Valid to Midnight Thursday 20 Jun 2013
Issued at 9:09am Thursday 20 Jun 2013
A significant pool of cold air aloft is expected to move onto the northern half of the North Island enhancing instability and showers. In Taranaki there is a moderate risk of thunderstorms this afternoon. These may be accompanied by heavy rain of 5-15 mm/hr, strong wind gusts of 90 to 100km/hr and small hail. In Auckland, the Coromandel Peninsula, Waikato, Waitomo and about coastal Bay of Plenty, there is a moderate risk of thunderstorms during the period. These could be accompanied by heavy rain of 5-15 mm/hr, hail and strong wind gusts of 90 to 110km/hr but possibly in excess of 110km/hr in the far north of Auckland. In Northland there is also a moderate risk of thunderstorms, accompanied by heavy rain of 5-15 mm/hr and hail, but there is additionally a moderate risk of severe wind gusts in excess of 110km/hr. A low risk of thunderstorms extends to inland areas of the North Island as indicated on the map. No thunderstorms or significant convection expected elsewhere.
MetService uses the following criteria for severe thunderstoms in New Zealand:
Heavy rain (from thunderstoms): 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.
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.