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On the second day of the battle to contain a wildfire in the Napa Valley, nearly 400 structures remained threatened and five had been destroyed.

Dubbed the Butts Fire, the blaze broke out about noon on Tuesday in an unincorporated area called Pope Valley, northwest of Lake Berryessa (map).

It spread quickly and was at 3,800 acres — or nearly 6 square miles — by Wednesday afternoon, up from 3,200 acres in the morning. It was 30 percent contained.

The blaze had died down thanks to an inversion layer earlier in the day but as weather warmed, the fire grew, according to Cal Fire.

A mandatory evacuation order was in effect for the area.

About 1,000 firefighters and other officials were working the blaze, which comes amid an extreme drought and steep increase in the number of California wildfires.

The fire was not threatening the region’s famed vineyards, the Los Angeles Times reported.