California's Caldor Fire is creeping closer to the Nevada border

The Caldor Fire, having already scorched more than 204,000 acres in Northern California, is moving east, with the head of the blaze getting close to the Nevada border.

There are more than 4,200 firefighters battling the Caldor Fire, and Gov. Gavin Newsom (D) said it is the state's "No. 1 priority." Crews worked overnight and Wednesday to keep the flames away from South Lake Tahoe, a popular resort town, but they are also concerned about the fire — which is threatening nearly 35,000 structures — heading south toward the community of Markleeville, the Los Angeles Times reports.

Craig Clements, a professor of meteorology and director of the Wildfire Interdisciplinary Research Center at San Jose State University, told the Times a computer model his team is using shows the fire reaching Nevada either late Wednesday night or early Thursday morning. The blaze is burning through an area with bone-dry vegetation amid windy conditions, making it "a difficult fire to forecast," Clements said.

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Firefighters are dealing with strong winds that are picking up embers, and fire behavior analyst Steven Volmer told the Times that because of the dry conditions, there's an "extremely high" probability an ember could spark a new blaze. The Caldor Fire has been burning for 18 days, with some exhausted firefighters working it the entire time. As of Wednesday morning, it was only 20 percent contained.