See this LA Times article, Emergency alerts from Santa Barbara County didn't go out until after mudslides began in Montecito. Claire Rubin shared this link.
The article's title is not quite right, in that warnings were issued of the "possibility" for mudslides based on rain projections. In the world we live in today, with people's attention span being very short and the fact that people get their news and information from a variety of sources, we have to use every warning tool in the toolbox.
Here in Washington state, there is another potential slide that is being watched carefully and people have been moved out of their homes. See Rattlesnake Ridge landslide: Yakima County declares disaster. Some say it is only a question of time for when a slide occurs. And what if all movement stops in the next few weeks? Will people be allowed back into their homes?
We don't have the luxury of knowing when hazards will strike.
I suggest you re-examine your alerting and warning procedures for your agency. It needs to be clear who can/will issue a warning, by name. What types of events will trigger a warning? What systems will be used to issue the warning? Then, you need to practice, practice, practice! Back in my King County days, the duty officer on duty for the week would do a practice (non-broadcast) Emergency Alert System (EAS) test. These were meant to ensure the equipment was working and people's mechanical skills for issuing the warning were rock solid.