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From the desert along the Mexican border to the central California coast, a distance of more than 300 miles, cellphones squawked and shook all at once on Wednesday night, and displayed a simple, serious message.

“Strong winds overnight creating extreme fire danger,” it said. “Stay alert. Listen to authorities.”

The alert went to most smartphones across seven counties that are home to more than 22 million people — by far California’s largest use of a disaster warning system created by Congress and activated in 2012.

“It is, for us, an unprecedented move,” said Kelly B. Huston, the deputy director of the state Office of Emergency Services.

The same alert system was not used in October, when fires swept through Napa and Sonoma Counties near San Francisco, destroying thousands of homes and killing dozens of people. Many survivors of those fires complained that they had little or no warning of the disaster.

That experience was uppermost in the minds of state officials who decided to send the message at 8 p.m. Pacific time on Wednesday, Mr. Huston said, though he would not second-guess the decisions made two months earlier.