The Oroville Dam is to be soaked by a foot of rain by Tuesday, ramping up the pressure on engineers who are frantically trying to repair 'patch and pray' quick-fixes which led to its near-collapse last week.
Up to five inches of rain will land on the Northern California dam this weekend in the first storm which will also bring winds of up to 33mph.
Another 10-12 inches will land at the start of next week and bring even stronger winds which may thwart repair works.
Meteorologists said it is that storm which will be a concern to engineers using helicopters to try to secure the dam's emergency spillway which nearly gave way last week to the dam's full reservoir.
200,000 people were suddenly evacuated on Sunday when the California Department of Water Resources dramatically announced that an emergency spillway at the dam was an hour away from collapse.
Residents nervously returned home as the evacuation order was downgraded to a warning on Tuesday but they have been told to be ready to flee again at a moment's notice if the volatile situation turns on its head again.
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The enormous scale of the damage - and the required work to fix it - at California's Oroville Dam became plain this week as 125 emergency crews worked round the clock to patch up its eroded spillways