Much-needed rain has begun to fall over Southern California, bringing relief to the drought-stricken region but also the threat of toxic runoff.

Pacific Coast Highway in the city of Santa Monica has been reopened for residents and businesses. The Santa Monica Police Department has also lifted all city street closures. They ask residents to remain vigilant in areas that were previously under evacuation warnings.

Forecasters in Southern California expect to issue a 'particularly dangerous situation' red flag warning for the coming week as the Santa Ana wind forecast worsens.

As winds across the Southern California area are calmer than their peak and firefighters are making progress, the threat to the fire-weary region remains with Santa Ana winds expected to continue in the coming days.

Southern California will continue to face "dangerous ... Santa Clarita Valley, western Santa Monica Mountains, Santa Susana Mountains and western San Gabriel Mountains are among the areas affected.

The Santa Ana winds are dry, powerful winds that blow down the mountains toward the Southern California coast. The region sees about 10 Santa Ana wind events a year on average, typically occurring from fall into January. When conditions are dry, as they are right now, these winds can become a severe fire hazard.

Santa Ana winds will continue whipping through Southern California through Thursday, sparking fears that progress made fighting wildfires that have scorched over 40,000 acres and left 28 dead could be reversed and more blazes could break out.

Wildfires in Southern California have prompted major discussions about water in the state. Here's an overview of where water comes from in the region.

After multiple days of heavy rains provided Southern California with significant relief from a spate of catastrophic wildfires, experts are now warning of potential mudslides as debris flows run

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Officials cautioned that ash in recent burn zones was a toxic mix of incinerated cars, electronics, batteries, building materials, paints, furniture and other household items.