon and early evening, and a broad low risk covering many other parts of central New Zealand as indicated on the chart. Any thunderstorms that develop will be accompanied by localised heavy rain and hail. No other thunderstorms or significant convection expected over 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.