Notification when a value exceeds a limit

Hi,

Is it possible to create some kind of a pop-up notification etc. if a value in the spreadsheet exceeds a limit?

For example:

Cell A1 contains the formula SUM(B1:B5)
Cell A2 contains the value 100.

If A1 exceeds A2 you get a notification that this has occured. I'm not talking about conditional formatting, but something that hits you right in the face when you're working on a big sheet and you can't necessarily keep a close eye on A1 all the time...

Anyone?