In reply to Chris F:
The past July was one of the driest ever measured at Squamish.
August has had a few days of rain.
There's much more to climbing in SW BC than just Squamish and Skaha. Marble Canyon has trad and sport limestone up to 15 pitches long, east of Pemberton 2 hours. Kelowna and Vernon have a developing cragging scene with about 500 routes between them, 1 hour north of Skaha.
For a remoter version of Squamish with plenty of new route potential, Eldred Valley near Powell River is well worth a visit.
Any of these places will be ideal for a visit in September, as will all of the alpine climbing venues near Vancouver (and it will be cold at night, killing off all the mosquitoes and blackflies)
The first three weeks of September are usually bone-dry. Fall monsoons usually begin reliably about October 15th.