Although it sounds like it belongs over at the EMP, A deformation band is just one more way it can rain around the Pacific Northwest.

The physics behind it is almost the opposite from a Convergence Zone, but the results are nearly the same.

In this case, there's usually an area of low pressure well north of the area, and an area of low pressure well south of the area.

Since air wants to head toward lower pressure, you have some air being pulled north, and some being pulled south. The void left behind creates a vacuum of sorts, pulling up air from near the ground to replace it.

As that air rises, it condenses, bringing clouds and rain. Where as with a Convergence Zone, it's colliding winds that force air upward and provides convection, deformation bands pull air upwards to provide convection.

Deformation bands tend to bring a steady, yet light-to-moderate rain. They're notoriously hard to predict in advance, and tend to be rather persistent -- as in can bring rain for a good 6-18 hours straight, even with no organized rain systems around for hundreds of miles.