Cyclone Cook vs Cyclone Debbie: The science of the storms

It's being called the most powerful storm to hit New Zealand in decades but how does Cyclone Cook compare to Debbie?

Generally, meteorologists say last week's storm, the remnants of an tropical cyclone, was a complex, widespread and longer-lasting system filled with an assortment of troughs and fronts.

With Cook, the system will be a more tightly packed beast, faster moving, and potentially much more devastating.

Debbie formed over the Coral Sea, morphed into a Category 4 system, and slammed into Airlie Beach, Queensland, on March 28, before weakening and travelling south.

Its remnants were pulled across the Tasman Sea towards New Zealand, a process that also helped form what Niwa meteorologists called an "atmospheric river".

Atmospheric rivers are relatively long, narrow regions in the atmosphere, like rivers in the sky, that transport most of the water vapour outside of the tropics.

Research indicates that for New Zealand, atmospheric rivers happen about 40 days a year and are associated with about half the country's rainfall extremes.

MetService meteorologist Lisa Murray said Debbie had died away when another low came in, soaked up its warm moist air and invigorated another low, which eventually hit New Zealand mid-last week.

"By the time it got to New Zealand, it was really a widespread thing. You had the low, but you also had these troughs and fronts moving around it, and these came in the bands of rain that we received all around us," Murray said.

"Yes, we had warm, moist air, and we had downpours, and because some of the activity was slow-moving, especially along some convective lines, we got some really large accumulations."

Auckland recorded exactly its normal April rainfall, 84.6mm, in just 14 hours between the evening of April 4 and the morning of April 5; Whakatane, near where Edgecumbe was overwhelmed by a deluge that poured through a collapsed stopbank, received a total 209mm.

But with Cook, the connection between its origins as a Category 2 cyclone that formed near Vanuatu early Sunday and what it is today was much more direct.

Cook had largely remained one low-pressure system, although one that had shifted to a mid-latitude low since it left the tropics.

Niwa meteorologist Ben Noll said the cyclone's energy had been harnessed by a low in the Tasman Sea, which had then hauled it south towards New Zealand.

"So this time, the Tasman low is kind of like a conductor and Cook is playing the notes."

Meteorologists expect Cyclone Cook to pack more of a punch: severe gales of gusts of 150km/h or more are possible when it makes landfall in Coromandel and Bay of Plenty this evening; in Auckland, gusts could reach 120km/h today.

"Rainfall will be as prolific, if not more prolific in some places this evening as the last one, but the main difference between the two is the wind that this system is likely to generate, along with the associated potential dangers it brings," Noll said.

Meteorologists were still referring to Cook as a cyclone "because we need people to sit up and listen", Murray said.

"Although it's fast moving, and will move over the country very quickly because the isobars are so tightly packed, the winds are really strong and they are going to come different directions as the low moves by.

"So we are just concerned about the impacts of this and are really telling people to batten down the hatches."

The fast-moving nature of Cyclone Cook also set it apart from the destructive Cyclone Bola in 1988, which lingered for days over the North Island, and also Cyclone Giselle in 1968, which caused the stormy seas that led to the Wahine disaster in the Cook Strait, and also hung about for days.