Severe Weather Outlook

Valid from Wednesday 12-May-2021 to Saturday 15-May-2021

Issued: 3:15pm Monday 10 May 2021

On Wednesday, a front moves northwards over the North Island bringing about a cool and unsettled southerly change, while the southerly flow across the South Island should ease as a ridge of high pressure builds across the Island. The southerly flow over the North Island should then ease on Thursday and the ridge should spread over the entire country. The ridge should begin to move away to the east later on Friday and early Saturday, ahead of a front approaching from the west. This front should cross the South Island on Saturday, and is preceded by a strong and moist northerly flow, then followed by disturbed westerlies.

On Saturday, there is a low risk of warning amounts of rain accumulating about the ranges of northwest Nelson, Buller and Westland, and also in Fiordland. In addition, there is a low risk of severe gale northerlies affecting exposed parts of Fiordland.

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, 11-May-2021

Issued: 7:41am Tuesday 11 May 2021

A strong moist northwest flow with embedded rain-bands covers New Zealand this morning. Later, a cold front moves northwards over the South Island bringing a cold southerly change. The southerly change reaches coastal Southland and Clutha late this morning with a period of rain that persists into the afternoon. This rain gives way to showers towards evening. Some showers are expected to be heavy with hail, and there is a low risk of thunderstorms there during the evening, as indicated on the chart. There is a moderate risk of thunderstorms embedded within broader areas of rain about Westland during the afternoon, and about Buller and Kahurangi National Park through much of the period. (The risks for these areas is considered low during the morning.) Any thunderstorms that occur will boost local rainfall rates to 15-25mm/h or possibly higher. Rain spreads onto the North Island during the second half of Tuesday along with a low risk of thunderstorms for western coastal areas from Waitomo southwards from evening onwards, and about the far North of Northland at night. No thunderstorms or significant convection expected elsewhere on Tuesday.



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.