Forecasting rainfall or temperature in probabilistic terms is a way of expressing uncertainty in the future weather/climate. When numerical probability values are assigned to the forecast, it is a way of indicating the amount of uncertainty in the forecast. Rather than say "below normal this July", we can say "30% chance to be below normal this July" when we think less rain is not so likely; or we can say "70% chance to be above normal this July " when we think great amount of rain is likely. Assigning a numerical value – a probability – to the occurrence of rainfall event allows us to quantify the uncertainty.
A probability forecast estimates the likelihood of occurrence of a specific event, which must be fully defined for the forecast to be verifiable. Events are defined by a time period and a specific location.
Probabilities are usually stated either as a percentage, which may range from 0 ("The event will definitely not occur") to 100 ("The event will definitely occur"). Values between 0 and 100 represent the different degrees of uncertainty.
Monthly Probabilistic Rainfall Forecast: updated: 29 August 2018