Display warning message box IF a post exists in x category

I want to allow a client to be able to display “Breaking News: School closed tomorrow b/c of bad weather” on their WP-powered home page, triggered when they write a post to the category named “alert”.

I would guess I need to use a conditional tag of some sort, that “asks”, If there exists a post in the category named “alert”, then write (“echo”?) the following HTML, enclosing the_content. This would lead to an “alert box” near the top of the home page.