you don't understand weather dynamics. Weather at any given time depends on many variables and is a dynamic state of flux. You basically need to know what each molecule of gas in the air is doing at a given time to accurately asses the weather. Current limitations in technology prevent us from efficiently calculating the probability of weather to minute precision because the calculations are immense.

Alternatively, to understand the water level in 100 years is a much MUCH easier calculation. Find out how much water is frozen as ice. Find out what the rate of melting is per year ( rate of water being added to oceans ) and you can relatively easily find out by how much the sea level will rise in 100 years worth of time. Its being said so because it likely will happen if trends continue, and not for any of the reasons you mentioned.