Thunderstorm Outlook
Issued at 7:06am 21/08/2026
Valid from 7:06am 21/08/2026 to Midnight 21/08/2026
There is minimal risk of thunderstorms or significant convection during this period.
Issued at 7:49pm 21/08/2026
Valid from Midnight 21/08/2026 to Midday 22/08/2026
A front moves northeast over the south of the South Island in the morning, bringing a risk of thunderstorms to the west. There is a moderate risk of thunderstorms in Fiordland throughout the period and a low risk for Westland in the morning. If these storms occur they may be accompanied by rainfall intensities of 10 to 25 mm/h. No other thunderstorms or significant convection expected elsewhere in New Zealand during this period.
Issued at 8:58am 21/08/2026
Valid from Midday 22/08/2026 to Midnight 22/08/2026
The front continues to move northeast Saturday afternoon and evening but a trough moves onto the south of the South Island. There is a moderate risk of thunderstorms with heavy rain, 10 to 25 mm/h, strong wind gusts, 90 to 100 km/h and small hail for Fiordland in the evening and at night, but the risk there is considered low in the afternoon. There is a low risk of thunderstorms with heavy rain for Grey, Westland, Stewart Island and the headwaters of the Canterbury and Otago lakes and rivers. No other thunderstorms or significant convection expected elsewhere in 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 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.
