Rain has begun falling in California where an "atmospheric river" is expected to bring heavy downpours, strong winds and flooding to areas ravaged by wildfires.
Ahead of the storm's arrival, officials distributed sandbags, put rescue services on stand by and told residents to pack a bag with essential items, ready for use during any emergency evacuation.
In the neighbouring state of Oregon, authorities coated the streets of the city of Portland with anti-ice liquid.
Officials in both Oregon and Idaho opened emergency shelters to assist people who may be affected by the conditions.
Southern California could see as much as 15cm of rain in the mountains and about 7.6cm in coastal areas and valleys, according to Brent Bower, a hydrologist with the US National Weather Service.
Strong wind gusts may bring down trees, cause power outages and delay flights.
Evacuation warnings have been issued in Mandeville Canyon and other areas ravaged by the Palisades Fire, the most destructive blaze in Los Angeles history, over concerns about potential debris flows during the upcoming storm.
There are also warnings for Trabuco Canyon and other areas near where one of the blazes, the Airport Fire, occurred.
All schools in the city of Malibu in California are closed.
The "atmospheric river" weather event is a long band of water vapour that can transport moisture from the tropics to more northern areas.
A climate scientist at the University of California, Daniel Swain, said the area is in desperate need of rain but it may fall too hard and fast.
The result could be debris flows and flash flooding around areas burned by the wildfires.
These are more at risk to debris flows because vegetation that helps to keep soil anchored has been destroyed and loose debris, including ash, soil and rocks, have been added, Mr Swain said.
Officials urged residents to be prepared should a mandatory evacuation order become necessary.
