Thunderstorm Outlook
Issued at 7:22am 17/05/2022
Valid from 7:22am 17/05/2022 to Midnight 17/05/2022
A front is expected to move north over the south of the South Island today, followed by an unstable and showery air mass. There is a moderate risk of thunderstorms in Fiordland and Stewart Island. Those storms may be accompanied by heavy rain, 10 to 15 mm/h, small hail and 90 km/h wind gusts. There is a low risk in a broader area including the south of Westland and coastal Southland. This afternoon and evening there is a low risk of thunderstorms in Dunedin and North Otago. No thunderstorms or significant convection expected elsewhere during this period.
Issued at 7:24pm 17/05/2022
Valid from Midnight 17/05/2022 to Midday 18/05/2022
Chart reviewed Tuesday evening. An unstable northwest flow affects the lower South Island. There is a low risk of thunderstorms in Fiordland south of Milford sound and Stewart Island, and a low risk further north to central Westland before dawn. A slow moving band of rain is expected to affect the far north of the North Island and there is a low risk of thunderstorms with heavy rain there in the morning. No thunderstorms or significant convection expected elsewhere during this period.
Issued at 7:42pm 17/05/2022
Valid from Midday 18/05/2022 to Midnight 18/05/2022
Chart reviewed Tuesday evening. A moist northwesterly flow is expected to destabilize further over the South Island during the afternoon and evening, bringing rain to Fiordland and parts of Westland. There is a high risk of thunderstorms in Fiordland and Stewart Island through the period, spreading to southern and central Westland from evening, including western parts of Otago and South Canterbury near the Alps. These storms may be accompanied by localised rainfall rates of 10 to 20 mm/h, hail of 5 to 15 mm diameter, strong wind gusts of 90 km/h, and a slight chance of a small coastal tornado. There is a low risk of thunderstorms in a broader area as indicated on the chart. A band of rain over Northland is expected to move into northern Auckland at night, bringing an associated low risk of thunderstorms. 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.
