This is the first presidential election King County will conduct entirely vote-by-mail, so it's hard to make apple-to-apple turnout and early voting comparisons. But if you're wondering about how much of the vote will be tallied in tonight's first and only ballot drop, it is fair to expect quite a bit more than years past.

For example, in 2010, King County Elections tallied only 254,261 ballots on election night out of the 766,477 that were ultimately cast in the US Senate race between Patty Murray and Dino Rossi. That's only 33 percent. But working from yesterday's ballot return statistics and assuming a turnout of about 84 percent, King County is on pace to report over 60 percent of the final vote at 8:15 p.m. tonight.

There are 1,181,700 registered voters in King County. As of the close of business yesterday, King County Elections reported 608,905 ballots verified. I'm waiting on confirmation on whether these are ballots that have simply been received and signature matched, or whether that number represents ballots that have actually been scanned. About 10,000 additional ballots have been received and set aside as "challenged" due to missing or mismatched signatures.

Either way, in past elections the number of ballots counted Tuesday night somewhat correlated with the number of ballots on hand Monday morning, and given the streamlined operations I witnessed yesterday at the county's new ballot processing facility in Renton, there is every reason to expect King County Elections to far exceed past performance. And tonight's ballot drop will be further padded by thousands of accessible voting machine results. So 608,905 strikes me as a conservative estimate of tonight's totals, especially considering that it doesn't include the processing that is being performed today.

At 81 percent turnout (which is what Secretary of State Sam Reed is projecting statewide), that would be 63.6 percent of total ballots cast. At 84 percent turnout (about what we saw in 2008), that would be 61.3 percent. At 87 percent turnout (which is what some observers are optimistically predicting given the pace of early returns), that would be 59.2 percent.

By any scenario, that's right in line with the 60 percent ballot drop Reed is projecting statewide, so we're sure to have a much better picture of winners and losers than we've had on election nights past.

But keep in mind that this also changes the expectations for tonight's ballot drop. For while we won't be able to account for any trend in late voters, Dems and progressive ballot measures can't count on the usual come from behind dynamic we've recently seen thanks to slow returns in King County. For the first time in years, we appear to be on pace to tally returns at a rate comparable to the state as a whole, and if that turns out to be true, tonights results should largely reflect the final results in all but the very closest statewide races.

UPDATE: Barbara Ramey at King County Elections emails me that "we expect to include at least 500,000 ballots in the count tonight." That would amount to over 50 percent of total ballots cast given an 84 percent turnout, short of my projection, but still far better than past years.