Big 'atmospheric river' wallops Western Australia, heads east

The once-in-a-decade storm event battering and soaking large areas off Western Australia sourced much of its moisture from a so-called "atmospheric river" that continues to head eastwards.

Such airborne rivers are characterised by narrow but concentrated corridors of moisture that can dump huge amounts of rainfall over short periods of time, often forming out of tropical cyclones. The events have been tracked in the northern hemisphere since the 1990s and are sometimes dubbed "pineapple express" storms.

Kimberley Reid, a PhD candidate studying atmospheric rivers for her dissertation, said the tempest over WA "very likely" has the characteristics of such storms.

Among the features are the relatively low altitude strong winds, reaching sustained averages of 80-100 knots (148-185 km per hour) at about 1.5 kilometres above the surface. The narrow band of moisture stretching as far as 2000 kilometres also formed out of ex-cyclone Mangga, Ms Reid said.

Most of the international study had focused on the northern hemisphere, particularly California where such rivers have regularly dumped large rainfall as the moisture hits the mountain ranges along the US west coast.

"We think they are more frequent than we first thought" for Australia as well, Ms Reid said.

At ground level, the wind speeds have reached as much as 132 km/hour at Cape Leeuwin, on the south-west coast of WA, the Bureau of Meteorology said.

In that region, rainfall totals have topped 50 millimetres, while parts of the Pilbara - normally dry at this time of year - have clocked up 30-50mm, the bureau said.

The ferocity of the event came as the atmospheric river interacted with a deep low-pressure system off the state's south.

Australia has been hit by other powerful rivers in recent years, including a storm that struck much of the east of Queensland just after Boxing Day, 2010, Ms Reid said.

Ms Reid said climate models indicate that the intensity and frequency of atmospheric rivers will increase in a warming world. "That's because there's more moisture available," Ms Reid said, noting that the physics of the atmospheric allow it to hold about 1 per cent more moisture per degree of warming.

Globally, temperatures have warmed more than 1 degree since pre-industrial times. The bureau puts the temperature for Australia at about 1.4 degrees higher over the past century.