Whenever you see a weather report with rain (or snow) in the forecast, it gives the chance of precipitation. How do they get that number, exactly?
Meteorologists are always collecting data on weather patterns, including the storms that develop across the country. Some of these storms produce precipitation, some don't, and there is record of all of it. When a new storm is developing, scientists match the incoming data with data of past storms and see how many times precipitation occurred given those parameters.
For instance, let's say you turn on the TV to see a report that there will be a 40% chance of rain tomorrow. This means that when clouds formed the same way in the past as they are now, it rained 40% of the time.
Since this number is calculated given previous records, the more data available, the more accurate the predictions can be.