Looking for a mod to popup a admin specified message to all users online.

[City Builder]

Hello,
If a mod like this is already available, accept my apologies and please direct me to it.

I would like a mod that will allow the administrator of my forum to be able to enter in a simple text message in the administration panel somewhere that would allow cause anyone who is on my website to receive a popup message when they move from the page they are on to the next page.

Basically the way that I want to use this, is to broadcast a message to anybody on my site and be able to say something like, "This site will be going offline in 10 minutes for scheduled maintenance.  Please log off now.  The site will be available to use again in 30 minutes".

Is there a mod like this already?  If not is there anyone interested in writing it up?

I hope I posted this to the right forum, if not please accept my apologies in advance.

Thank you,