just because its raining in the city doesnt mean it will hit as hard in the valley. There has been tons of times when its raining hard in Surrey/Langley and been sunny or at least not raining in Chilliwack.
Last time i hit the flow it was supposed to rain a decent amount. We got sprinkled on for 5 mins around noon, as we were leaving though the sky opened up and let go. We seem to have excellent timing to just miss the onslaught the last few years
get the water graphs and put them into your favourites list. If it rises a good amount really quick then expect the water to be dirtier in colour. If the water is brown look for soft water and fish almost right against shore and out to where current is rougher, shallower is usually better as well in dirty water.
water graph -
http://www.wateroffice.ec.gc.ca - click "my station list" on the left side and then go through the list adding the rivers you fish or want to keep an eye on