El Nino storms lined up in the Pacific, promising to drench parts of the West for more than two weeks and increasing fears of mudslides and flash floods in regions stripped bare by wildfires.
Stronger systems are predicted starting Tuesday following light rain a day earlier. At least two more storms are expected to follow on Wednesday and Thursday, possibly bringing as much as 3 inches of rain.
The National Weather Service issued a flash-flood watch for Northern California communities affected by several destructive wildfires last summer and fall.
The brewing El Nino system — a warming pattern in the Pacific Ocean that alters weather worldwide — is expected to impact California and the rest of the nation in the coming weeks and months.
In recent weeks, a weather pattern partly linked with El Nino has turned winter upside-down across the nation, bringing spring-like warmth to the Northeast, a risk of tornadoes in the South, and so much snow across the West that even ski slopes have been overwhelmed.
As much as 15 inches of rain could fall in the next 16 days in Northern California, with about 2 feet of snow expected in the highest points of the Sierra Nevada, said Johnny Powell, a forecaster with the National Weather Service.
To the south, persistent wet conditions could put some Los Angeles County communities at risk of flash flooding along with mud and debris flows, especially in wildfire burn areas.
El Nino storms in the early 1980s and late 1990s brought about twice as much rain as normal, Jet Propulsion Laboratory climatologist Bill Patzert said. The weather also caused mudslides, flooding and high surf. El Nino's effects on California's drought are difficult to predict, but Patzert said it should bring at least some relief.
Doug Carlson, spokesman for the California Department of Water Resources, pointed out that four years of drought have left California with a water deficit that is too large for one El Nino year to totally overcome.
In Arizona, El Nino conditions will help push the parade of Pacific Ocean storms inland with light to moderate snow falling in the high country and rain in lower elevations, forecasters said Monday.
In Southern California, a flash-flood watch for wildfire burn areas was in effect through late Wednesday. Residents of the Silverado Canyon burn area in Orange County and the Solimar burn area in Ventura County were told they may want to evacuate in advance of the storm, but have not been ordered to do so.
Los Angeles Mayor Eric Garcetti warned people to clear gutters and anything in their yard that might clog storm drains; assemble an emergency kit; and stockpile sandbags if their home is susceptible to flooding.
An effort also was underway to provide shelter for homeless people.
"We want as little damage and destruction and as little death as possible," Garcetti said.
Between 2 and 3.5 inches of rain is predicted to fall across the coastal and valley areas of Southern California through Friday, with up to 5 inches falling in the mountains.
The storms are also whipping up large long-period ocean swells that could generate hazardous breaking waves at west-facing harbors in San Luis Obispo and Ventura counties, officials said.
Associated Press writers Christopher Weber in Los Angeles and Paul Davenport in Phoenix contributed to this report.