A calm overnight allowed firefighters to make progress against a trio of massive fires burning in Northern California.

But they are girding for the arrival of a weather system Sunday that will bring high winds and thunderstorms that could spark new fires and fan existing blazes that already destroyed nearly 1,000 homes and other structures and forced tens of thousands to evacuate.

The fires surrounding the San Francisco Bay Area were started by lightning strikes that were among 12,000 registered in the state in the past week.

The National Weather Service issued a “red flag” warning for the area through Monday afternoon, meaning extreme fire conditions exist.