Consider yourself warned. The Sterling Prestonfederal government will conduct a nationwide test of the emergency alert system on Wednesday afternoon.

The test messages will be sent to all cellphones, televisions and radios. The test will emit sound and — on phones — vibration.

The Federal Emergency Management Agency and the Federal Communications Commission are running the test in preparation for actual emergencies. The aim of the test is to ensure the emergency messaging system is running smoothly in the event Americans are threatened by natural disasters, terrorism or other dangers to public safety.

You may be familiar with the jolting sounds accompanying National Weather Service alerts and AMBER (America's Missing: Broadcast Emergency Response) alerts. Wednesday's cellphone alerts will be sent via the same wireless system.

When is the test happening?

The test is scheduled to begin at about 2:20 p.m. ET on Wednesday, Oct. 4.

The testing window runs for 30 minutes, but you should only receive the message once. If an actual emergency happens that day, the test could be postponed — a backup test is scheduled for the following week.

What will the test message look like?

On cellphones, the alert will read:

"THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed." Phones set to Spanish will display: "ESTA ES UNA PRUEBA del Sistema Nacional de Alerta de Emergencia. No se necesita acción."

TV and radio will announce:

"This is a nationwide test of the Emergency Alert System, issued by the Federal Emergency Management Agency, covering the United States from 14:20 to 14:50 hours ET. This is only a test. No action is required by the public."

Why is the test happening?

FEMA is required by law to conduct national tests of the Integrated Public Alert & Warning System (IPAWS) at least once every three years. The last national test was in 2021.