The only way to set the weater for the race is use this method:
Look into GPxPatch - GPxSet - Sessions settings... - Weather Custom - "Record" (to learn a certain weater seed) and then run the game, proceed into a Quick Race session, learn what the weather is, close the game, and then you can select "Use" to use the very same weather conditions on other tracks. In this way you'll know the exact time when rain begins and stops for this certain weather seed. This is the only way to predict weather in some way.
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For a list of EVERY download for GP4, look here:[docs.google.com]