The Announcement Notification plugin allows staff members to create a fake notification that sits on top of the existing, legitimate ones.
You can choose what text shows in the notification, what background and side-highlight color it should have, and what the text in the "NEW" icon should say.
Members can click "clear notification" to remove it from their list of notifications, or "mark as read" to prevent it from creating or incrementing their notification bubble. Clicking the [?] button will pop open a dialog explaining that the notification is produced by a plugin.
This plugin uses a key to keep track of whether or not a member has chosen to clear the notification. With that in mind, whenever you need to put up a new notification, you should configure it in the plugin settings and then go to Admin > Plugins > Manage > Keys tab and clear out the key data for the Announcement Notification plugin (by clicking "Clear" next to the appropriate key), so the notification shows for everybody on the forum again. If you don't clear out this data this way, the users that have chosen to clear a notification in the past will not receive your new notification.