An Upper Low will move onshore today over Northern Washington. Moisture has already moved up into Southern BC around this circulation. You may have noticed a little more cloud yesterday and rain showers through the Okanagan Valley; this was the start of a shift away from sunny/dry to cloudy/wet for a couple days.
This morning's water vapour images, capture the moisture from yesterday wrapping around to southwest of Kamloops, and more is on the way later today for Southern BC. Expect showers to start late afternoon/early evening and continue overnight and into Friday with the chance of isolated thundershowers as well.
The slow moving nature of this system will keep instability and therefore showers around for the August Long weekend. However, this rain will help reduce the Wildfire Hazard.