I have been visiting Bourg St Maurice, almost every year since 1983. It’s nearly always been hot and sunny for every summer visit, with temperatures around 30 degrees Celsius or even more (38C once!). It’s cooler high in the mountains of course – something akin to a spring day in England. Bourg is in the valley bottom, relatively low at 850 metres altitude, and you certainly feel the heat as you descend from a day’s riding in the mountains. The main swimming pool is open all year round, with the open air part open from 1st July. There are additional pools attached to mountain restaurants, and a small lake further down the valley for sun-bathing & splashing around in. You will need your shades & suncream. In 20 years, rain has only been a problem once. (update, the 2006 tours saw some rain as well)
The weather is not guaranteed of course, but I’ve only known one summer trip badly affected by days and days of rain.
I wish I had NEVER said this back in 2003. In 2004, we got heavy rain on BOTH Alps tours. It was a close call, but rain didn't stop play. Yes, it rained for about half the week, but we still got clear weather in the latter half of both trips, and still completed the high passes in sunshine. (although an early run up the Col saw snow and sub-zero temperatures!)
One benefit of a night time shower of rain in these mountainous areas is, you’ll wake up to a fresh covering of snow on the highest peaks – a stunning sight in the morning sunshine.