Hi Rossi and welcome to the forums!
Unfortunately August comes into the rainy season for Chiang Mai (generally July to October).
That was the bad news out of the way, the good news is that you should find it affects your game very little and the cooler temperatures make for a more pleasant experience.
- It is often possible to wait out downpours in a nearby drinks stop (most will last for around 30 mins).
- Most courses in Chiang Mai drain fast.
- The rain is warm!

Try to leave some flexibility in your schedule as there can be bad days.
Wishing you a pleasant trip and excellent weather,
Benzil