The alert came in to thousands of phones just after 10:30 p.m. Wednesday: The National Weather Service had issued a flash flood warning for the Washington region.

But for most people, the message arrived unbidden, because they had never signed up to receive emergency alerts.

"I was on the computer and then it went off, and I didn't even know what was going on," said Jim Van Buren, of Springfield. "It definitely frightened me."

Chalk it up to the federal government and wireless companies.

The brief message that put the area on notice was thanks to a new program that allows authorities to "push" free alerts to people with phones equipped with a specific type of software.

"More and more people are turning to modern technology, and so many people in society now carry cellphones," said Susan Buchanan, of the National Weather Service. "What this does is deliver weather warnings to them no matter where they are."

Customers -- who are automatically signed up for the service when they purchase a phone with the necessary technology -- can opt out of most messages by contacting their wireless providers. But customers won't be able to avoid "presidential alerts," which authorities say will be used only for national emergencies.

Most alerts will come from forecasters, who said they will send messages for weather emergencies like tornado and flash flood warnings. They won't notify customers of watches or thunderstorm warnings because they are so frequent.

"It's just intended to be a bell ringer to say, 'You're in an area with a weather warning in effect,' " Buchanan said.

State and local authorities can also send messages, as can the National Center for Missing and Exploited Children.

And to keep information flowing in crisis situations, the system uses a type of software that allows the alerts to go through even when networks are clogged.

Officials also say the technology will allow them to send alerts that are specific to where a person is with their phone, meaning an area resident visiting Alabama could receive a warning about a tornado in Athens, Ala., but not a flood in Alexandria.

"You'll get the alert for wherever you're located in the moment," Buchanan said. "Your phone will receive the signal from the nearest radio transmitter, and the warning will be in effect for wherever you are."

Although wireless providers serving about 97 percent of the country's users have agreed to participate, plenty of phones don't have the technology to receive the alerts.

The compatible phones vary among providers, but they include the two newest iPhone models on Verizon's network, four BlackBerry products offered by AT&T and several of Sprint's Samsung phones. Some companies also offer software upgrades so customers can receive the alerts.