The forecast for Silverstone has been a bit of a moving target, with periods of rain shifting from one day to the next. My original forecast had Sunday staying dry, but now it looks like a weak low pressure trough will be moving across England from the northwest to southeast. It doesn't appear to be a very strong system that will bring a large amount of rain to the track, but it will create some wet conditions through most of the race. If you're going to the race, be sure to bring some rain gear along.
There could be some light sprinkles around in the morning for the support races, but I think the bulk of the rain will start to move in early in the afternoon, probably between noon and 2 pm. Periods of light to moderate rain will likely continue for most of the race. I don't think that the rain will be so heavy that the race will need to be stopped. The rain may become more scattered in the closing hours, and there may be a few breaks in the clouds as well. I expect that the total amount of rain will be between 0.08 and 0.15 inches (2 to 4 mm). It will be a little breezy all day too, with a west wind of 10 to 15 mph in the morning and early afternoon shifting to the northwest. Gusts of 20 to 25 mph will be possible at times.