That’s basically the first order argument for more precip in rainy season here. The general circulation will continue to deliver us a steady diet of storms and it’s going to be fully saturated because it’s winter in the PNW. So they will produce more rain when the added moisture slams into the mountains or is lifted by the fronts (which may intensify as well due to the added latent heating…).
In summer you are still increasing dew point (and probably RH) as plants transpire more to stay cool, but there are no weather systems to produce rain. So that excess moisture from our soil gets shipped off to a different region where it increases rainfall in areas that get rain in summer. Robbing Peter to pay Paul.
That’s a bit oversimplified but I think it’s the gist of it.
I feel like changes in RH are going to be the lynchpin of this case. If temperatures increase but RH remains steady, that's more water vapor, more latent heat, more water to potentially squeeze out.