10 day forecasts are useless in general. They basically predict persistence - ie if it is hot and sunny likely to stay that way, if cool then stay cool etc.
5 day forecasts are a bit better - but it pretty much depends on the type of weather - ie much easier to predict high pressure, hot and still weather for a few days (because highs persist for up to 10 days normally).
Better to find out what August weather in Paris is normally like. I would say probably hot, probably humid, maybe some rain, probably some thunderstorms (from experience of Paris in July, and knowing climate for that area). If it is thundery it tends to be in the afternoon and evening as a result of daytime heating. Also sometimes you get storms moving up from the south (just like we do). Storms can be quite spectacular on the continent (at least in Rhine area of GErmany/France).
Hope that helps.