By GREGG WYCHERLEY
The freak hail that hit parts of Auckland on Wednesday night was caused by a "breeding ground" of thunderstorms in the Pacific Ocean.
Cars were damaged and houses flooded as about 150mm of hail fell in the Albany and Torbay area before 8.30 pm.
Meteorologist Bob McDavitt said he sent out a warning on Tuesday after seeing a "dangerous looking" storm containing baseball-sized hail on radar. Nothing happened that night but a day later the hailstorm hit.
"We were invaded by a marauding horde of thunderstorms."
Mr McDavitt said the system had been building over the Pacific for the past week.
"These storms are all related to the polar blast that hit Tasmania last week.
"It was only a matter of time before someone got hit."
The storm was one of a group of thunderstorms that rolled over the upper North Island early in the week, caused by warm sea air meeting freezing air from the Antarctic.
"The warm sea air rises very quickly but has to stop when it reaches a ceiling of cold polar air - it then freezes and sinks again," he said.
"This creates a vigorous up-and-down motion leading to up-draughts and down-draughts snaking around each other - like a chimney or a tower." He said the storms were associated with cumulo-nimbus cloud - large "tower clouds" which are often icy at the top and "rolling" at the bottom.
Mr McDavitt said cumulo-nimbus cloud systems were also responsible for thunder and lightning, tornadoes, hailstorms and shower storms.
Weatherwise Auckland coordinator Carla Salinger said the storm was very localised and intense, with 28mm of hail falling in a 20-minute period in Torbay about 8 pm.
In some places, the hail was piled a metre deep.
She said records compiled since 1930 showed that Auckland could expect four or five hailstorms a year, normally from May to December.
One couple who felt the full brunt of the storm were Murray and Lynley Faulkner, owners of Centrepoint Nurseries in Albany.
Lynley Faulkner said the storm extensively damaged their stock of young plants and piles of hail were left on the ground. She said it was too early to assess the cost of the damage, but expected that up to 80 per cent of some lines could be wiped out.
Mr McDavitt said the "breeding ground" syndrome had gone and the MetService was forecasting cold weather for Monday and Tuesday next week - but no hail.
Hail progeny of marauding Pacific thunderstorms
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