I'm not 100% on the LML rivers, but assuming you're after steelhead this time of year, it's all about water levels. Cutties, it more about where the fry are at... and if there's enough viz for them to hunt. I've had great cuttie days on blown out rivers on side channels and sloughs.
In general, if it's rained hard, I try to find a dam controlled or large lake headed system that can absorb or buffer high water and at least give you a chance. But under ideal weather and water conditions, I'd rely on my bank of "secret spots" to get away from the hoard, and get out earlier than normal and hike further than normal. Steelheading is hunting more than fishing...
Decisions become easier the more familiar you are with an area and the more you have your season mapped out as far as run timing and spots. Then it's all about checking forecasts, water levels and rain gauges across a few favoured systems.
I'm on the far end of VI, so it's a little easier - no dam controlled rivers, lots of remoteness, but still plenty of popular spots that can be busy-ish, but never anything that compares with the LML (Vedder) or down island (Stamp or Cowichan). Trouble is, if it's a big enough storm, everything blows out and that's that...
If I were to systematize it, I'd sort out what rivers I like to fish, and then the run timing. Then learn a few proven spots or beats to fish on each river. Once you have a repertoire built over a few hard years, and get to know how each system behaves with different rain events or cold snaps, then you can fine tune it. Eventually you wont even look at rain gauges and will just know that a 3 days of 20mm with a taper to 5-10mm for a few days is ok and can even be ideal if it's snowing in the high country, but one day of 50+mm and a warm snap is a usually disaster on most systems. That being said, 15 days of sun and below freezing temps can be even worse... depending on the system.
Fall salmon is a little simpler, but follows the same learning curve.
Enjoy the journey!
