Something never achieved by
meterologists.
But seriously... 100% certainty is the knowledge that you know something and you are not wrong, you know this, despite the fact that there is some aspect of the system of which you are not sure (such as the future) but you know this anyway. People who have no doubts do not use the phrase "100% certain." It more often crops up in places such as:
"Are you sure?"
"I'm 100% certain."
Generally, I hear some fear behind that phrase, some worry that someone somewhere is not going to believe the speaker. It isn't enough to report that they are certain, that is, that their knowledge is sure. No, they have to report the percent certainty, as though some roll of the dice and random chance was still involved and yet was guaranteed to come up their way, as if the dice were loaded.
The use of "100% certain" in conversation is the surest sign that everyone involved should double check all their facts and assumptions.