Dangerous winds returned to the region Tuesday as new wildfires broke out in San Diego and a pair of major Los Angeles-area blazes burned for a third week, while officials made preparations to protect scorched neighborhoods from toxic ash runoff ahead of potential rain this weekend.

Santa Ana winds are whipping Southern California on Tuesday, sparking fresh fears that progress made fighting wildfires that have scorched over 40,000 acres and left 27 dead could be reversed and more blazes could break out.

Containing wildfires—never mind extinguishing them—in parched and windy conditions can be monstrously difficult. Dry vegetation is rocket fuel for blazes and wind both feeds the flames and spreads the embers. “The winds are like pointing an air blower at a fireplace,” says Gershunov, “except it’s not contained and it’s on a much larger scale.”