Washington • Beginning at 2:18 p.m. Eastern time Wednesday, Americans across the country will be interrupted by an urgent notification on their cellphones.

"Presidential Alert," the message will read. "THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed."

As it's blasted out by cell towers nationwide over a 30-minute period, the message is expected to reach some 225 million people in an unprecedented federal exercise.

Many cellphone users may be familiar with alerts they receive as warnings about flash flooding or missing children. But those emergency weather alerts and AMBER alerts can only be sent on a regional or state-wide basis, not nationally and all at once.

Wednesday's test of the Wireless Emergency Alert system will mark the first time that emergency management officials will have used the nationwide alerting capabilities reserved for the office of the president.

Unlike other types of emergency alerts, the presidential alert is designed to let the White House inform the entire country almost instantly of grave public emergencies, such as a terrorist attack or an invasion, according to the Federal Emergency Management Agency. And while Americans can choose not to receive weather and AMBER alerts, they cannot opt out of presidential ones.

Despite its name, Wednesday’s presidential alert will not be issued by President Donald Trump directly. But in general, the process calls for the president — or his representative — to authorize FEMA to send an alert on the White House’s behalf.

Before it shows up on cellphones, the alert must be routed to wireless carriers such as AT&T and Verizon through an online system called the Integrated Public Alert and Warning System, or IPAWS.

The cellphone alert will be accompanied by a similar test of the Emergency Alert System, an older system that handles radio and television.

The federal government is required by law to test its alerting systems every three years. FEMA last tested the EAS in 2016 and 2017, and the first wireless emergency alerts were sent in 2012.

Not all cellphones may receive Wednesday's alert, however. You won't get it if you aren't in range of a cell tower, for instance, or if your phone is switched off. But that's expected to be the case for only about a quarter of the nation's cellphones.

That 3 out of 4 phones will likely get the warning is a reflection of our always-on, technologically connected times. Yet the same technology that has helped connect millions of people also has the potential to cause chaos at Internet speeds.

In January, America’s emergency management technologies made headlines when a state-level official mistakenly sent an alert to Hawaii residents warning them of an inbound ballistic missile attack. The report was false, but it sent Hawaiians scrambling for cover, seeking shelter in gymnasiums, mountain bunkers and even in manholes.

A subsequent investigation by the Federal Communications Commission found that the staffer responsible for the message had thought an attack was truly underway, having misinterpreted a drill.

FEMA officials have said the Hawaii incident played no role in the scheduling of Wednesday’s test.