The test will take place at 11 a.m., last approximately 30 seconds, and will be aired on every television and radio station, including cable TV and satellite radio.
The message people will see and hear on television and radio will be similar to that of regular regional testing systems with the familiar phrase, "this is a test."
It was originally slated to last nearly three minutes.
FEMA spokesman Paul Luke said the change was made because the longer the test runs, the greater the likelihood that some cable systems in place around the country might not recognize the signal as a test and send out an actual emergency message.
"It might create undue confusion," Luke said.
The new EAS would allow the president to address the nation during extreme emergencies, FEMA officials said.
The test is to help "determine the reliability of the system and its effectiveness in notifying the public of emergencies and potential dangers nationally and regionally," FEMA officials said on the agency's website.