The men's cycling time trials this afternoon, where Team GB's Bradley Wiggins is favourite for gold, should escape any rain.
However the women's event, which begins at 10.30am, may start amid light drizzle.
Harmless white cloud is expected to dominate the skies over Wimbledon for most of the day. However, a brief downpour is forecast between 12pm and 1.30pm. Andy Murray is due on Centre Court at around 4pm.
Persistant rain is forecast throughout the day in Cardiff, however it should subside in time for Team GB's crucial football match with Uruguay at the Millennium Stadium (kickoff 7.45pm).
Great Britain's bid for rowing gold at Eton Dorney, which begins at 9.30am, is expected to be under rainy skies.
