Rain trapped 'like sponge against wall'

A satellite image in the Visible spectrum from the Geostationary weather satellite MSAT showing a large high to the east of New Zealand. Photo / Supplied

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A rare "blocking high" to the east of New Zealand trapped a severe weather system over the upper South Island and made flooding inevitable, experts say.

The main cause of the 48 hours of torrential rain was a wall of air pressure which trapped rainclouds over the Nelson region and held them in the same spot while they were fuelled by humid air from the Coral and Tasman seas.

The wall of high pressure off the east coast of the country blocked the jetstream - high altitude air currents - which steers most of New Zealand's weather.

MetService weather ambassador Daniel Corbett said: "Imagine having a stream of water moving across the Pacific. Put a big couple of boulders in the stream and what happens? All the water just gets blocked up. Think of throwing a sponge against the wall and squeezing it. That's how you can squeeze out 400-500mm of rain and get intense rain rates over the higher ground."

In colliding with the blocking high, the westerly-flowing jetstream buckled. This meant the air which flowed over New Zealand came from further north, near Australia.

This warmer, damper air was channelled into the trapped weather system, creating heavier rain which burst river banks and caused landslides.

In the far northwest of the Tasman district, local hydrologists recorded such heavy rainfall - 30mm an hour in some places - that they have called it a 1-in-100-year event.

Mr Corbett explained that the weather system was not literally trapped, but the jetstream was forced to "take the long way around" the blocking high.