The UK Met Office has been making use of the remains of Hurricane Bertha to publicise the probabilistic aspects of their forecast. In particular, they have been publishing probabilities for various tracks of the storm across the UK.
Why is the forecast being made in terms of probabilities?
The classical idea of a forecast is a prediction of the precise value of something (e.g., temperature) at a particular point in space and time. This is referred to as a deterministic forecast. In a probabilistic forecast, this is expressed as a probability instead. For example, a forecast could say that there is a 60% chance that the temperature will fall between 21 and 25 degrees Celsius, a 20% chance that it will be below this range, and a 20% chance that it will be above.