I've heard all the weather jokes. “You can be right 50% of the time and still keep your job.” Or, “How can you tell a weatherman is lying… his lips are moving.” And “50% chance of rain? So it's either going to rain or it's not.”
There is a common misconception about the chance of precipitation. A 50% chance of rain does not mean a 50% chance that it will not rain. In a weather forecast, the percentage you see by precipitation indicates the likelihood of measurable precipitation occurring in a specific area during a forecast period.
A 50% chance means half of a gridded forecast area will receive measurable precipitation over the course of the next 12 hours. If there is a 100% chance of precipitation, the entire gridded forecast area will see that rain or snow. If there is only a 20% chance, 1 in 5 locations in that forecast area will see isolated precipitation.
The probability percentage is comprised of two main details - the confidence level in precipitation developing and how widespread or spotty it may be.
Precipitation chances are fluid and may change with regard to a storm system's track. The chance of precipitation does not indicate the amount of rain or how long it will last; it simply reflects the likelihood in that gridded forecast area.
