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A flash flood warning was issued in the mountains of San Diego Monday afternoon.
At about 3 p.m., National Weather Service meteorologists detected thunderstorms in the areas around Julian.
Many residents received a text message warning them of the flood dangers.
Later, San Diego County officials announced the startling tone and message was a national emergency alert system, never before used in San Diego.
The Commercial Mobile Alert System (CMAS) was activated at about 4 p.m. to most wireless carriers. The system is operated by the Federal Emergency Management Agency and the Federal Communications Commission.
“When a disaster occurs, we will want to reach residents through this federal mass notification system,” said Holly Crawford, director of the County Office of Emergency System in a press release. “So it’s important that the messages are targeted to the right population.”
The office hopes to target the alerts geographically if a warning only affects one area.
The flood warning remained in effect into Monday evening, the National Weather Service said.
Foothills beneath the mountains experienced floods following the rainfall, trained storm spotters reported.
The storm was capable of producing damaging winds of up to 60 MPH as well.