Thunderstorm Outlook
Issued at 7:26am 4/04/2025
Valid from 7:26am 4/04/2025 to 1:00am 5/04/2025
A front moving slowly eastwards continues to bring heavy rain to parts of northern and central New Zealand today, and Severe Weather Warnings and Watches remain in force for many regions. Another front spreads a south to southwest wind change up the South Island from afternoon, while upper temperatures cool, making the atmosphere more unstable. There is mostly a low risk of thunderstorms for the west of the South Island from Westland to western Tasman after midday. However, the risk is considered moderate for the Westland District this afternoon and evening, and for the Grey and Buller Districts from evening. Any thunderstorms that occur will likely boost local rainfall rates to 10 to 20 mm/h, especially about the ranges. In the east of the South Island there is a low risk of thunderstorms from Clutha to the Rakaia River this afternoon and evening, and from the Rakaia River to Cape Campbell from evening, as depicted on the chart. However, for Dunedin, Central Otago east of Alexandra, North Otago, and the Canterbury Plains south of Ashburton, the risk is considered moderate this afternoon and evening. Any thunderstorms that occur will bring localised heavy rain with intensities of 10 to 25 mm/h, and possibly small hail. No thunderstorms or significant convection expected elsewhere in New Zealand today.
Issued at 10:07am 4/04/2025
Valid from 1:00am 5/04/2025 to 1:00pm 5/04/2025
A front embedded in an unstable west to southwest flow is forecast to move northeastwards over central New Zealand. There is a low risk of thunderstorms for coastal areas of the South Island between Christchurch and Cape Campbell before dawn. There is also mostly a low risk of thunderstorms for the Grey and Buller Districts, the Tasman District west of Motueka, the Marlborough Sounds, and southern parts of the North Island, as drawn on the chart. However, the risk is considered moderate for the Buller District before dawn. Any thunderstorms that occur will likely boost local rainfall rates to 10 to 20 mm/h, especially about the ranges. No thunderstorms or significant convection expected elsewhere in New Zealand during Saturday morning.
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 25mm/h, or more.
Large hail:
Hailstones 20mm in diameter, or larger.
Strong wind gusts (from thunderstorms):
Gusts of 110km/h (60 knots) or stronger.
Damaging tornadoes:
Fujita F1 - i.e. wind speeds greater than 116km/h (63 knots) or stronger.
Note: some tornadic systems such as funnel clouds, waterspouts and small land-based tornadoes are possible with thunderstorms that may not be classified as severe.
