A look at the radar this morning across France and Belgium shows a line of showers and thunderstorms moving east. This will affect Spa-Francorchamps through the morning. It will likely produce between a tenth and a quarter inch of rain, or between 3 and 6 mm. The chance of rain is expected to taper off in the late morning and early afternoon, and I expect all the rain to be out of the area by around 2 pm. This should give dry conditions for the start of the race at 4:30 pm.
There will be quite a bit of cloud cover remaining overhead through the overnight hours. There will be a full moon, but it likely won't be seen very much. The cloud cover should start to diminish late Sunday morning and Sunday afternoon, with partly cloudy skies by the end of the race.