Forecasting is based on the rise and fall of the barometric pressure.
Units with a History Bar Graph - The bar graph shows the recorded air pressure over the past few hours. The graph is read from Left to Right with oldest data on the left. If the bars are rising (higher on the right than the left) then the air pressure has a rising trend, and the weather should improve. Below the pressure history bars you will see a time frame for that column. If the bars are lowering (higher on the left than the right) then the air pressure has a lowering trend, and the weather will likely worsen.
Units that Display the Current Air Pressure Numbers – you can get the pressure number off your local weather forecast or from one of the national weather forecast websites. This is the only forecast value that can be calibrated.
The Forecast icons will change with the rise and fall of the barometric pressure. The picture you see on your unit is not a picture of what you are seeing out your window, but is instead, a prediction of what is going to be happening about 6-12 hours from now. These predictions should become more accurate over time. In some units, you may also see a weather tendency arrow, indicating if the weather is expected to improve, remain the same, or worsen. The number of different forecast icons your station has varies with the model. These predictions are correct about 60-80% of the time, the same as your local weather forecast.
These forecasts are accurate up to approximately 6000 feet or below. Above 6000 feet there are fewer changes to the pressure so the forecast icons will not change as easily.