The National Weather Service says the rain will complicate the search and recovery effort and add moisture to the already unstable slope.
Forecasters say rainfall in the valley also is likely to increase the flow in the river. A flood warning remains in effect for pooling in the area immediately upstream of where the landslide blocked the river.
The river has cut a channel through the blockage, but a flash flood watch is in effect for the area downstream, in case it suddenly gives way.
Forecasters expect showers Saturday and drier, cloudy weather Sunday in Western Washington.