Because Washington state is now totally a vote by mail jurisdiction (with one very small exception in one county), the only results that can be announced tonight for Referendum 71, on whether to validate a civil union-style partner registration law, will be those ballots that are received by today. However, ballots will be counted if they are postmarked by today, so there could be thousands of outstanding ballots.
From the yes on 71 campaign:
Election night trends may not be meaningful unless they show Referendum 71 being approved, in which case, the outcome will be almost certain. This is because King County will be slow to report its ballots. ... [E]lection night returns may not show Referendum 71 being approved, or showing as wide a margin of support as will be reflected in the final results. Washington is a vote-by-mail state. King County, which includes Seattle, is Washington’s most populous county and is expected to strongly favor approving Referendum 71. King County will not have many of its ballots counted until several days after the election. During the 2007 election, a similar dynamic occurred with an education measure supported by the progressive community known as “Simple Majority.” On election night, this measure appeared to be failing by 38,000 votes, but ultimately passed by over 10,000 votes. And in this election, voters seem to be holding onto their ballots longer because they are undecided about the Seattle mayoral race.