The weather pattern over western Europe during the middle of the week will feature an area of low pressure that will be drifting eastward. Belgium will still be under the influence of this system on Thursday, so there will likely be numerous showers around the track for the opening practice sessions. With wet and cloudy conditions, temperatures will only be in the 40s on Thursday. Most of the rain showers should be fairly light, and I expect that rainfall amounts on Thursday will be between one and two tenths of an inch (2.5 to 5 mm).
The good news is that conditions should be improving through the weekend. The low pressure system will be tracking away on Friday, taking any chance of rain with it. Clouds will linger into Friday morning, but there should be at least a little sun by the afternoon which will bring temperatures up into the 50s. There will be a stationary front across central France over the weekend, with high pressure over England. I expect that any rain will stay to the south of Belgium near that front, and the high pressure over England will have the greater influence on the weekend's weather. So my forecast for the weekend will be dry and partly cloudy, with highs in the upper 50s. However, there is some uncertainty about the positioning of the front, and if it ends up being farther north, I may need to add a chance of rain to the weekend with later forecast updates. Let's hope that doesn't happen.