AMBER ALERT is never issued on a nationwide basis.
42-page 2015 AMBER ALERT report
Detailed information, including charts and map, at this link:
https://www.amberalert.gov/pdfs/2015AMBERAlertReport.pdf
Some pertinent points on page 6:
Once an AMBER Alert is activated, the alert may be issued in another state/territory at the request of the originating states AMBER Alert coordinator, thus creating a multistate/territory activation.
Although an AMBER Alert can be issued in multiple states or territories, it is
never issued on a nationwide basis.
AMBER Alerts are geographically targeted based on law enforcements investigation.
And page 9:
Multistate/territory AMBER Alerts
When an AMBER Alert is issued an abductor may take the child outside the jurisdiction of the issuing law enforcement authority. In some cases the AMBER Alert coordinator in the state/territory where the AMBER Alert originated may request an AMBER Alert be extended into another state/territory.