In an effort to warn the public about the path of a dangerous storm, smartphone users will soon begin receiving text messages about severe weather from a government system that can send a warning to mobile devices.

USA Today reported that the new Wireless Emergency Alerts system gives the National Weather Service a new way to warn Americans about bad weather, even if they are away from a television or radio. When a warning is issued for a specific county, a message of no more than 90 characters will cause capable smartphones in that area to sound a special tone and vibrate. Those who prefer not to get the warnings can chose to opt out of the system.

"These alerts will make sure people are aware of any impending danger and provide them with the information needed so they can be safe until the threat is over," said Amy Storey, spokeswoman for CTIA-The Wireless Association, a trade group that helped set up the weather alert system.

Currently, the system does not work with all smartphones or in all areas. It is part of a broad alert network that the Federal Emergency Management Agency launched in April that can send public-safety warnings from the President as well as state and local governments. 

The weather service estimates that over 90 percent of the messages within this alert system will be about storms warnings including tornadoes, hurricanes, typhoons, tsunamis, flash floods, extreme winds, blizzards and ice and dust storms. According to weather service spokesman Susan Buchanan, designers limited the messages to warnings, not watches, and excluded severe thunderstorm warnings because they were concerned about overloading users with too much information.

Wireless carriers serving about 97 percent of U.S. subscribers are participating including AT&T Inc., Verizon Wireless, Sprint Nextel Corp. and T-Mobile USA. Sprint, Verizon and T-Mobile who say it offers the service nationwide, while AT&T offers it in New York City, Washington, D.C., and Portland.

Government officials don’t know exactly how many capable devices are already in use, but Damon Penn, assistant administrator for national continuity programs at FEMA, thinks the number is in the millions. FEMA's system carries three kinds of alerts including presidential alerts, imminent-threat alerts and Amber Alerts. Phone users can opt out of the imminent threat and Amber Alerts, usually just by changing their settings, but they can't opt out of presidential alerts. Twenty-eight state or local emergency management agencies in a dozen states are authorized to send imminent-threat alerts.

According to Sprint spokeswoman Crystal Davis, most Sprint smartphones now in use can receive the alerts thanks to recent automatic software upgrades.

There is question as to when Apple iPhone users will be able to receive these alerts. Buchanan said iPhones are supposed to join the system in the fall, but she didn't know if that means only new iPhones, or if software upgrades will make older models capable too.

In Minneapolis, some smartphone users were open to receiving the warnings. "I spend enough time reading junk on my phone that's of no real benefit to me. I might as well read something useful," said Bob Burns, a Minnetonka attorney.

Dan Smith, a photographer from Reston, Va., said he was worried that the messages could become intrusive. "It's like e-mail. It used to be you only got stuff you wanted. Now you get 20 junk messages for every good one," he said.

The system doesn't use the satellite-based global positioning system to determine a phone's location. Instead, carriers send an alert from every cell tower in the affected county and capable smartphones pick it up. That feature sets the system apart from weather apps that deliver information based on users' ZIP code, but don't automatically update locations when users travel.