Your Homepage was designed to be the first thing that your website visitors see. HOWEVER, in times of emergencies you may deicide that you want all traffic to be pushed to an interior page that has been built for a specific purpose "Evacuation Warning" . 

1. Build and Publish the "Emergency Home page" just like you would any other page
2. Set that page as "Emergency Home Page" in the Configurations > System Variables
3. When the Emergency is over, "Expire" the Emergency Page