Depends on the system

The “Yo dog, a tidal wave is hitting right now. That is why it looks like it is super duper low tide.” alert? Yeah, you’ll get that. Whether that is time to meaningfully act or not depends.

But most regions have additional services you can and should sign up for that will give the early warnings. So “Seismic activity detected a mile or three off shore or however tidal waves work. We are monitoring the situation” is a good indicator of “maybe today is not the day I go to the beach”. And “We are no longer monitoring the situation. Please proceed as normal” is a sign that maybe you do want to go for swimmies after all.

Same with other disasters. I live in a region that has a lot of wildfires. We tend to get the early warnings and even the “We might say to evacuate in a few days” through a different service. We get the “Get the fuck out of town immediately” alerts through the normal emergency alert system.