Maintenance countdown warning message – modal pop-up window? is it a good practice for such a important message

I'm currently designing a Maintainance countdown message. I think to create it as a pop-out modal window when the user logs-in and afterward place the countdown with a short header on a "sticky" position on the bottom of the screen / or under navigation. Does someone have any experience designing it? Do you believe that the pop-out modal window is appropriate in this case?