A 100% chance of rain means that the chance of rain is over 99.5%. That is, typically, the probability is rounded to the nearest integer. It is not possible for the probability of rain for tomorrow to be exactly 100%, since there is a non-zero probability that the Earth could be destroyed today. One exception is when it is raining, then the probability of rain presently is 100%.

A 100% chance of rain means that the chance of rain is over 99.5%. That is, typically, the probability is rounded to the nearest integer. It is not possible for the probability of rain for tomorrow to be exactly 100%, since there is a non-zero probability that the Earth could be destroyed today. But, 99% would imply the chance is less than 99.5%, and would be wrong if the chances were 99.9%. One exception is when it is raining, then the probability of rain presently is 100%.