Storm damage in Merced, CaliforniaEPA

California is braced for more heavy rain after being lashed by a relentless series of storms and downpours that have caused death and destruction across the state.

More extreme weather is expected to roll in from the Pacific after weeks of high winds, floods and landslides that have devastated neighbourhoods – from towns to rural communities like Chualar in Monterey County.

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Storms have lashed coastal cities such as Santa Cruz and San Francisco, opening sinkholes in roads and cutting power to thousands of homes.

The Central Valley towns of Planada and Merced were hit by widespread flooding, forcing some people to leave their homes.

Residents in Planada clear homes after floods

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Several people had to be rescued after their cars fell into a sinkhole near Chatsworth

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Between 26 December and 9 January, parts of California saw up to six times more rain than usual, according to the National Weather Service.

In the last week, some areas of Santa Barbara received more than 410mm (16 inches) of rain in two days.

Map showing precipitation over the last 14 days in California

Forecasters say many of the same areas of the state can expect up to 200mm (8 inches) of rain in the coming five days.

Some of the rain will fall on ground which is heavily saturated, causing more flooding.

Map showing rain forecast over the next five days

The rain has raised the water level in rivers across the state. Among those starting to flood is the Salinas River in Monterey County.

Many low-lying areas along the Salinas River Valley are under evacuation orders “until further notice”.

Chart showing water level in the Salinas River

Local officials are warning that flooding may cut off the Monterey Peninsula from the rest of the state.

“Northern California has been hammered with heavy precipitation events over the past couple weeks, and any additional rainfall could pose a threat of flash flooding,” the National Weather Service said.

Atmospheric rivers

Driving some of the rain are weather phenomena called atmospheric rivers – water vapour evaporating from the ocean and carried along by the wind like a river in the sky.

As it reaches higher ground, the moisture is released as rain or snow.

Annotated satellite image of the storm heading for California

Although these rain rivers can bring much needed water to drought-prone areas like California, in recent weeks they have coincided with other severe weather systems – such as low pressure “bomb cyclones”, causing severe storms.

California has suffered years of drought – hardening the surface of the soil and reducing the ground’s ability to absorb water. That, in turn, makes run-off and flooding much more likely.

Although a lot more rain would be needed to reverse drought conditions across the state, the US Drought Monitor shows the storm appears to have almost eliminated extreme drought conditions.

Design by Erwan Rivault and Gerry Fletcher

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