Thunderstorm Outlook
Issued at 8:43am 22/08/2026
Valid from 8:43am 22/08/2026 to Midnight 22/08/2026
A front moves northeast over the South Island West coast during Saturday, with a second trough moving onto Fiordland at night. There is a moderate risk of thunderstorms about Fiordland during the afternoon, evening and night, with heavy rain of 10 to 20 mm/h, strong wind gusts of 90 to 100 km/h and small hail. In addition, there is a low risk of thunderstorms with heavy rain for Westland, Stewart Island and the headwaters of the Canterbury and Otago lakes and rivers, this extends into Grey and Buller in the evening. No other thunderstorms or significant convection expected elsewhere in New Zealand during this period.
Issued at 7:01pm 22/08/2026
Valid from Midnight 22/08/2026 to Midday 23/08/2026
A series of fronts and very cold air aloft move across the South Island during the first half of Sunday. There is a moderate risk of thunderstorms about the western South Island, from Stewart Island to Farewell Spit. Any thunderstorms are likely to bring heavy rain, strong wind gusts of 90 to 110 km/h and hail. There is also a low risk of localised SEVERE thunderstorms with damaging wind gusts of more than 110 km/h and small tornadoes. During the morning, there is a moderate risk over coastal Southland and Clutha, likely bringing strong wind gusts of 90 to 110 km/h and hail of 5 to 15 mm diameter. A broader low risk extends farther east, and also north to Horowhenua Kapiti Coast and Taranaki during the morning. No thunderstorms or significant convection expected elsewhere during this period.
Issued at 9:59am 22/08/2026
Valid from Midday 23/08/2026 to Midnight 23/08/2026
During the second half of Sunday, the cold air aloft moves rapidly eastwards, and a very cold showery flow gradually stabilises. During Sunday afternoon, there is still a moderate risk of thunderstorms from Clutha to Westland. Any thunderstorms here could bring strong wind gusts of 90 to 110 km/h, and hail up to 10 mm diameter. A broader low risk extends farther east and north. Not shown on the chart, there is also a moderate risk of thunderstorms over the Chatham Islands Sunday evening, with gusts of 90 to 100 km/h and small hail likely. No thunderstorms or significant convection expected elsewhere 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 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.
