Hello,
The weather you see in iGP is based on real-time data courtesy of weather provider, Open Weather. 
Link to the OpenWeather site - 
https://openweathermap.org/
Once on Openweather, input your race location (eg, Silverstone) and you will get the weather forecast. Some race circuit locations are more difficult to find, however a quick Google search should get you relatively close.
NOTE: There is a slight delay (~15 minutes iirc) between what you see on Openweather and what happens in the game. 
Additionally, relying on weather forecasts can be risky.