I attended a lecture a few years back at my university by someone(cant remember who) from Weather SA. He pointed out that the 30% rain fall area they show on the news is not as random as most people think. That 30% means they have a 99% confidence based on the model that it would rain over 30% of the given area. But it is not random, because of the time window they give. He showed a simulation with a storm moving from right to left over the area. He said the models could predict that the storm would reach critical mass and start raining anytime in the next 10 hours. If it started to rain in the first hour or so the most right hand side of the given area would receive rain whereas if it reached critical mass at 9 hours the most left hand side of the area would receive rain, based on the size and speed they know the storm would only last long enough to precipitate about 30% of the area.