MINNEAPOLIS —  Millions of smartphone users will soon begin receiving text messages about severe weather from a sophisticated government system that can send a blanket warning to mobile devices in the path of a dangerous storm.

The new Wireless Emergency Alerts system gives the National Weather Service a new way to warn Americans about menacing weather, even if they are nowhere near a television, radio or storm sirens.

Beginning Thursday, the system will notify people about approaching tornadoes, hurricanes, blizzards and other threats. When a warning is issued for a specific county, a message of no more than 90 characters will cause late-model smartphones in that area to sound a special tone and vibrate.

Users do not have to sign up for the service or pay for the text message. And people who prefer not to get the warnings can 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, an industry trade group that helped set up the system.

The system does not yet work with all smartphones or in all areas. It is part of a broader alert network the Federal Emergency Management Agency launched in April that can also send public-safety warnings from the president and participating state and local governments. But the weather service estimates that more than 90 percent of the messages will be about storms.

The weather warnings will include tornadoes, hurricanes, typhoons, tsunamis, flash floods, extreme winds, blizzards and ice and dust storms. Designers were concerned about overloading users with too much information, so they deliberately limited the messages to warnings, not watches, and excluded severe thunderstorm warnings, weather service spokeswoman Susan Buchanan said.

Wireless carriers serving almost 97 percent of U.S. subscribers have agreed to participate, including the biggest nationwide companies — AT&T Inc., Verizon Wireless, Sprint Nextel Corp. and T-Mobile USA. Each of the four offers at least some phones capable of receiving emergency alerts, with more on the way.

Sprint, Verizon and T-Mobile say they offer the service nationwide. AT&T only offers it in New York City, Washington, D.C., and Portland, Ore., at the moment. Spokesman Michael Balmoris said the company will add additional markets over time but declined to say which ones or when.

Government officials don’t have a good handle on exactly how many capable devices are already in use, but Damon Penn, assistant administrator for national continuity programs at FEMA, said the number is probably in the millions.

He said smartphone users should check with their carriers to find out whether service is available and if their device is able to use it. He said many people own phones equipped to get the new alerts but don’t know it yet.

Sprint spokeswoman Crystal Davis said most Sprint smartphones now in use can receive the alerts thanks to recent automatic software upgrades. All new models will be equipped, as will all new tablet devices.

One unanswered question is when the legions of Apple iPhone users will be able to receive 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. Representatives of Apple Inc., which is highly secretive about its product upgrades, did not respond to several messages seeking details.

FEMA’s system carries three kinds of alerts: presidential alerts, which might deal with national security information such as terrorist attacks; imminent-threat alerts, which include weather warnings as well as public-safety messages from local authorities; and Amber Alerts issued by law enforcement agencies for kidnapped children.

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 about a dozen states are authorized to send imminent-threat alerts. Eighty-three others are in the process of getting certified.

Agencies have different ideas for the system. Minnesota is considering using it for chemical spills or nuclear accidents. In southern Florida’s Miami-Dade County, it might convey hurricane evacuation information.

Curt Sommerhoff, Miami-Dade’s director of emergency management, said the alerts will permit authorities to distribute urgent information to people in danger “whether you’re a resident, employee or visitor.”

On the streets of downtown Minneapolis, a couple of smartphone users were open to receiving the unsolicited weather and other 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 sitting at a sidewalk cafe as he worked on both an iPhone and an iPad. “It’s putting technology to use for the public good.”

Dan Smith, a photographer from Reston, Va., who was in Minneapolis for a convention, said he was worried that the messages could became intrusive.

“It’s like email. It used to be you only got stuff you wanted. Now you get 20 junk messages for every good one,” Smith said.

The system doesn’t use the satellite-based global positioning system to determine a phone’s location. Participating carriers just send an alert out from every cell tower in the affected county, and capable smartphones pick it up.

So if a user from Minneapolis travels to Kansas City, Mo., that person would get local warnings for Kansas City, not their home city. That feature sets the system apart from weather apps that deliver information based on users’ ZIP code but don’t automatically update their locations when users travel.

Greg Carbin, the warning coordination meteorologist at the national Storm Prediction Center in Norman, Okla, said in a typical year most smartphone users will probably receive relatively few weather alerts.

“Even in those areas of the country where there’s a lot of severe weather, the frequency with which you would be alerted is pretty low,” Carbin said.

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Associated Press Writer Patrick Condon contributed to this report.

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Online:

NOAA FAQs on Wireless Emergency Alerts: http://www.nws.noaa.gov/com/weatherreadynation/wea.html

FEMA alert system site:  http://www.fema.gov/emergency/ipaws/index.shtm