The Department of Homeland Security is unveiling its new two-level terror alert system goes into effect today, replacing the color-coded version that has been in effect since 9/11.

DHS Secretary Janet Napolitano will announce the new system at Grand Central Terminal in New York City. It will be in place by next week.

DHS describes the new threat levels:

Elevated: Warns of a credible terrorist threat against the United States

Imminent: Warns of a credible, specific, and impending terrorist threat against the United States.

"In some cases, alerts will be sent directly to law enforcement or affected areas of the private sector, while in others, alerts will be issued more broadly to the American people through both official and media channels," DHS says on its website.

The new alerts will include maps or other graphics showing the areas affected and, unlike the color-coded system, will include a specific date when the alert expires.

The alerts will be issued through normal news channels, as well as via the DHS web page, by email for those who sign up, and via Facebook and Twitter.