Washington, like California, uses the top-two primary system, so both Democrats and Republicans run on the same ballot. That essentially makes tonight like a massive poll of the general election -- a chance to check in and see how many people vote for Democratic candidates (in total) vs. Republican ones (in total).
Keep an eye on the 3rd and 5th Districts. We think they lean toward Republicans right now, but maybe tonight's results will show unexpected Democratic strength. And the 8th District is expected to be a toss-up; this will be a good sanity check to see if it really is 50-50.