From The GPRO Wiki
Understanding Weather and the Weather Forecast are fundamental to planning your race strategy.
- The weather for Q1 and Q2 will remain static for the entire qualification period, so there is no point in waiting and hoping that the rain will stop before you qualify.
- The weather for Q2 will also be the exact weather (temperature, humidity & rain or dry) as for the start of the race.
- The 30 minute intervals shown in the weather forecast relate to the leading driver in the Elite race.
- All weather changes (temperature, humidity, rain) happen on the same lap in all races, not dependent on time. If the Elite race has rain starting on lap 24, it will start in all races on lap 24, even if the race time is 10 minutes later than the Elite race.
- In the unlikely event that the race continues past the 2 hours mark, the weather in the last quarter of the weather forecast will apply.
0% rain probability means you can be sure that it won't start raining during the period. If it is already raining, you can be sure that it will stop raining during the period, but you don't know when it will happen (it can stop after one minute or after 29 minutes).
100% rain probability means that rain won't stop during the period. If it is dry at the start of the period, you can be sure it will start raining during the period - but again you don't know when that will happen.
Weather and Car Setup
It is likely your driver would prefer a different car setup when driving in the wet, opposed to driving under dry conditions.
A little thought regarding how cars would need to be changed in real life should help give you an idea of what parts would need to be changed in what ways.
There are also methods in-game which should help you work out this puzzle more accurately.