Strongest atmospheric river seen by California, Washington and Oregon this season knocks out power and downs trees

• California and Pacific north-west brace for an atmospheric river – but what is it?

What was expected to be one of the strongest storms in the north-west US in decades arrived on Tuesday evening, knocking out power and downing trees across the region.

The Weather Prediction Center issued excessive rainfall risks beginning on Tuesday and lasting through Friday as the strongest atmospheric river – a large plume of moisture – that California and the Pacific north-west has seen this season bears down on the region. The storm system is considered a “ bomb cyclone”, which occurs when a cyclone intensifies rapidly.