Weather forecasting and uncertainty
There will always be some uncertainty in a weather forecast. This is partly due to the fact that we do not know the state of the atmosphere right now exactly, that the numerical models are a simplified description of reality, and that the atmosphere is a chaotic system that is difficult to predict. This means that even if the current weather forecasts are generally good, especially the first days of the forecast period, the weather will not always be as exactly as forecast.
Scientific studies have shown that most people are well aware that weather forecasts are uncertain, and that they are more uncertain further into the forecast period. This is true on a general basis, but there are situations where the weather forecast for today may be uncertain (eg thunderstorms in the summer), while we are fairly sure of high pressure and sun a week ahead. Communication of uncertainty is therefore an important part of the weather forecast, whether it takes place orally or in writing through the way meteorologists word themselves for example on TV or Twitter, numerically based on probability forecast ("30 percent chance of rain"), or various graphical representations.
Uncertainty on Yr
On Yr, the uncertainty in the warning is communicated in several different ways.
In the tables you can see intervals that indicate forecast precipitation (in blue in the table below). The first value is the minimum amount of precipitation expected, while the last value in the interval indicates the largest amount of precipitation that can be expected during that time period. If the interval is narrow (eg 1-2 mm), then the warning is more certain than if the interval is wide (eg 5-15 mm).
The precipitation forecast is also plotted graphically. In the graph below, the dark blue color illustrates the expected amount of precipitation, while the light blue color indicates the uncertainty in how much precipitation can occur. Similar to the interval, a high light blue bar will indicate a warning that is more uncertain than a low light blue bar.
If you go to the tab "Details" for a given location, you will find a column that indicates the chance of precipitation in percent, for each hour. Similarly, on a weather forecast as a PDF, you will find a column that indicates the chance of precipitation in percent, for a given day. The higher the percentage, the greater the chance of precipitation.
What should you do when the forecast is uncertain?
No matter what form the uncertainty is presented in, an uncertain forecast means that it may change. In that case, you may want to stay regularly updated on developments before laying plans. If you do not have the opportunity to keep updated on the weather forecast, but must use the information you have there and then, it may be a good idea to keep in mind that the weather forecast may change and, for example, dress accordingly.