Background information: The issue often comes up whether forecasts should be based on probability or based on what the forecaster thinks will happen.
Reasons to give forecasts using probability:
1. It is a fact that it is impossible to predict the future with 100% certainty and this uncertainty increases with time.
2. You may not be believed if you make your forecasts with certainty because forecasts are going to be wrong from time to time. The general public will not forget those wrong forecasts.
3. Model data is not very useful beyond 3 days. Because of this forecasts can not have any kind of certainty especially in the extended forecast.
4. There are less busted forecasts using this method. The public hopefully will also understand that there is uncertainty in weather predictions.
5. Other meteorologists' will find you more believable and honest.
Reasons to state certainty in the forecast:
1. People need to plan for what they need to do. People do not like to be unsure of what will happen.
2. To be a great forecaster and a famous forecaster you need to be able to forecast with certainty and be correct.
3. You are more likely to be believed if you are confident in what will happen.
4. A great forecaster needs to be better than the models and MOS.
5. The general public doesn't generally think in percentages. They think it will either happen or it will not. For example a 50% chance of rain sounds like you do not know at all if it will rain or not.